An illustrated atlas of the vertebral morphology of extant non-caenophidian snakes, with special emphasis on the cloacal and caudal portions of the column

Zbigniew Szyndlar; Georgios L. Georgalis

doi:10.3897/vz.73.e101372

Research Article

An illustrated atlas of the vertebral morphology of extant non-caenophidian snakes, with special emphasis on the cloacal and caudal portions of the column

Zbigniew Szyndlar^‡, Georgios L. Georgalis^‡

‡ Institute of Systematics and Evolution of Animals, Polish Academy of Sciences, Kraków, Poland

Corresponding author: Georgios L. Georgalis ( dimetrodon82@gmail.com )

Academic editor: Uwe Fritz

This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Citation: Szyndlar Z, Georgalis GL (2023) An illustrated atlas of the vertebral morphology of extant non-caenophidian snakes, with special emphasis on the cloacal and caudal portions of the column. Vertebrate Zoology 73: 717-886. https://doi.org/10.3897/vz.73.e101372

ZooBank: urn:lsid:zoobank.org:pub:8F3D5EDA-2F18-4E5C-A53E-2F7741FF1339

Abstract

We here present a thorough documentation of the vertebral morphology and intracolumnar variation across non-caenophidian snakes. Our studied sample of multiple individuals covers a large number of genera (67) and species (120), pertaining to almost all extant non-caenophidian families. Detailed figuring of multiple vertebrae across the trunk, cloacal, and caudal series for many different individuals / taxa documents the intracolumnar, intraspecific, and interspecific variation. An emphasis is given in the trunk-to-caudal transition and the pattern of the subcentral structures in that region of the column. Extant non-caenophidian snakes show an astonishing diversity of vertebral morphologies. Diagnostic vertebral features for extant families and many genera are given, though admittedly vertebral distinction among genera in certain groups remains a difficult task. A massive compilation of vertebral counts for 270 species, pertaining to 78 different genera (i.e., almost all known valid genera) and encompassing all extant non-caenophidian families, is provided based on our observations as well as an extensive literature overview. More particularly, for many taxa, detailed vertebral counts are explicitly given for the trunk, cloacal, and caudal portions of the column. Extant non-caenophidian snakes witness an extremely wide range of counts of vertebrae, ranging from 115 up to 546. A discussion on the diagnostic taxonomic utility and potential phylogenetic value of certain vertebral structures is provided. Comparisons of the subcentral structures of the cloacal and caudal vertebral series are also made with caenophidian lineages. We anticipate that this illustrative guide will set the stage for more vertebral descriptions in herpetological works but will also be of significant aid for taxonomic identifications in ophidian palaeontology and archaeozoology.

Keywords

Intracolumnar variation, osteology, Serpentes, Squamata, taxonomy, vertebral counts, vertebral morphology

Introduction

It is truly wonderful to see the work of hands, feet, fins, performed by a simple modification of the vertebral column.

Richard Owen (1849–1884: 153)

To be sure, such “modification” is far from “simple”, but nevertheless, with this phrase, Richard Owen demonstrated his fascination about the morphology and adaptations of the snake axial skeleton. Indeed, the prominent scientist was one of the first to foray into the morphology of snake vertebrae, coining the terminology of classical structures, such as the zygosphene and the zygantrum (Owen 1850). In this latter monograph principally devoted to fossil remains, Owen (1850) described in detail (and supplemented with nice illustrations), vertebrae of a few living ophidian species, spanning different regions of the vertebral column. Of course, snake vertebrae had attracted the interest of reptile naturalists even earlier. Indeed, certain preliminary attempts to describe and/or figure snake vertebrae had been already made by the end of the 18^th and the first half of the 19^th centuries (Lacépède 1789; Bonnaterre 1790; Gray 1831; D’Alton 1836; Schlegel 1837; Grant 1841; Wagner 1841), including also a more preliminary work by Owen (1841). As a matter of fact, some of these early anatomical works contain really interesting figures and early descriptions (e.g., D’Alton 1836), setting the stage for subsequent comprehensive works on ophidian vertebrae in the next decades.

Important descriptions of snake vertebral morphology were subsequently made by Fischer (1857) and again, Owen (1853, 1857, 1866, 1877). A few decades later, Rochebrune (1880 and especially 1881) conducted a rather detailed, for his time, survey of the vertebral morphology of multiple snake taxa and provided important features for taxonomic identifications and further analyzed intracolumnar variation. Unfortunately though, the illustrated vertebrae of Rochebrune (1881) originate exclusively from the anterior portion of the column, while moreover, some data are apparently not very reliable. For instance, he overestimated the number of cloacal vertebrae in most snakes (see the entry “Parts of the vertebral column” below). Other works in the late 19^th century also dealt with the vertebral morphology of snakes (e.g., Salle 1880; Nicholson and Lydekker 1889; Hoffmann 1890; Boulenger 1893).

Since the onset of the 20^th century, an overwhelming number of studies have dealt extensively with the vertebral morphology of snakes, diagnostic features of each group, and/or the recognition of intracolumnar vertebral position and variation (e.g., Abel 1919; Gilmore 1938; Hoffstetter 1939a; Sood 1948; Johnson 1955; Auffenberg 1963; Bullock and Tanner 1966; List 1966; Hoffstetter and Gasc 1969; Gasc 1974; Dowling and Duellman 1978; Szyndlar 1984; LaDuke 1991; Holman 2000; Venczel 2000; Szyndlar and Rage 2003; Ikeda 2007; Smith 2013; Zaher et al. 2019).

Among these studies on the morphology of snake vertebrae it is important to highlight the works of Robert Hoffstetter and his co-authors and followers, who contributed significantly to the development of our understanding of ophidian postcranial osteology, both in extant and extinct snakes. The up-to-date knowledge on the vertebrae of modern reptiles was summarized in the monographic study by Hoffstetter and Gasc (1969), a work that stands as a key reference up to this day. That work described the morphology of the entire vertebral column from the atlas to the tip of the tail and also considered adaptive features of snake vertebrae; it lacked, however, a systematic description of vertebrae in particular of higher taxa of snakes. The eleventh volume of the “Handbuch der Paläoherpetologie”, i.e., the monumental treatise on Serpentes by Jean-Claude Rage (Rage 1984), bridged in part this gap, providing brief characteristics of the vertebral morphology of all then-known (both extant and extinct) ophidian families. Worth mentioning is also the monograph of Walter Auffenberg (1963), where he gave a very useful “résumé” of terminology and diagnostic features found in isolated ophidian vertebrae. However, it should be stressed that the interest of most North American authors of the 20^th century, including Auffenberg (1963) himself as well as his predecessors (e.g., Gilmore 1938) and successors (e.g., Holman 1979, 2000) was restricted exclusively to mid-trunk vertebrae.

Besides, important studies on the vertebral morphology of individual taxa have also been conducted (e.g., Baumeister 1908; Hoffstetter 1939b; Sood 1941; Gans 1952; Dowling 1959; Williams 1959; Bogert 1964, 1968a, 1968b; Hoffstetter and Gayrard 1964; Holman 1967; Thireau 1967, 1968; McDowell 1968; Hoffstetter and Rage 1972; Hecht and LaDuke 1988; Szyndlar 1988, 1994; Szyndlar and Schleich 1994; Walker 2003; Albino 2011; Pinto et al. 2015; Onary and Hsiou 2018; Koch et al. 2019, 2021; Weinell et al. 2020; Head 2021; Martins et al. 2021a, 2021b; Smith and Scanferla 2021; Georgalis and Szyndlar 2022). It has to be emphasized that, notably, many of these works in the past and current decades, have been substantially aided by the usage of modern, non-invasive technologies, primarily micro-CT (μCT) scanning and 3D imaging, which allowed skeletal investigation of rare taxa preserved in wet collections and/or tiny specimens which would otherwise be difficult to skeletonize (see also discussions in Bell and Mead 2014; Bell et al. 2021).

Furthermore, analytical approaches and quantitative analyses (e.g., morphometrics, landmark analyses, etc.) and/or study of the functional morphology on snake vertebrae have been applied (e.g., Johnson 1955; Gasc 1974, 1976; Lindell 1994) but more rapidly in the past two decades (e.g., Baszio and Weber 2002; Polly and Head 2004; Lawing et al. 2012; Houssaye et al. 2013; Bochaton et al. 2019; Jurestovsky et al. 2020; Bochaton and Hanot 2021; Herrel et al. 2021). The internal anatomy of snake vertebrae has received significantly less attention (e.g., Sood 1948), but eventually more recently, attempts have also been made to decipher several aspects on their microanatomy and histology (e.g., Buffrénil and Rage 1993; Houssaye et al. 2010, 2013). Finally, the embryology and developmental mechanisms of the snake postcranial skeleton have also been the focus of several different studies (e.g., Winchester and Bellairs 1977; Cohn and Tickle 1999; Polly et al. 2001; Gomez et al. 2008; Vonk and Richardson 2008; Woltering et al. 2009; Di-Poï et al. 2010; Tsuihiji et al. 2012; Head and Polly 2015; Werneburg and Sánchez-Villagra 2015; Aires et al. 2016; Guerra-Fuentes et al. 2023).

Difficulties surrounding the study of snake vertebrae arise due to the high variability of vertebral morphology even within a single species (e.g., ontogenetic or, more rarely, sexual variation), but also even within a single individual (intracolumnar variation) (Bogert 1964; Keiser 1970; Gasc 1974; Szyndlar 1984, 1991a, 1991b; Szyndlar and Rage 2003; Petermann and Gauthier 2018; Georgalis and Scheyer 2019; Georgalis and Szyndlar 2022). An analysis made by Johnson (1955) revealed that vertebral shape was associated with phylogenetic relationships rather than mode of life of particular ophidian lineages. But Gasc (1974, 1976) showed that vertebral form is highly correlated with biomechanical functions and therefore some features may be adaptations overlying phylogenetic characters, a view that has been reinforced by recent research as well (e.g., Herrel et al. 2021). This may explain why, except for ophidian palaeontology, where the majority of fossil record is restricted exclusively to isolated vertebrae, the use of postcranial elements in extant snake classification schemes has been and still remains infrequently applied, compared to the undoubted diagnostic utility of cranial elements.

As a matter of fact, the most common vertebral feature fairly often employed or discussed by herpetologists in snake classifications since the time of Edward Drinker Cope (Cope 1864, 1886, 1893, 1895) and George Albert Boulenger (Boulenger 1893) and until the onset of the 21^st century, has been the presence or absence of hypapophyses on posterior trunk vertebrae (e.g., Rosén 1905; Smith 1943; Hoffstetter 1946; Malnate 1972). “The condition of the posterior trunk vertebrae in respect of hypapophyses” was the only vertebral feature applied in the classification of Underwood (1967: 25) who considered it “one of the classical systematic characters of snakes”. Notably, some herpetologists also characteristically named new snake ranks exclusively on the basis of this feature (e.g., Hypophysia and Anhypophysia of Smith 1943), while others prompted that the presence or absence of hypapophyses on posterior trunk vertebrae should not be considered alone as the most optimum taxonomic criterion, as this could be variable within a species (Rosén 1905).

However, the taxonomic importance of other vertebral structures has been more rarely raised among herpetologists, and this is particularly true for 20^th century workers. As summarized by McDowell (1987: 4), “closely related forms, however, may sometimes have quite different vertebrae, and differences within the column of a single individual may exceed those between corresponding vertebrae of different genera”; this opinion has been widespread, although true in part only. Anyway, that is the main reason that vertebral characters have been largely omitted in constructing classifications or in phylogenetic analyses of particular lineages of snakes. Cadle (1987) further pointed out that the inadequate survey of vertebral morphology and variability of extant snakes has hindered an effective use of the fossil record (see also Szyndlar 1991b). This is definitely true, as vertebral characters must necessarily be employed in palaeontology, since the vast majority of ophidian fossil record consists of isolated vertebrae (Rage 1984; Szyndlar 1984, 1991a, 1991b; Venczel 2000; Georgalis et al. 2021a; Head et al. 2022; Ivanov 2022; Smith and Georgalis 2022).

In this present monograph, we attempt to collectively decipher the vertebral morphology of non-caenophidian snakes, focusing on important diagnostic features of each lineage and intraspecific variability. For the first time, this study covers all main parts of the vertebral column, based on observations of numerous specimens (mostly complete postcranial skeletons) belonging to 67 genera and 120 species, encompassing almost all families of non-caenophidian snakes (see Appendices 1–2 for complete lists of taxa). Nevertheless, contrary to former works, the focus of attention in the present study are vertebrae situated around the cloacal region of the body, i.e., a group of posteriormost trunk to anterior caudal vertebrae. As demonstrated, this portion of the vertebral column displays interesting diagnostic features and can be the basis for differentiating major lineages of snakes and identifying isolated fossil remains. The remaining parts of the column are also considered, although (contrary to standard papers on ophidian osteology), descriptions of vertebral elements are here reduced to very parsimonious statements. Instead, the text is supplemented with numerous detailed figures concentrated mainly at the vertebrae surrounding the cloacal region, but showing also those coming from other portions of the column. Vertebral counts (either total or more specifically detailed across the trunk, cloacal, and caudal series) are provided for many non-caenophidian genera and species (in total 78 genera and 270 species), either from our direct observations or our thorough review of the existing literature and personal communications with other colleagues (see Appendices 3–4 for complete lists of taxa). Besides, we make an attempt to employ some vertebral characters for interpreting interrelationships within and among particular lineages of snakes, although we emphasize the strong degree of convergence among certain distantly related groups. We anticipate that this illustrated guide with the detailed depiction of the intracolumnar variability of multiple non-caenophidian snake taxa will permit more reliable taxonomic identifications of isolated vertebrae (fossil or recent), setting the stage for a more comprehensive understanding of the evolution, functional morphology, and potential phylogenetic and systematic values of the ophidian vertebral column.

Material and methods

A large number of dry skeletons, pertaining to almost all families of non-caenophidian snakes was studied. In addition, we studied 3D models of μCT scanned anomalepidid, gerrhopilid, and typhlopid skeletons, which were otherwise unavailable as dry skeletons, while we also surveyed additional μCT scans of other snake taxa for counting numbers of vertebrae. These 3D models and μCT scans were directly provided by colleagues or were available from Morphosource (https://www.Morphosource.org) – see details for each of these specimens in their respective entries or also in the Acknowledgements section below. In total, our studied sample includes numerous specimens, pertaining to 67 genera and 120 species (see Appendices 1–4 for complete lists of taxa). The only families that are lacking from this work are Xenotyphlopidae and Xenophidiidae, for which no specimen was accessible for study; also, for Anomochilidae, only an X-ray was available for the posteriormost trunk, cloacal, and caudal portion of the column of a skeleton. Accordingly, for these few families that we were lacking skeletal material for study, we are constrained to discuss about already published figures and/or descriptions from (if available).

The illustrations show vertebrae of many of the species examined. In most cases, all vertebrae belonging to the last trunk, cloacal, and anterior caudal portions of the vertebral column are figured. Nevertheless, we also present those coming from other parts of the body (anterior trunk, anterior / mid-trunk, mid-trunk, posterior trunk, and posterior caudal).

The most commonly presented views of the vertebrae are lateral views. Customarily, the left side is figured – the right side is shown in the cases when the left part of a vertebra is damaged or if the bone displays an asymmetry. Note that in many skeletons, lymphapophyses (and sometimes pleurapophyses) are broken out; in some cases they are also partly omitted in the figures. Also, in the few figures of 3D models of scolecophidians, ribs are occasionally present or when they are not shown in the images, then the exact shape of the paradiapophyses is reconstructed.

The way of describing illustrated bones differs from conventions met in other osteological papers. Different views of the same vertebrae are linked by stripes. The illustrated vertebrae are numbered one by one from the beginning (atlas or V 1) to the end of the column. Serial numbers of each vertebra, preceded with the letter V, are put on the figures at the neural spine of its lateral view (e.g., V 150 means the 150^th vertebra in the column). Besides, the exact positions in the column of the last trunk, first cloacal, and first caudal vertebrae are indicated as in the following examples: V 245 = last T; V 246 = S 1; V 250 = C 1. When a skeleton is fragmentary or the position of vertebrae in the column uncertain, figures are described in a more general way, e.g., ant T (= anterior trunk vertebra), mid C (= middle caudal vertebra), and so forth.

Nomenclature of extant taxa follows Wallach et al. (2014) and Boundy (2021), the decisions of ICZN (1988, 2020), and the recommendations of Frétey (2019) and Frétey and Dubois (2021), coupled with an exhaustive and detailed survey of all primary literature that established all taxa treated and mentioned in this paper, i.e., the works of: Abalos et al. (1964), Adalsteinsson et al. (2009), Amaral (1924, 1955), Apesteguía and Zaher (2006), Aplin (1998), Aplin and Donnellan (1993), Bailey and Carvalho (1946), Baird and Girard (1853), Barbour (1914a, 1914b), Barbour and Loveridge (1928), Barbour and Shreve (1936), Beddome (1863a, 1863b, 1867, 1877, 1886), Berg (1901), Bianconi (1849), Blainville (1835), Bocage (1866, 1873, 1890), Boettger (1887, 1913), Bogert (1968a), Boié (1827), Bonaparte (1831, 1845), Bosch and Ineich (1994), Bostanchi et al. (2006), Boulenger (1884, 1888, 1889, 1890a, 1892, 1893, 1895a, 1895b, 1896a, 1896b, 1898, 1906, 1912), Bowdich (1819), Broadley and Wallach (2000, 2007, 2009), Brongersma (1951, 1953), Burmeister (1861), Chabanaud (1917), Cope (1861a, 1861b, 1862a, 1862b, 1866, 1868a, 1869, 1875, 1879, 1896), Cundall et al. (1993), Cuvier (1829), Das et al. (2008), Daudin (1803a, 1803b), Degerbøl (1923), Duméril (1856), Duméril and Bibron (1844), Duméril and Duméril (1851), Dunn (1944, 1946), Dunn and Conant (1936), Esquerré et al. (2020), Eversmann (1823), Fischer (1856), Fitzinger (1826, 1843), FitzSimons (1932, 1962), Gans and Laurent (1965), Garman (1884), Georgalis et al. (2019), Gmelin (1789), Gow (1977), Grandidier (1872), Gray (1825, 1842, 1845, 1849, 1858a, 1858b, 1860), Griffin (1916), Guibé (1952), Günther (1862, 1864, 1868, 1875, 1877), Günther and Manthey (1995), Haas (1979), Hallowell (1848), Head et al. (2009), Hedges (2008, 2011), Hedges and Thomas (1991), Hedges et al. (2014), Hemprich (1820), Hoffstetter (1946, 1959), Hoffstetter and Rage (1972, 1977), Hoge (1954), Holman and Case (1992), Hoogmoed (1977), Hornstedt (1787), Hsiou et al. (2014), Hubrecht (1879), Jan (1861, 1863, 1864), Jan and Sordelli (1860–1866), Joger (1990), Kelaart (1853), Klauber (1939), Klein et al. (2017), Kluge (1993a), Koch et al. (2019), Kraus (2017), Krefft (1864), Kuhl (1820), Lacépède (1801, 1804), Lataste, (1887), Laurent (1949, 1956, 1964), Laurenti (1768), Lidth de Jeude (1890), Linnaeus (1758), Loveridge (1932, 1941, 1942), Machado (1944), Martins et al. (2023), Massalongo (1859), McCartney and Seiffert (2016), McDowell (1987), Merrem (1820), Meyer (1874), Mocquard (1897, 1905), Montague (1914), Müller F (1880), Müller J (1831), Nopcsa (1923), O’Shea et al. (2015); Oken (1816), Oppel (1811), Orejas-Miranda (1961), Orejas-Miranda and Peters (1970), Orejas-Miranda and Zug (1974), Orejas-Miranda et al. (1970), Orlov et al. (2022), Ortega-Andrade et al. (2022), Owen (1841), Pallas (1773), Parker (1930, 1939), Passos et al. (2006), Peracca (1896, 1910), Peters (1854, 1857, 1858, 1860a, 1860b, 1863, 1864a, 1864b, 1865, 1868, 1873, 1874, 1875, 1876, 1877, 1878, 1879, 1880, 1881), Peters and Doria (1878), Pinto and Fernandes (2012), Pyron et al. (2014), Rage (1983, 1988, 2008a), Rage and Augé (2010), Rage and Escuillié (2000, 2002), Rage et al. (2003, 2008), Ramón de la Sagra (1838–1843), Reynolds et al. (2014), Richmond (1955, 1964), Robb (1972), Romer (1956), Roux-Estève (1974a, 1980), Russell (1802), Salazar et al. (2015), Sauvage (1880), Scanferla and Smith (2020a), Scanlon (1992), Schlegel (1837, 1837–1844, 1848, 1872), Schmidt (1923, 1933), Schneider (1801), Schwartz (1957), Scortecci (1929, 1939), Shaw (1802), Shea (2015), Smith A (1838–1849), Smith MA (1940), Smith MJ (1976), Smith KT and Scanferla (2021), Steindachner (1880), Stejneger (1889, 1891, 1894, 1901, 1904, 1907), Sternfeld (1908, 1910, 1913), Storr (1981), Stull (1928, 1938, 1956), Szyndlar and Böhme (1996), Taylor (1919, 1939, 1940), Tchernov et al. (2006), Theobald (1876), Thomas O (1913), Thomas R (1965), Thomas R et al. (1985), Vanzolini (1976, 1996), Vidal et al. (2007, 2010), Villiers (1956), Vullo (2019), Wagler (1824, 1828, 1830), Waite (1893, 1894, 1897, 1918a), Wall (1921), Wallach (1995), Wallach and Glaw (2009), Wallach and Günther (1998), Wallach and Ineich (1996), Wells and Wellington (1984, 1985), Werner (1901, 1909), Witte (1953), Woodward (1901), Zaher et al. (2009), and Zhao (1972). Following the recent recommendations of Benichou et al. (2022) about the best best practices for the citation of authorities of scientific names in taxonomy, we provide all authorships for all genera and species upon their first appearance in the text and include also these authorships in the references list.

Institutional abbreviations

AMNH, "American Museum of Natural History, New York, USA; AMS, Australian Museum, Sydney; CAS, California Academy of Sciences, San Francisco, USA; CM, Carnegie Museum of Natural History, Pittsburgh, Pennsylvania, USA; CPZ-UV, Colección de Prácticas Zoológicas Universidad del Valle, Cali, Colombia; FMNH, Field Museum of Natural History, Chicago, USA; HNHM, Hungarian Natural History Museum, Budapest, Hungary; ISEZ, Institute of Systematics and Evolution of Animals, Polish Academy of Sciences, Kraków, Poland; IVPP, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Beijing, China; KUBI, Kansas University Biodiversity Institute and Natural History Museum, Herpetology collection, Lawrence, USA; LSUMZ, Louisiana State Museum of Natural History, Baton Rouge, USA; MCZ, Museum of Comparative Zoology, Harvard University, Cambridge, USA; MGPT-MDHC, Massimo Delfino Herpetological Collection, Department of Earth Sciences, University of Torino, Italy; MNCN, Museo Nacional de Ciencias Naturales, Madrid, Spain; MNHN, Muséum national d’Histoire naturelle, Paris, France; MNHW, Museum of Natural History, Wrocław, Poland; MSNS, Museo di Storia Naturale “La Specola”, Florence, Italy; MVZ, Museum of Vertebrate Zoology, University of California, Berkeley, California, USA; NCSM, North Carolina Museum of Natural Sciences, Raleigh, North Carolina, USA; NHMC, Natural History Museum of Crete, Herakleion, Greece; NHMUK, Natural History Museum, London, United Kingdom; NHMW, Naturhistorisches Museum Wien, Vienna, Austria; NMNH, Smithsonian National Museum of Natural History, Washington, DC, USA; PIMUZ, Palaeontological Institute and Museum of the University of Zurich, Switzerland; QM, Queensland Museum, Brisbane, Australia; RMNH, Naturalis-Nationaal Natuurhistorisch Museum [formerly Rijksmuseum van Natuurlijke Historie], Leiden, Netherlands; SAMA, South Australian Museum, Adelaide, Australia; SIZNASU, Schmalhausen Institute of Zoology of National Academy of Sciences of Ukraine, Kiev, Ukraine; SMF, Herpetology collection, Senckenberg Research Institute and Natural History Museum, Frankfurt am Main, Germany; SMF-PH, Paleoherpetology collection, Senckenberg Research Institute and Natural History Museum, Frankfurt am Main, Germany; Tokar Coll., Herpetological Collection of Anatoly Tokar, Kiev, Ukraine; UCM, University of Colorado Museum of Natural History; UF Herp, Florida Museum of Natural History, Herpetology collection, Gainesville, USA; UMMZ, University of Michigan, Museum of Zoology, Ann Arbor, USA; UMZC-R, University Museum of Zoology, Reptile collection, University of Cambridge, Cambridge, United Kingdom; USNM, Smithsonian Institution, National Museum of Natural History, Washington, USA; UTACV, University of Texas at Arlington, USA; YPM, Yale Peabody Museum of Natural History, Herpetology collection, New Haven, Connecticut, USA; ZFMK, Zoologisches Forschungsmuseum Alexander Koenig, Bonn, Germany; ZMB, Museum für Naturkunde, Berlin, Germany.

Abbreviations on the position of vertebrae in the column

ant, anterior; C [followed by a number], serial number of a caudal vertebra; last T, last trunk vertebra; mid, middle; post, posterior; S [followed by a number], serial number of a cloacal vertebra; V [followed by a number], serial number of a vertebra.

The snake vertebral morphology

Parts of the vertebral column

Snakes demonstrate a great variability in their total number of vertebrae compared to limbed amniotes (Head and Polly 2007; Gomez et al. 2008; Vonk and Richardson 2008; Müller et al. 2010; Bergmann and Irschick 2011). The total number of vertebrae varies significantly among different snake species: the lowest known number pertains to the extant Afrotyphlops cuneirostris (Peters, 1879) (115 vertebrae in total; Gans and Laurent 1965; Alexander and Gans 1966; Roux-Estève 1974a); on the other hand, the highest number is achieved by the extinct Eocene taxon Archaeophis proavus Massalongo, 1859 (around 565 total; Janensch 1906). Among extant snakes, the highest vertebral counts are observed in the leptotyphlopid genus Rhinoleptus Orejas-Miranda, Roux-Estève & Guibé, 1970, which achieves the extraordinary number of 546, followed by pythonids, some boids, the typhlopid genus Letheobia Cope, 1869, and some other scolecophidians. Worth noting here is the case of the Australian pythonid Nyctophilopython oenpelliensis (Gow, 1977), for which Scanlon (1996: 299) mentioned that it is very elongate and could probably have “more vertebrae than any other animal”. Scanlon (1996) based this assumption on the ventral and subcaudal scale counts of this taxon (Gow 1977), and estimated that the total number of vertebrae could be around 600, with up to 445 trunk vertebrae! Although we consider that this assumption could be probable, nothing has been formally published on this and we had not available any X-ray or μCT scan of this taxon to test this extreme count.

The correlation of the number of ventral scales (also known as ventral annuli or just ventrals) with the respective number of vertebrae has received the attention of several works (Jourdran 1904; Ruthven and Thompson 1913; Bellairs and Underwood 1951; Gans and Taub 1965; Alexander and Gans 1966; Guibé et al. 1967; Roux-Estève 1974a, 1974b; Voris 1975; Gower and Ablett 2006; Lee et al. 2016). Indeed, although in most snakes, the number of ventrals correlates 1:1 with the number of vertebrae, this is not the case with few other groups, primarily fossorial taxa (i.e., typhlopoids and anomalepidids), where this number can vary between 1.4:1 and 2.3:1 (see Alexander and Gans 1966; Roux-Estève 1974a, 1974b; Wallach and Ineich 1996). Note that the 2:1 correlation number has been in the past also suggested for leptotyphlopids and uropeltids by Bellairs and Underwood (1951) but was shown to be erroneous by Alexander and Gans (1966). As a matter of fact, leptotyphlopids, aniliids, tropidophiids, uropeltids, cylindrophiids, and Constrictores are known to maintain the 1:1 ratio of number of ventrals to vertebrae (Alexander and Gans 1966). In any case, we refrain from using ventral scale numbers for inferring any vertebral counts, and we only use them for the case of xenophidiids, where no other data are available.

There is no general consensus on how to divide the ophidian vertebral column and how to name its particular parts. In the present paper, we employ the subdivision of the entire vertebral column into four main parts: atlas-axis complex, trunk portion, cloacal portion, and caudal portion (Fig. 1). Note, however, that many authors have expanded the term “caudal” also to the cloacal vertebrae (e.g., anterior caudal region of Sood 1941; caudal versus precaudal vertebrae of Holman 1967), whereas some others dealt the trunk and cloacal vertebrae jointly (e.g., Alexander and Gans 1966). Other criteria for divisions and terminologies have also been applied in the literature: certain authors divided the vertebrae on the basis of the absence or presence of lymphapophyses (pre-lymphapophyseal etc.) (e.g., Petermann and Gauthier 2018), or divided them into lumbar and not lumbar (Bullock and Tanner 1966; Holman 1973), or also called the trunk vertebrae as thoracolumbar (e.g., Pinto et al. 2015). Owen (1850) divided the vertebral column into the atlas, axis, trunk vertebrae (that supported moveable ribs), and caudal vertebrae. Rochebrune (1880, 1881) divided the skeletal column of snakes into five distinct areas: cervical (“cervicale”; including though solely the atlas and axis), thoracic (“thoracique”), pelvic (“pelvienne”), sacral (“sacrée”), and coccygeal or caudal (“coccygienne”, “caudale”). Simpson (1933) slightly modified Rochebrune’s arrangement and proposed three areas: cervical, thoracic (divided into anterior and posterior thoracic), and caudal (divided into anterior and posterior caudal). Gasc (1974) further divided the trunk region with respect to the position of the vertebrae with the heart, i.e., a “précardiaque” sub-region for those vertebrae anterior to the heart (roughly corresponding with the anterior trunk) and a “post-cardiaque” sub-region for those posterior to the heart (roughly corresponding with the mid- and posterior trunk).

Figure 1.

Nomenclature of anatomical structures in snake vertebrae (A–M Epicrates cenchria [ISEZ R/437]; N Tropidophis jamaicensis [NHMUK 1964-1239]). A–B anterior trunk vertebra; C–G mid-trunk vertebra; H–I posteriormost trunk vertebra; J–K cloacal vertebra; L–M caudal vertebra; N posterior trunk vertebra. Views: A, C, H, J, L left lateral views; D dorsal view; E ventral view; B, F, I, M, N anterior views; G posterior view. Anatomical abbreviations: c centrum; cd condyle; ct cotyle; d diapophysis; h hypapophysis; ha haemapophysis; hk haemal keel; ir interzygapophyseal ridge; lf lateral foramen; ls lymphapophysis; na neural arch; nc neural canal; ns neural spine; p parapophysis; pcf paracotylar foramen; pd paradiapophysis; pl pleurapophysis; po postzygapophysis; poa postzygapophyseal articular facet; pr prezygapophysis; pra prezygapophyseal articular facet; prp prezygapophyseal accessory process; pt pterapophysis; sf subcentral foramen; sg subcentral groove; sr subcentral ridge; z zygosphene; zy zygantrum. For abbreviations of anatomical structures of the erycid caudal skeleton, see Fig. 106.

In fact, the greatest controversies in naming parts of the ophidian vertebral column concern just the region situated around the cloaca (pericloacal vertebrae sensu Smith 2013). Interestingly, Robert Hoffstetter himself, for many decades a leading authority in the postcranial osteology of snakes, changed the nomenclature in different stages of his career. Hoffstetter (1939a) used the term “région caudale” for all vertebrae located behind the “région dorsale”. He figured (Hoffstetter 1939a: fig. 10E–G) cloacal and caudal (in the meaning accepted in the present paper) vertebrae of the colubrid Dolichophis jugularis (Linnaeus, 1758); he named the former “vertébre caudale antérieure”, whereas the latter “vertébre caudale moyenne” and “vertébre caudale postérieure”. The same figures redrawn later by Hoffstetter and Gasc (1969: figs 77–78) were described as “cloacal vertebra” and “post-cloacal (caudal) vertebrae”, respectively. The system employed here follows that proposed by Hoffstetter and Gasc (1969).

The first and second vertebrae, termed atlas and axis (also known in the literature as “epistropheus”) respectively, connect the column with the occipital area of the skull. Except for the uropeltids, which are characterized by a unique atlas-axis morphology and articulation (Baumeister 1908; Williams 1959), both elements display a relatively homogenous morphology in virtually all snakes. Nevertheless, the few existing studies on this subject showed that there is some variability and distinctiveness in these elements across snake taxa (e.g., Ruben 1977; Garberoglio et al. 2019; Martins et al. 2021a, 2021b). Moreover, it has to be highlighted that the squamate atlas-axis complex has been recently demonstrated to be an important source of new morphological characters, particularly for lizards (Čerňanský 2016).

The trunk portion of the column is subsequently subdivided into anterior trunk vertebrae (called by certain workers as cervical vertebrae), mid-trunk vertebrae, and posterior trunk vertebrae (Fig. 1A–I, N). The anteriormost trunk vertebrae, following the atlas-axis complex, are provided with hypapophyses and accompanied by ribs. There is no clear and definite border between the anterior and mid-trunk vertebrae in snakes possessing hypapophyses throughout the trunk portion of the column. In the remaining snakes (with hypapophyses restricted to the anterior trunk region), the term “anterior trunk vertebrae” is usually attributed just to the “cervical vertebrae”. The number of anterior trunk vertebrae provided with hypapophyses differs considerably in various groups of snakes: in scolecophidians the presence of hypapophyses is restricted to a few anterior trunk vertebrae only, whereas in some pythons they disappear at the level of the 80^th to 90^th vertebrae. The anterior (in particular anteriormost) trunk vertebrae, are of little use for purposes of identification. It is worth noting that certain species of the extinct enigmatic group of Palaeophiidae, possessed two hypapophyses (an anterior one [at the anterior portion of the centrum] and a posterior one [at the posterior portion of the centrum]) in their anterior trunk vertebrae (Rage et al. 2003; McCartney and Seiffert 2016; Georgalis et al. 2021b; Georgalis 2023).

The mid-trunk (or middle trunk) vertebrae are most numerous and at the same time morphologically most homogenous in the column. Those following immediately the anterior portion of the column are usually characterized by the highest neural spines and greatest absolute dimensions (length and width) (see also graphs in Hoffstetter 1960: figs 1–4). Mid-trunk vertebrae are the most often reported (and perhaps best known) vertebral elements of ophidians, both extant and extinct.

There is no clear border between the mid- and posterior trunk vertebrae. The latter term is usually attributed (semi)arbitrarily to a number (one or more dozens) of the vertebrae situated prior to the cloacal region. Generally, the posterior trunk vertebrae are smaller and relatively longer than those situated more anteriorly in the column and possess, among other features, distinctly lower neural spines, more depressed neural arches, deeper subcentral grooves, wider haemal keel, and shorter (if any) hypapophyses. Additionally, we use the term last trunk vertebra(e) for one or two vertebra(e) preceding directly the cloacal vertebrae. The last trunk vertebra is articulated with bifurcated ribs. Besides, in many snakes, in which the presence of hypapophyses is restricted to the anterior trunk portion of the column, a short hypapophysis reappears on the last (or two last) trunk vertebra(e).

The cloacal vertebrae (known in the literature also as sacral vertebrae) are those bearing forked ribs, which are typically known as lymphapophyses (also known in the literature as “processi costo-transversii”) and are typically fused to the centrum (Fig. 1J, K). Sometimes the differentiation between the cloacal and caudal portions is unclear, because the last cloacal (or first caudal) vertebra is provided with a lymphapophysis on one side and a pleurapophysis on the other side, i.e., the process is forked on one side and not on the other. The total number of cloacal vertebrae in snakes usually ranges between three and seven: it is most commonly four, often three, and seldom five or even rarer seven. Notably, in two cases (specimens of Lichanura orcutti Stejneger, 1889 [CAS Herp 200860] and Trachyboa boulengeri Peracca, 1910 [NHMUK 1907.3.29.26.77]), skeletonized personally by one of us [ZS]), only two cloacal vertebrae were present (but both snakes possessed bifurcated ribs on two last trunk vertebrae). Two cloacal vertebrae were also present in a single specimen of Corallus caninus (Linnaeus, 1758) (SMF PH 182), but as this was an already prepared skeleton, we cannot exclude that some cloacal became missing during its skeletonization process. In any case, these low numbers of cloacal vertebrae in the aforementioned snake specimens should not be considered as so extraordinary, as deviations in the numbers of sacral vertebrae are also known to occur as well even in other reptiles, where this number is most usually standard (see Scheyer et al. 2019). It is also worth noting that Rochebrune (1881) mentioned extremely large counts of cloacal vertebrae for an array of extant species (reaching up to 8, 9, or even 10 cloacal vertebrae)! Apparently, these extreme values of cloacal vertebrae given by Rochebrune (1881) are erroneous and obviously some of these pertain instead to parts of the trunk or the caudal portions of the column.

The caudal vertebrae (= postcloacal vertebrae of other authors), following the cloacal ones, are those in which the lymphapophyses are replaced by non-forked pleurapophyses (Fig. 1L, M). A single species, the leptotyphlopid Mitophis leptepileptus (Thomas, McDiarmid & Thompson, 1985), is the only snake that lacks pleurapophyses in its caudal vertebrae (Martins et al. 2021a). Erycids and charinaids possess a distinctively complex morphology in their caudal vertebrae, with several unique structures, otherwise absent from all other snakes. The posteriormost few caudal vertebrae can be fused together in certain taxa (e.g., several scolecophidians and uropeltids, certain constrictors, certain caenophidians). Smith (2013) proposed to subdivide the tip of the tail (i.e., the posterior portion of the caudal series) into two parts: the proximal (= anterior) part, where the vertebrae, even if they are fused, can still be individuated and their morphology and distinct processes can be discerned, and the distal (= posterior) part, where only the anteriormost portion can be “homologized” with a vertebra and the remainder is a poorly differentiated mass from which there cannot be discerned any individual structures. Smith (2013: 171) further noticed that the vertebrae of that proximal part of the tip of the tail “are abruptly shorter than preceding vertebrae”.

Vertebral structures and characters

The anatomical terminologies in current use in the (mainly palaeontological) literature, follows those proposed by Auffenberg (1963) and Hoffstetter and Gasc (1969), respectively; the latter is practically the English version of the French terminology introduced earlier by Hoffstetter (1939a). Auffenberg (1963) provides the information about synonyms as well as authors of particular terms. There are minor differences between the terminologies of Auffenberg (1963) and Hoffstetter and Gasc (1969): accessory process = prezygapophyseal process; subcentral ridge = margo ventralis; interzygapophyseal ridge = margo lateralis; pedilar foramina = lateral foramina, respectively. The terminology employed in this paper follows the terms used in the aforementioned works (see Fig. 1).

In the present paper the descriptions of vertebral morphology refer to adult specimens (the only exception is the ungaliophiid Ungaliophis continentalis Müller, 1880, for which we had available only a single subadult specimen, but still we provide also descriptions of already published adult specimens of that taxon). All descriptions are brief and restricted to the most distinctive features only. In the case of trunk vertebrae, usually these are the relative centrum length, shape of cotyle and condyle, height of neural arch and shape of its posterior border, relative height of neural spine and (possibly) its shift to the posterior portion of the vertebra, presence or absence and relative length of prezygapophyseal accessory processes, presence or absence and shape of hypapophyses or haemal keel, and presence or absence of paracotylar foramina. If necessary, the above information is supplemented with additional remarks on other important peculiarities, if such (e.g., expansions of neural spines or accessory apophyses) occur on the vertebrae. Any remaining morphological features (general appearance of the vertebral body, shape of zygosphenes or paradiapophyses, presence of lateral or subcentral foramina, etc.), that are undescribed in the text, can be easily traced on the accompanying illustrations. Unless otherwise stated, the descriptions of trunk vertebrae concern those coming from the middle portion of the column. Terms describing shapes and proportional dimensions of trunk vertebrae follow in part (with some minor modifications) the definitions introduced by LaDuke (1991).

Vertebral length. A vertebra can be as long as wide when the centrum length to centrum width (= neural arch width) ratio is approximately 1.0; shorter than wide when the ratio is < 1.0; longer than wide when the ratio is > 1.0. It is considered elongate if this ratio is approximately 1.2 or greater.

Cotyle and condyle. They can be depressed (moderately or strongly), when the width distinctly exceeds their height; they are orbicular (circular), when they are more or less as high as wide.

Neural arch. In posterior view, it is depressed (flattened), vaulted (high) or moderately vaulted (intermediate in height). Georgalis et al. (2021a) recently proposed a quantitative approach on the degree of vaulting of the neural arch in order to facilitate its intracolumnar and interspecific analysis: they introduced a “vaulting ratio”, which accounts for the height of the roof of the zygantrum to the line connecting the outer edges of the postzygapophyses (PO-PO), measured on the midline, to the half-width of PO-PO (hPO-PO) measured in posterior view.

In dorsal view, the posterior median notch of the neural arch can be absent, shallow (the posterior border of the neural arch is weakly concave), or deep.

Neural spine. In lateral view, it is located more or less centrally or restricted to the posterior portion of the neural arch. If the anterior edge of this structure (measured from the dorsal margin of the zygosphene to the dorsal margin of the spine) is approximately half its dorsal length, then it is considered of medium height. If the anterior edge is distinctly shorter than this, it is referred to as low. If distinctly greater, it is termed as high as long or higher than long. In some snakes the spine can be vestigial (reduced) or even absent.

Zygosphene. We avoid characterizing the shape of the zygosphene in the descriptions and it can be directly observed from the figures. Note that the shape of this element can be subjected to a degree of intracolumnar but also intraspecific variation (see Szyndlar 1984; Szyndlar and Rage 2003). The thickness, length, and overall shape of the zygosphene can be significantly modified across ontogeny (see Georgalis and Scheyer 2019). As such, we mention the zygosphene in the description only for a few taxa for which some morphologies of this element appear to be indeed consistent and characteristic.

Prezygapophyseal accessory processes. If the length of the process is about half as long or longer than the greatest length of the prezygapophyseal articular facet, it is termed long; if scarcely seen or not seen in dorsal aspect, it is short or vestigial, respectively; the process of an intermediate length is termed moderate in length.

Hypapophysis. It can be restricted to the anterior trunk portion or can occur throughout the trunk portion of the column. As discussed also above, in certain extinct palaeophiid species, the hypapophyses can be doubled in anterior trunk vertebrae, however, this structure is single in all other snakes. The hypapophyses present on the anterior trunk vertebrae are usually relatively long and slender and do not differ greatly from one another in most groups of snakes. That is why more detailed descriptions in the following text refer to the hypapophyses of the posterior trunk vertebrae (if present). The hypapophysis may be spine-like (in lateral view more or less straight, as long as wide or longer), sigmoidal, or plate-like (square-shaped or subsquare-shaped in lateral view). The distal tip may be pointed distally or rounded; it can be produced caudally to near or beyond the level of the condyle end. In one species, Xenopeltis unicolor Reinwardt in Boié, 1827, there is a distinct notch in the ventral edge of the hypapophyses (in lateral view) of the anterior trunk vertebrae, a feature unique among snakes (see Description in the entry of Xenopeltis below).

Haemal keel. In ventral view, it can be ridge-like (narrow) or flattened (broad). In lateral view, it can be well-developed (relatively tall, projecting), moderately-developed or absent. In some cases, distinguishing between a short hypapophysis and a well-developed haemal keel is somehow arbitrary (see also Discussion below).

Subcentral structures around the cloacal region

A few preliminary works in the 19^th century dealt with vertebrae located at the level of cloaca (e.g., Salle 1880). Subsequently, some important observations on these vertebrae in different groups of snakes were made by Hoffstetter (1960, 1968, and other papers), Hoffstetter and Gasc (1969), and more recently by Szyndlar and Böhme (1996), Szyndlar and Rage (2003), Szyndlar et al. (2008), Smith (2013), Martins et al. (2021a, 2021b), Smith and Scanferla (2021), Georgalis and Szyndlar (2022), and Alfonso-Rojas et al. (2023). Besides these works, this region (including last trunk, cloacal, and anterior caudal vertebrae) has been generally poorly studied.

Most snake lineages display a characteristic sequence of subcentral (ventral) structures in the transition between the last trunk vertebrae and the anterior caudal vertebrae (“pericloacal vertebrae” in the terminology of Smith [2013]). Generally speaking, these are unpaired hypapophyses and/or haemal keels (or no distinct structures are present), followed in more posterior vertebrae by paired haemapophyses. The main difference between the Colubroides Zaher et al., 2009, and most remaining snakes is the presence of haemapophyses in cloacal vertebrae in the former group, whereas in the latter these structures appear first in caudal vertebrae. There are also other peculiarities and differences connected with the occurrence of the subcentral structures in members of particular ophidian lineages, for example in the caudal vertebrae of some snakes the paired haemapophyses are replaced by unpaired hypapophyses, whereas in some others these are vestigial or lost entirely.

Anyway, the most important aspect of the following descriptions of cloacal and caudal vertebrae is to point out the presence or absence of particular subcentral structures as well as their relative position in the column; the shape of the subcentral structures in the cloacal and caudal vertebrae is considered less important (but in any case, this can also be deduced from the figures). Descriptions of other morphological structures of the cloacal and caudal vertebrae are omitted as (except for erycid and charinaid snakes) these elements resemble closely their homologues occurring in the posterior trunk vertebrae. For example, if the neural spine, for instance, becomes reduced in posterior trunk vertebrae, the reduction will be retained also in cloacal and caudal vertebrae. The only important difference is that the vertebrae at the cloacal level have relatively shorter centra than those situated more anteriorly in the column, but more posterior caudal vertebrae become again longer and smaller (except for “erycines”). The morphology of a few terminal caudal vertebrae can be strongly simplified, moreover, they can be even fused into a single element.

Taxonomy

Despite recent advances in snake systematics and phylogenetics, encompassing either external morphological characters, or either skeletal anatomy, or either molecular data, or either a combination of some or all these methods, there is still not a consensus about the exact interrelationships and taxonomy of certain non-caenophidian snake groups (e.g., Lee and Scanlon 2002; Rieppel et al. 2002; Slowinski and Lawson 2002; Wilcox et al. 2002; Lawson et al. 2004; Gower et al. 2005; Lee et al. 2007; Vidal et al. 2007, 2010; Wiens et al. 2008, 2012; Vidal and Hedges 2009; Scanlon and Lee 2011; Gauthier et al. 2012; Pyron and Burbrink 2012; Zaher and Scanferla 2012; Pyron et al. 2013; Reynolds et al. 2014; Hsiang et al. 2015; Reeder et al. 2015; Figueroa et al. 2016; Scanferla et al. 2016; Streicher and Wiens 2016; Zheng and Wiens 2016; Harrington and Reeder 2017; Ruane and Austin 2017; Miralles et al. 2018; Burbrink et al. 2020; Palci et al. 2020; Scanferla and Smith 2020b; Zaher and Smith 2020; Ortega-Andrade et al. 2022; Zaher et al. 2022, 2023). Nevertheless, especially with the use of molecular data, an important degree of consensus has been achieved for a number of groups, the topology and taxonomic composition of which was radically different during the 19^th and 20^th centuries’ exclusively morphological / anatomical studies. One such prominent example is the case of Tropidophiidae and Ungaliophiidae, which were once united into an expanded concept of Tropidophiidae and treated as booids (e.g., Rage 1984). However, there is now a general agreement that Tropidophiidae lie instead close to aniliids, while Ungaliophiidae are true booids, close to charinaids (e.g., Zaher 1994; Wilcox et al. 2002; Lawson et al. 2004; Vidal et al. 2007; Wiens et al. 2008, 2012; Reynolds et al. 2014; Streicher and Wiens 2016; Burbrink et al. 2020; Smith and Georgalis 2022; Ortega-Andrade et al. 2022; Zaher et al. 2023). In addition, the monophyly of Scolecophidia has been challenged, with recent studies demonstrating that it is paraphyletic (Wiens et al. 2008; Vidal et al. 2010; Miralles et al. 2018; Burbrink et al. 2020; Zaher et al. 2023). Nevertheless, there is still much work to be conducted in order to fully resolve the precise relationships of groups such as Bolyeriidae, Xenophidiidae, Erycidae, Charinaidae, and Uropeltoidea.

Taxonomy of extant snakes follows Pyron et al. (2014), Zheng and Wiens (2016), Burbrink et al. (2020), and Georgalis and Smith (2020). An idealized hierarchy of the families and suprafamiliar groups of the extant non-caenophidian snakes discussed herein is presented in Table 1.

Table 1.

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A taxonomical hierarchy of non-caenophidian snakes.

Serpentes

Scolecophidia

Leptotyphlopidae

Typhlopoidea

Gerrhopilidae

Xenotyphlopidae

Typhlopidae

Anomalepididae

Alethinophidia

Amerophidia

Aniliidae

Tropidophiidae

Afrophidia

Uropeltoidea

Anomochilidae

Cylindrophiidae

Uropeltidae

Constrictores

Bolyeriidae

Xenophidiidae

Booidea

Calabariidae

Sanziniidae

Boidae

Candoiidae

Erycidae

Charinaidae

Ungaliophiidae

Pythonoidea

Xenopeltidae

Loxocemidae

Pythonidae

Caenophidia

Descriptions

In the following, a few comments about the taxonomic composition, affinities with other snakes, geographic distribution, and fossil record of each family or suprafamiliar group are provided. In addition, brief information about previous studies of the vertebral osteology (if any) is given for each family. It is not, however, the aim of this paper to gather all possible comments or mentions that can be found in the literature. These are followed by lists of examined skeletons and descriptions of (trunk and succeeding) vertebrae of particular snake genera. The only exception are scolecophidians, where the lists of examined skeletons and descriptions of vertebrae are made collectively for each family and not each genus separately.

The descriptions below are accompanied by the information (obtained by our direct observations and/or taken also from an extensive survey of the existing literature) about the number of vertebrae in the column. For each examined complete (or almost complete) skeleton the following data are given: the total amount of vertebrae and (in parentheses) the count of trunk (including atlas and axis), cloacal, and caudal vertebrae. For instance, the statement “250 (190+4+56)” means that a given skeleton contains 250 vertebrae altogether, and within this number are 190 trunk, 4 cloacal, and 56 caudal ones. If in some skeletons terminal vertebrae are lacking (it occurs fairly often in dry skeletonized specimens), the total number of vertebrae as well as the number of caudal vertebrae are followed by the sign “+”, for instance “250+ (190+4+56+)”. If the number of vertebrae is uncertain because of any reason or some of them are lost, it is indicated by a question mark (“?”) placed before the total count and repeated before the number of the appropriate columnar portion; for instance, the phrase “?250 (?190+4+56)” means that the precise number of trunk vertebrae is unknown or doubtful. Unfortunately, the above system cannot be reliably employed to most data cited from the literature, where cloacal vertebrae are counted either together with trunk vertebrae or together with caudal ones. Notable exception to this literature rule are recent vertebral counts on several scolecophidian taxa, where the numbers of trunk, cloacal, and caudal vertebrae were explicitly given (e.g., Koch et al. 2019; Martins et al. 2021a, 2021b). Note also that for the several snake taxa that possess fused tail tips, it is difficult to assess the precise number of caudal vertebrae – in those cases we mostly counted as last vertebrae the “proximal part” (sensu Smith 2013) of the fused tail tip (as there the vertebrae could still be individualized and counted properly) and added the note that this number does not contain the final fusion or alternatively we attempted to count even the “distal part” (sensu Smith 2013) but stated that the final fusion is included in that number.

Scolecophidia Duméril & Bibron, 1844

General information

Commonly called “worm snakes” (as their name in Greek literally translates [σκώληξ + ὀφίδια]) or “blind snakes”, Scolecophidia was for a long time considered a monophyletic assemblage of basal snakes (e.g., Underwood 1967; Rage 1984). Certain 19^th century workers even included them within lizards and not snakes (e.g., Bonaparte 1839; Gray 1845). Notably, a similar view was held also more than one century after, when McDowell and Bogert (1954) suggested that Scolecophidia do not belong to snakes, and instead represent a lineage of limbless lizards that became convergent to snakes, a view accepted also by few others (Robb 1960; Goin and Goin 1962)! Nevertheless, the true ophidian nature of scolecophidians is currently universally accepted, though their monophyly has been seriously challenged, primarily based on molecular data (Vidal and Hedges 2005; Wiens et al. 2008; Vidal et al. 2010; Pyron and Burbrink 2012; Pyron et al. 2013; Hsiang et al. 2015; Reeder et al. 2015; Figueroa et al. 2016; Zheng and Wiens 2016; Harrington and Reeder 2017; Streicher and Wiens 2017; Miralles et al. 2018; Burbrink et al. 2020), as well as total evidence analyses (Zaher et al. 2023). More specifically, recent analyses have suggested that anomalepidids either represent the sister group of alethinophidians, being thus not sharing an exclusive common ancestor with the remaining scolecophidians, or lie more basal to leptotyphlopids and typhlopoids (Wiens et al. 2008; Pyron and Burbrink 2012; Miralles et al. 2018; Burbrink et al. 2020; Zaher et al. 2023).

The fossil record of scolecophidians is poor, consisting exclusively of vertebrae spanning across a small number of Cenozoic localities (Szyndlar 1991a; Mead 2013; Georgalis et al. 2017; Ivanov 2022; Smith and Georgalis 2022) and a potential Cretaceous occurrence (Fachini et al. 2020; but see Head et al. 2022 for an alternative interpretation of the latter Cretaceous form). In the vast majority of cases, scolecophidian fossil vertebrae cannot be referred more precisely to the family level, and this is especially the case for the Paleogene and Neogene remains (Szyndlar 1991a; Mead 2013; Georgalis et al. 2017; Ivanov 2022; Smith and Georgalis 2022). Nevertheless, some Quaternary fossil vertebrae have been identified to the family or even the genus level, aided mainly by a geographic rationale (e.g., Bochaton et al. 2015; Syromyatnikova et al. 2021; Peralta and Ferrero 2023; Ramello et al. 2023). Despite this very poor fossil record, divergence date estimates suggest that scolecophidians have split from other snakes around the Early Cretaceous (Pyron and Burbrink 2012; Zheng and Wiens 2016; Miralles et al. 2018; Fachini et al. 2020).

A considerable amount of skeletal studies in scolecophidians has focused primarily on their cranial anatomy already since the 19^th century (e.g., Müller 1831; Jan and Sordelli 1860–1866; Boulenger 1893; Waite 1918b; Mahendra 1936; Dunn 1941; Dunn and Tihen 1944; List 1966; Wallach et al. 2007; Broadley and Wallach 2009; Rieppel et al. 2009; Pinto et al. 2015; Kraus 2017; Chretien et al. 2019; Marra Santos and Reis 2019; Linares-Vargas et al. 2021; Lira and Martins 2021; Chuliver et al. 2023; Graboski et al. 2023; Martins et al. 2023) revealing indeed important differences among the different families. In contrast, their vertebral morphology has received very little attention (e.g., Mookerjee and Das 1933; Mahendra 1936; List 1966; Fabrezi et al. 1985), though there seems to be growing interest recently, substantially aided by the usage of non-invasive technologies, such as μCT scanning (e.g., Martins et al. 2019, 2021a, 2021b; Herrel et al. 2021; Koch et al. 2019, 2021; Lira and Martins 2021).

All scolecophidians (leptotyphlopids, typhlopoids, and anomalepidids) seem to display a very simple and relatively homogenous vertebral morphology, which at most times renders it almost impossible to differentiate members of particular families based on postcranial osteology. They are all characterized by an elongate centrum, depressed cotyle and condyle, depressed neural arch, presence of relatively long prezygapophyseal accessory processes, the direction of the major axis of the prezygapophyseal articular facets approximating the direction of the major axis of the vertebra, absent or very shallow median notch of the neural arch, absent haemal keels in mid- and posterior trunk vertebrae, vestigial neural spine (present only in the anterior trunk vertebrae) shifted posteriorly, absence of any subcentral structures in the cloacal and caudal portion of the column, and very low number of caudal vertebrae.

Vertebral distinction among scolecophidian families

Despite the morphological homogeneity of scolecophidian families, certain features have been addressed in the literature to distinguish them or of possessing potential diagnostic utility for family level determination. These are summarized and assessed in this section.

Mookerjee and Das (1933) and Mahendra (1935) highlighted the presence of a single subcentral foramen in trunk vertebrae of typhlopids, with the latter author regarding it as unique among extant snakes. Since then, the diagnostic utility of the subcentral foramen in typhlopids has been repeatedly highlighted (List 1966; Lee and Scanlon 2002; Wallach 2009, 2020; Fachini et al. 2020). Wallach (2020) even suggested that this foramen was unusual among typhlopids and could be used as a characteristic of Indotyphlops braminus (Daudin, 1803). Fachini et al. (2020) regarded “the presence of asymmetrical subcentral foramina” as diagnostic of scolecophidians. We acknowledge the presence of such single large subcentral foramen in typhlopids, which, as it can be seen in the figures, it is not constantly present in all vertebrae and in all species. In our leptotyphlopid sample, this feature was not observed, but, judging from the published literature, we have to note that these foramina can be present in all trunk and cloacal vertebrae of certain leptotyphlopid taxa (e.g., Trilepida Hedges, 2011; see Pinto et al. 2015) as well as in anomalepidids, where they are also intracolumnarily variable (i.e., present in the anterior trunk vertebrae of Anomalepis Jan, 1860 in Jan and Sordelli 1860–1866 but absent posteriorly in this taxon [Dunn 1941] and in the posterior trunk vertebrae of Liotyphlops Peters, 1881 [Dunn and Tihen 1944]). It should be noted also that such foramina can be variably present in aniliids, uropeltids, and calabariids as well (see the respective entries below). Certain fossorial lizards also possess distinctive subcentral foramina in their vertebrae (e.g., amphisbaenians, see Georgalis et al. 2018 and Xing et al. 2018; dibamids, see figs in Xing et al. 2018). This feature should therefore be more properly and quantitatively assessed across many different scolecophidian genera, before it can obtain any diagnostic utility for taxonomic determinations.

In his monographic treatise, List (1966) noted that some species of Typhlopidae possessed a peculiar rodlike ossification associated with the fused terminal vertebrae (not occurring in other snakes), which he named as urostyle, but besides he did not find any identifying vertebral characters.

List (1966) also stated that zygosphenes and zygantra of leptotyphlopids are well dorsal to the level of the zygapophyseal joint. Holman (2000) stated that vertebrae of leptotyphlopids are more elongate than those of typhlopids. We consider that these features are variable and more widespread across scolecophidians, and so they cannot be used for determination.

One interesting feature that was addressed by Pinto et al. (2015) is that leptotyphlopids seem to possess a higher number of caudal vertebrae than typhlopids and anomalepidids, a fact further highlighted subsequently by Martins et al. (2021a). Based on our observations and the extensive literature overview presented below, we here tentatively concur with this statement, but we have to emphasize that data on caudal vertebral counts are missing from most typhlopid and leptotyphlopid species, in particular the African taxa. It would be interesting indeed to see how this number of caudal vertebrae ranges in the genera Rhinoleptus (Leptotyphlopidae) and Letheobia (Typhlopidae), which possess the highest total vertebral counts among all extant snakes (see below).

Fachini et al. (2020) further highlighted two additional characters, the “high position of the paradiapophyses (synapophyses in their terminology), which are located dorsal to the ventral margin of the cotyle” and the “smooth ventral margin of the centrum” as characteristic of typhlopids (and scolecophidians in general), considering these features as absent in all other snakes and therefore exclusively present for scolecophidians. We confirm the former feature as present (and sometimes distinct) in scolecophidians, but we have to note that this is also (intracolumnarily) variable present in few vertebrae of aniliids and uropeltids. As for the latter feature, we confirm that typhlopids (and scolecophidians as a whole) all possess a smooth centrum devoid of subcentral structures in their trunk vertebrae.

Finally, one feature that was highlighted by Herrel et al. (2021) is that (at least certain) typhlopids possess more robust cotyles, condyles, prezygapophyses, and postzygapophyses in their anterior trunk vertebrae, compared to those of leptotyphlopids and anomalepidids, which was explained as an adaptation for higher pushing forces during burrowing of the former group. This would of course prove to be of importance for taxonomic determination but its utility should be further quantified and studied across a high number of taxa.

Leptotyphlopidae Stejneger, 1891

General information

Leptotyphlopidae comprise the smallest known snakes, with the record holding a species of the genus Tetracheilostoma Jan, 1861, which achieves a maximum snout-vent length of only 105 mm (i.e., Tetracheilostoma carlae [Hedges, 2008]; see Hedges 2008). At the same time, as their etymology also suggests (from the Greek “λεπτός”, meaning “thin”), this group also comprises the snakes with the minimum body width (diameter), i.e., a species of the genus Mitophis Hedges, Adalsteinsson & Branch in Adalsteinsson et al., 2009 (around only 2.5 mm; see Adalsteinsson et al. 2009). They consist of more than 140 species, distributed in Africa, the Middle East and parts of southern Asia, and the Americas (Adalsteinsson et al. 2009; Boundy 2021). Most leptotyphlopid species were once placed into the genus Leptotyphlops Fitzinger, 1843, however, recent molecular studies coupled with evidence from external morphology have suggested their partitioning into several different genera (Adalsteinsson et al. 2009; see also Wallach et al. 2014 and Boundy 2021). Divergence date estimates suggest that leptotyphlopids split from other scolecophidians already during the Early Cretaceous (Zheng and Wiens 2016; Miralles et al. 2018; Sidharthan and Karanth 2021). Leptotyphlopidae were variously known, especially during the 19^th and early 20^th centuries, under the names Catodontia (or Catodonta or Catodontiens) (e.g., Duméril et al. 1854; Cope 1864, 1887, 1894, 1895, 1898; Carus 1868), a term apparently evoking to the fact that they possess teeth (Greek “οδόντες”) solely on their lower (Greek “κάτω”) jaws, as well as Stenosomata (Ritgen 1828), evoking to their narrow bodies (Greek for narrow [“στενός”] and body [“σώμα”]), Stenostomidae (or Stenostomatidae or Stenostomata or Stenostomina or Stenostomi or Sténostomiens) (e.g., Bonaparte 1845, 1852; Boulenger 1882; Peters 1881, 1882; Boettger 1884; Cope 1887), evoking to their narrow mouth (Greek for narrow [“στενός”] and mouth [“στόμα”]), or more commonly as Glauconiidae (e.g., Boulenger 1890b, 1893; Bocage 1895; Cope 1898; Gadow 1901; Janensch 1906; Lydekker 1912; Werner 1916; Abel 1919; Mahendra 1936).

The leptotyphlopid vertebral morphology is generally reminiscent of other scolecophidians (see Description and figures below).

Previous figures of vertebrae of extant Leptotyphlopidae have been so far presented by List (1966), Dowling and Duellman (1978), Fabrezi et al. (1985), Pinto et al. (2015), Koch et al. (2019, 2021), Martins et al. (2019, 2021a, 2021b), Herrel et al. (2021), Alfonso-Rojas et al. (2023), and Peralta and Ferrero (2023). Among these, vertebrae from the cloacal and/or caudal series were presented by List (1966), Fabrezi et al. (1985), Pinto et al. (2015), Koch et al. (2019), and Martins et al. (2021a, 2021b). Quantitative studies on the intracolumnar variability of leptotyphlopid vertebrae have been conducted by Head (2021).

Material examined

Epacrophis boulengeri (Boettger, 1913) (SMF 16700 [holotype]); Epictia albifrons (Wagler, 1824) (MCZ Herp R-17393 [Morphosource.org: Media 000077182, ark:/87602/m4/M77182]); Epictia ater (Taylor, 1940) (USNM 580323 [Morphosource.org: Media 000165850, ark:/87602/m4/M165850]); Epictia borapeliotes (Vanzolini, 1996) (MCZ Herp R-182170 [Morphosource.org: Media 000059305, ark:/87602/m4/M59305]); Epictia columbi (Klauber, 1939) (MCZ Herp R-48773 [Morphosource.org: Media 000077341, ark:/87602/m4/M77341]); Epictia guayaquilensis (Orejas-Miranda & Peters, 1970) (X-ray of ZMB 4508 [holotype]); Leptotyphlops nigricans (Schlegel, 1839 in Schlegel 1837–1844) (CAS Herp 173933); Leptotyphlops scutifrons (Peters, 1854) (UF Herp 187225 [Morphosource.org: Media 000099311, ark:/87602/m4/M99311]); Mitophis pyrites (Thomas, 1965) (MCZ Herp R-77239 [Morphosource.org: Media 000060820, ark:/87602/m4/M60820]); Myriopholis longicauda (Peters, 1854) (MCZ Herp R-184447 [Morphosource.org: Media 000063946, ark:/87602/m4/M63946]); Rena humilis Baird & Girard, 1853 (AMNH R 73716); Rena myopica (Garman, 1884) (UCM Herp 64598 [Morphosource.org: Media 000439463, ark:/87602/m4/439463]); Rena segrega (Klauber, 1939) (UCM Herp 16011 [Morphosource.org: Media 000444494, ark:/87602/m4/444494]); Siagonodon borrichianus (Degerbøl, 1923) (MCZ Herp R-15899 [Morphosource.org: Media 000407561, ark:/87602/m4/407561); Siagonodon cupinensis (Bailey & Carvalho, 1946) (MCZ Herp R-142653 [Morphosource.org: Media 000397266, ark:/87602/m4/397266]); Siagonodon septemstriatus (Schneider, 1801) (MNHN L20.3177b); Tetracheilostoma bilineatum (Schlegel, 1839 in Schlegel 1837–1844) (MCZ Herp R-10693 [Morphosource.org: Media 000397689, ark:/87602/m4/397689]; USNM 222954 [Morphosource.org: Media 000165848, ark:/87602/m4/M165848]); Tricheilostoma bicolor (Jan, 1860 in Jan and Sordelli 1860–1866) (MCZ Herp R-48934 [Morphosource.org: Media 000408221, ark:/87602/m4/408221]).

Description (Figs 2–4)

Trunk vertebrae . The morphology of all vertebrae is very simple. Centrum elongate and cylindrical; cotyle and condyle strongly depressed; neural arch depressed; posterior median notch of the neural arch absent or very shallow; neural spine (except for a few anteriormost vertebrae) vestigial and restricted to the posteriormost part of neural arch or absent; prezygapophyseal accessory processes very long; paradiapophyses situated at a high position, higher than the ventral margin of the cotyle; the presence of hypapophyses restricted to a few anteriormost vertebrae (up to V 5); haemal keel absent in more posterior vertebrae, where the centrum is flattened and smooth, with no subcentral structures – but Hoffstetter (1968) highlighted that only in Siagonodon septemstriatus the haemal keel exists up to V 37; paracotylar foramina absent. Subcentral foramina were absent in all studied leptotyphlopids but they have been well documented as present throughout the trunk vertebrae of Trilepida (Pinto et al. 2015).

Figure 2.

Leptotyphlopidae: Siagonodon septemstriatus (MNHN L20.3177b), trunk vertebrae.

Figure 3.

Leptotyphlopidae: Siagonodon septemstriatus (MNHN L20.3177b), trunk vertebrae and ?cloacal vertebrae.

Figure 4.

Leptotyphlopidae: Rena humilis (AMNH R 73716), trunk, cloacal, and caudal vertebrae.

Trunk/caudal transition. No subcentral structures occur in cloacal and caudal vertebrae. In some caudal vertebrae zygosphenes and zygantra may be missing.

Following the published literature, a range of 2–6 cloacal vertebrae are known in leptotyphlopids, and Mitophis is observed to show the maximum (Martins et al. 2021a), while this number is also variable in Epictia Gray, 1845 (3–5 in Epictia rioignis Koch, Martins & Schweiger, 2019; Koch et al. 2019). In a few small species (Tetracheilostoma spp.), pleurapophyses of the caudal vertebrae are extremely reduced. Moreover, in a single species (i.e., Mitophis leptepileptus), pleurapophyses seem to be totally absent, a feature that is unique among all snakes, extant or extinct (see Martins et al. 2021a). The posteriormost two or three caudal vertebrae can be fused in several species, with this fused unit being conical with a bifurcated posterior tip (Martins et al. 2021b).

Number of vertebrae. Epacrophis boulengeri (SMF 16700 [holotype]): 185 (160+3+22, including a final fusion); Epictia ater (USNM 580323): 242 (220+4+18, including a final fusion); Epictia albifrons (MCZ Herp R-17393): 251 (231+4+16, including a final fusion); Epictia borapeliotes (MCZ Herp R-182170): 263 (244+4+15, including a final fusion); Epictia columbi (MCZ Herp R-48773): 254 (225+4+25, including a final fusion); Epictia guayaquilensis (ZMB 4508 [holotype]): 246 (222 trunk vertebrae plus 24 cloacal and caudal vertebrae, including a final fusion); Leptotyphlops scutifrons (UF Herp 187225): 269 (244+3+22, including a final fusion); Mitophis pyrites (MCZ Herp R-77239): 269 (252+4+13, including a final fusion); Myriopholis longicauda (MCZ Herp R-184447): 307+ (273+3+31+ [posteriormost caudal vertebrae lacking]); Rena myopica (UCM Herp 64598): 210 (193+3+14, including a final fusion); Rena segrega (UCM Herp 16011): 289 (271+4+14, including a final fusion); Siagonodon borrichianus (MCZ Herp R-15899): 289 (273+3+13, including a final fusion); Siagonodon cupinensis (MCZ Herp R-142653): 268 (252+4+12, including a final fusion); Siagonodon septemstriatus (MNHN L20.3177b): 185+ (184+1+0+ [most cloacal and all caudal vertebrae missing]); Tetracheilostoma bilineatum (MCZ Herp R-10693): 179 (161+3+15, including a final fusion); Tetracheilostoma bilineatum (USNM 222954): 178 (159+4+15, including a final fusion); Tricheilostoma bicolor (MCZ Herp R-48934): 266 (251+3+12, including a final fusion).

Data from literature and unpublished data from personal communications: Epictia albipuncta (Burmeister, 1861): 205–228 trunk vertebrae plus 2–5 cloacal vertebrae plus 22–23 caudal vertebrae (Fabrezi et al. 1985); Epictia munoai (Orejas-Miranda, 1961): 207 trunk vertebrae plus unknown number of cloacal and caudal vertebrae (Koch et al. 2019); Epictia rioignis: 231–248 trunk vertebrae plus 3–5 cloacal vertebrae plus 14–21 caudal vertebrae (Koch et al. 2019); Epictia magnamaculata (Taylor, 1940): 227 trunk vertebrae plus 4 cloacal vertebrae plus 19 caudal vertebrae (posteriormost 3 caudal vertebrae are fused) (List 1966); Epictia magnamaculata: 199 trunk vertebrae plus unknown number of cloacal and caudal vertebrae (Koch et al. 2019); Epictia phenops (Cope, 1875): 213–246 trunk vertebrae plus unknown number of cloacal and caudal vertebrae (Koch et al. 2019); Epictia tenella (Klauber, 1939): 190–204 trunk vertebrae plus unknown number of cloacal and caudal vertebrae (Koch et al. 2019); Epictia tricolor (Orejas-Miranda & Zug, 1974): 282 trunk vertebrae plus unknown number of cloacal and caudal vertebrae (Koch et al. 2019); Habrophallos collaris (Hoogmoed, 1977): 132–144 trunk vertebrae plus 3–4 cloacal vertebrae plus 14–17 caudal vertebrae (posteriormost 2 caudal vertebrae are fused) (Martins et al. 2019); Leptotyphlops conjunctus (Jan, 1861): 191–223 trunk and cloacal vertebrae plus ?21–26 caudal vertebrae (Alexander and Gans 1966); Leptotyphlops emini (Boulenger, 1890): 220 trunk vertebrae plus 4 cloacal vertebrae plus 25 caudal vertebrae (posteriormost 3 caudal vertebrae are fused) (List 1966); Leptotyphlops nigricans: 195 trunk vertebrae plus 3 cloacal vertebrae plus 29 caudal vertebrae (posteriormost 3 caudal vertebrae are fused) (List 1966); Leptotyphlops nigricans: 130 trunk vertebrae plus 3 cloacal vertebrae plus 9 caudal vertebrae (Rochebrune 1881; potentially erroneous counts); Mitophis asbolepis (Thomas, McDiarmid & Thompson, 1985): 279 trunk vertebrae plus unknown number of cloacal and caudal vertebrae (Martins et al. 2021a); Mitophis calypso (Thomas, McDiarmid & Thompson, 1985): 350–352 trunk vertebrae plus unknown number of cloacal and caudal vertebrae (Martins et al. 2021a); Mitophis leptepileptus: 354–391 trunk vertebrae plus 6 cloacal vertebrae plus 19–21 caudal vertebrae (Martins et al. 2021a); Mitophis pyrites: 259–260 trunk vertebrae plus unknown number of cloacal and caudal vertebrae (Martins et al. 2021a); Myriopholis longicauda: 208–215 trunk and cloacal vertebrae plus ?25–?32 caudal vertebrae (Alexander and Gans 1966); Myriopholis phillipsi (Barbour, 1914): 335–343 trunk and cloacal vertebrae plus ?41–49 caudal vertebrae (Alexander and Gans 1966); Namibiana occidentalis (FitzSimons, 1962): 268–302 trunk and cloacal vertebrae plus 24–29 caudal vertebrae (Claudia Koch, unpublished data, personal communication to GLG); Rena dissecta (Cope, 1896): 212–214 trunk vertebrae plus 4 cloacal vertebrae plus 16 caudal vertebrae (posteriormost 3 caudal vertebrae are fused) (List 1966); Rena dulcis Baird & Girard, 1853: 209 trunk vertebrae plus unknown number of cloacal and caudal vertebrae (Tsuihiji et al. 2012); Rena humilis: 254–265 trunk vertebrae plus 4–5 cloacal vertebrae plus 20–21 caudal vertebrae (posteriormost 2 caudal vertebrae are fused) (List 1966); Rena maxima (Loveridge, 1932): 198 trunk vertebrae plus 4 cloacal vertebrae plus 17 caudal vertebrae (posteriormost 2 caudal vertebrae are fused) (List 1966); Rhinoleptus koniagui (Villiers, 1956): 546 vertebrae in total (Guibé et al. 1967); Siagonodon exiguum Martins et al., 2023: 239–263 trunk vertebrae plus 3–4 cloacal vertebrae plus 18–20 caudal vertebrae in males and 248–260 trunk vertebrae plus 4–5 cloacal vertebrae plus 18 caudal vertebrae in females (Martins et al. 2023); Tetracheilostoma bilineatum: 152–160 trunk vertebrae plus 3 cloacal vertebrae plus 16 caudal vertebrae (Martins et al. 2021a); Tetracheilostoma breuili (Hedges, 2008): 155–162 trunk vertebrae plus 3 cloacal vertebrae plus 16 caudal vertebrae (Martins et al. 2021a); Tetracheilostoma carlae: 165–167 trunk vertebrae plus 3–4 cloacal vertebrae plus 15–16 caudal vertebrae (Martins et al. 2021a); Tricheilostoma bicolor: 240–253 trunk and cloacal vertebrae plus 13–16 caudal vertebrae (Claudia Koch, unpublished data, personal communication to GLG); Trilepida affinis (Boulenger, 1884): 192 trunk vertebrae plus 4 cloacal vertebrae plus 21 caudal vertebrae (posteriormost 3 caudal vertebrae are fused) (Martins et al. 2021b); Trilepida brasiliensis (Laurent, 1949): 172–198 trunk vertebrae plus 4 cloacal vertebrae plus 18–20 caudal vertebrae (posteriormost 2–3 caudal vertebrae are fused) (Martins et al. 2021b); Trilepida dimidiata (Jan, 1861): 180 trunk vertebrae plus 4 cloacal vertebrae plus 17–18 caudal vertebrae (posteriormost 3 caudal vertebrae are fused) (Martins et al. 2021b); Trilepida fuliginosa (Passos, Caramaschi & Pinto, 2006): 179–190 trunk vertebrae plus 4 cloacal vertebrae plus 22 caudal vertebrae (posteriormost 2 caudal vertebrae are fused) (Martins et al. 2021b); Trilepida jani (Pinto & Fernandes, 2012): 154–165 trunk vertebrae plus 3–4 cloacal vertebrae plus 22 caudal vertebrae (posteriormost 2 caudal vertebrae are fused) (Martins et al. 2021b); Trilepida joshuai (Dunn, 1944): 158–162 trunk vertebrae plus 3–5 cloacal vertebrae plus 15–19 caudal vertebrae (posteriormost 3 caudal vertebrae are fused) (Martins et al. 2021b); Trilepida macrolepis (Peters, 1857): 196–219 trunk vertebrae plus 3–4 cloacal vertebrae plus 19 caudal vertebrae (posteriormost 3 caudal vertebrae are fused) (Martins et al. 2021b); Trilepida macrolepis: 206 trunk vertebrae plus 21 cloacal and caudal vertebrae (Nopcsa 1923); Trilepida nicefori (Dunn, 1946): 135 trunk vertebrae plus 4 cloacal vertebrae plus 16 caudal vertebrae (posteriormost 3 caudal vertebrae are fused) (Martins et al. 2021b); Trilepida pastusa Salazar-Valenzuela et al., 2015: 176–184 trunk (?and cloacal) vertebrae plus 23–24 caudal (?and cloacal) vertebrae (posteriormost 3 caudal vertebrae are fused) (Salazar-Valenzuela et al. 2015); Trilepida salgueiroi (Amaral, 1955): 203–228 trunk vertebrae plus 3–5 cloacal vertebrae plus 21–28 caudal vertebrae in males, and 211–231 trunk vertebrae plus 3–5 cloacal vertebrae plus 17–27 caudal vertebrae in females (Pinto et al. 2015).

In general, the vertebral counts are highly variable with a single species, even subjected to sexual variation. With 546 vertebrae in total (see Guibé et al. 1967), the African monotypic genus Rhinoleptus possesses by far the highest count of vertebrae among leptotyphlopids, and also the largest recorded among all extant snakes, actually being surpassed only be a few vertebrae from the record holder snake of all time, the Eocene Archaeophis (565 vs. 546); it is unclear how many of these vertebrae of Rhinoleptus pertain to the trunk, cloacal, and caudal region. Mitophis leptepileptus possesses the second highest number of trunk vertebrae known among leptotyphlopids, reaching 391, with the same number in all other species of Mitophis being also high (above 252). The next highest number of trunk vertebrae is recorded in Myriopholis Hedges, Adalsteinsson & Branch in Adalsteinsson et al., 2009, where it can surpass 330, in Namibiana Hedges, Adalsteinsson & Branch in Adalsteinsson et al., 2009, where it ranges between around 270 to 300, in a few species of Epictia, in which it can surpass 240 (E. borapeliotes, E. rioignis, and E. phenops) and reach up to 282 (E. tricolor), in two species of Siagonodon Peters, 1881, where it can reach up to 263 (Siagonodon exiguum) or even 273 (S. borrichianus), in Tricheilostoma Jan, 1860 in Jan & Sordelli 1860–1866, ranging between around 240 and 250, in one species of Leptotyphlops where it can reach up to 244 (L. scutifrons), and in Rena Baird & Girard, 1853, where it is almost always above 200 and can reach up to 271. In contrast, the number of trunk vertebrae in the remaining taxa is around 200, with some genera even possessing as few as around 135 trunk vertebrae or less (i.e., Habrophallos Martins et al., 2019, and some species of Trilepida). The highest caudal vertebral counts in leptotyphlopids are observed in Myriopholis, where it ranges between 25 and 49, followed by Namibiana (24 to 29), and Epacrophis Hedges, Adalsteinsson & Branch in Adalsteinsson et al., 2009 (22), while instead that number in the remaining genera ranges between 12 and 26.

Typhlopoidea Gray, 1825

General information

This superfamily represents the sister group to leptotyphlopids and comprises Typhlopidae, Xenotyphlopidae, and Gerrhopilidae (Vidal et al. 2010; Pyron and Wallach 2014; Miralles et al. 2018). They were once all lumped together in an expanded Typhlopidae, however, recent molecular but also anatomical evidence attests for the distinction in different families (e.g., Vidal et al. 2010; Kornilios et al. 2013; Pyron and Wallach 2014; Zheng and Wiens 2016; Chretien et al. 2019; Lira and Martins 2021). Nevertheless, as with all scolecophidians, vertebral morphology of typhlopoids is relatively uniform.

Typhlopidae Gray, 1825

General information

Typhlopidae is the most speciose family of scolecophidians and is currently distributed over large parts of Africa, Asia, the Americas, Oceania, and southeastern Europe (Hedges et al. 2014; Pyron and Wallach 2014). For several decades, all typhlopids had been referred to a single genus, Typhlops Schneider, 1801, or to two genera, Typhlops and Rhinotyphlops Fitzinger, 1843 (e.g., Roux-Estève 1974a, 1974b), but recent advances in molecular phylogenetics and more thorough knowledge of external morphologies have suggested instead an assignement into several other genera (Broadley and Wallach 2007, 2009; Hedges et al. 2014; Pyron and Wallach 2014). Divergence date estimates suggest that typhlopids split from other scolecophidians already during the Late Cretaceous (Zheng and Wiens 2016; Sidharthan and Karanth 2021; Tiatragul et al. 2023). Typhlopidae were also called during the 19^th century under the name Epanodontia (or Epanodonta or Épanodontiens) (Duméril et al. 1854, 1870–1909; Carus 1868; Cope 1887, 1894, 1895, 1898), a term apparently evoking to the fact that they possess teeth (“οδόντες”) solely on their upper (“επάνω”) jaws, i.e., the name counterparting Catodontia (i.e., Leptotyphlopidae).

Previous figures of vertebrae of extant Typhlopidae have been so far presented by Rochebrune (1881), Mookerjee and Das (1933), Mahendra (1935, 1936), Hoffstetter (1939b), Evans (1955), List (1966), Hoffstetter and Gasc (1969), Lee and Scanlon (2002), Nakamura et al. (2013), Palci et al. (2013a), Xing et al. (2018), Fachini et al. (2020), Hawlitschek et al. (2021), Herrel et al. (2021), Lira and Martins (2021), and Peralta and Ferrero (2023), including also from individuals of earlier ontogenetic stages (Xing et al. 2018). Among these, vertebrae from the cloacal and/or caudal series were presented by Evans (1955) and List (1966). Figures of the microanatomy and histology / transverse sections of typhlopid vertebrae were presented by Mookerjee and Das (1933) and Sood (1948). Quantitative studies on the intracolumnar variability of typhlopid vertebrae was conducted by Gasc (1974) and Head (2021).

Material examined

Acutotyphlops kunuaensis Wallach, 1995 (FMNH 44800 [Morphosource.org: Media 000052992, ark:/87602/m4/M52992]); Afrotyphlops punctatus (Leach in Bowdich, 1819) (NHMUK 1911.6.9.2); Afrotyphlops steinhausi (Werner, 1909) (MNHN-ZA-AC-1964.0042); Amerotyphlops brongersmianus (Vanzolini, 1976) (FMNH 195928 [Morphosource.org: Media 000065157, ark:/87602/m4/M65157]); Amerotyphlops microstomus (Cope, 1866) (UCM Herp 40750 [Morphosource.org: Media 000438912, ark:/87602/m4/438912]); Anilios erycinus (Werner, 1901) (UF Herp 113561 [Morphosource.org: Media 000448363, ark:/87602/m4/448363); Anilios torresianus (Boulenger, 1889) (UMMZ 83512 [Morphosource.org: Media 000386190, ark:/87602/m4/386190]); Antillotyphlops hypomethes (Hedges & Thomas, 1991) (MCZ Herp R-38322 [Morphosource.org: Media 000048876, ark:/87602/m4/M48876]); Argyrophis diardii (Schlegel, 1839 in Schlegel 1837–1844) (MNHN-AC-1908.0115); Argyrophis muelleri (Schlegel, 1839 in Schlegel 1837–1844) (FMNH H 259200 [Morphosource.org: Media 000069969, ark:/87602/m4/M69969]); Cubatyphlops biminiensis (Richmond, 1955) (MCZ Herp R-69444 [Morphosource.org: Media 000393020, ark:/87602/m4/393020]); Grypotyphlops acutus (Duméril & Bibron, 1844) (MCZ Herp R-3849 [Morphosource.org: Media 000350220, ark:/87602/m4/350220]); Indotyphlops braminus (MNHN-AC-1911.0030; UF Herp 29433 [Morphosource.org: Media 000493259, ark:/87602/m4/493259]); Madatyphlops arenarius (Grandidier, 1872) (UMMZ 241854 [Morphosource.org: Media 000070129, ark:/87602/m4/M70129]); Rhinotyphlops lalandei (Schlegel, 1839 in Schlegel 1837–1844) (UMMZ 61525 [Morphosource.org: Media 000083066, ark:/87602/m4/M83066]); Typhlops gonavensis Richmond, 1964 (YPM VZ Her 003003 [holotype] [Morphosource.org: Media 000495769, ark:/87602/m4/495769]); Typhlops lumbricalis (Linnaeus, 1758) (YPM VZ Her 003000 [Morphosource.org: Media 000068126, ark:/87602/m4/M68126]); Xerotyphlops syriacus (Jan, 1864) (ISEZ R/407); Xerotyphlops vermicularis (Merrem, 1820) (MGPT-MDHC 197; MGPT-MDHC 293; NHMW 33838).

Description (Figs 5–13).

Trunk vertebrae . The morphology of the trunk vertebrae is very similar to leptotyphlopids. See the respective part in Leptotyphlopidae above.

Figure 5.

Typhlopidae: Afrotyphlops punctatus (NHMUK 1911.6.9.2), anterior trunk vertebrae.

Figure 6.

Typhlopidae: Afrotyphlops punctatus (NHMUK 1911.6.9.2), trunk vertebrae.

Figure 7.

Typhlopidae: Afrotyphlops punctatus (NHMUK 1911.6.9.2), trunk vertebrae.

Figure 8.

Typhlopidae: Afrotyphlops punctatus (NHMUK 1911.6.9.2), trunk vertebrae.

Figure 9.

Typhlopidae: Afrotyphlops punctatus (NHMUK 1911.6.9.2), cloacal and caudal vertebrae.

Figure 10.

Typhlopidae: Argyrophis diardii (MNHN-AC-1908.0115), trunk vertebrae.

Figure 11.

Typhlopidae: Argyrophis diardii (MNHN-AC-1908.0115), trunk and cloacal vertebrae.

Figure 12.

Typhlopidae: Amerotyphlops brongersmianus (FMNH 195928), trunk vertebrae.

Figure 13.

Typhlopidae: Xerotyphlops syriacus (ISEZ R/407), trunk vertebra.

Worth noting here is that, contrary to most leptotyphlopids, a single asymmetrical subcentral foramen is present in several (but not all) trunk vertebrae of typhlopids (see above “Vertebral distinction among scolecophidian families”).

Trunk/caudal transition. The morphology of these vertebrae is overall similar to leptotyphlopids. See the respective part in Leptotyphlopidae above.

Number of vertebrae. Acutotyphlops kunuaensis (FMNH 44800): 280 (267+5+8, including a final fusion); Afrotyphlops punctatus (NHMUK 1911.6.9.2): 188+ (183+5+0+); Amerotyphlops brongersmianus (FMNH 195928): 149 (138+4+7, including a final fusion); Amerotyphlops microstomus (UCM Herp 40750): ~310 vertebrae in total; Anilios erycinus (UF Herp 113561): 183 (173+3+7, including a final fusion); Anilios torresianus (UMMZ 83512): 221 (208+4+9, including a final fusion); Antillotyphlops hypomethes (MCZ Herp R-38322): 218 (205+3+10, including a final fusion); Argyrophis diardii (MNHN-AC-1911.0030): 192+ (190+2+0+); Argyrophis muelleri (FMNH H 259200): 176 (167+4+5, including a final fusion); Cubatyphlops biminiensis (MCZ Herp R-69444): 273 (260+4+9, including a final fusion); Grypotyphlops acutus (MCZ Herp R-3849): 267 (255+5+7, including a final fusion); Indotyphlops braminus (UF Herp 29433): 270 (244+3+23, including a final fusion); Madatyphlops arenarius (UMMZ 241854): 180 trunk vertebrae (cloacal and caudal vertebrae are missing, perhaps also some posteriormost trunk vertebrae); Rhinotyphlops lalandei (UMMZ 61525): 199 (187+3+9, including a final fusion); Typhlops gonavensis (YPM VZ Her 003003 [holotype]): 247 (236+4+7, including a final fusion); Typhlops lumbricalis (YPM VZ Her 003000): 179 (167+4+8, including a final fusion); Xerotyphlops vermicularis (MGPT-MDHC 197): 211+ vertebrae in total.

Data from literature and unpublished data from personal communications: Acutotyphlops infralabialis (Waite, 1918): 289 trunk and cloacal vertebrae plus 13 caudal vertebrae (Alexander and Gans 1966); Acutotyphlops solomonis (Parker, 1939): 219–222 trunk and cloacal vertebrae plus 13–16 caudal vertebrae (Alexander and Gans 1966); Afrotyphlops angeli (Guibé, 1952): 332 vertebrae in total (Roux-Estève 1974a); Afrotyphlops angolensis (Bocage, 1866): 209–236 vertebrae in total in males and 213–236 vertebrae in total in females (Roux-Estève 1975); Afrotyphlops angolensis: 206–246 vertebrae in total (Broadley and Wallach 2007); Afrotyphlops anomalus (Bocage, 1873): 184–203 vertebrae in total in males and 198–206 vertebrae in total in females (Roux-Estève 1974a); Afrotyphlops bibronii (Smith, 1846 in Smith 1838–1849): 182–212 vertebrae in total in males and 197–223 vertebrae in total in females (Roux-Estève 1974a); Afrotyphlops blanfordii (Boulenger, 1889): 233 trunk vertebrae plus 4 cloacal vertebrae plus 11 caudal vertebrae (posteriormost 3 caudal vertebrae are fused) (List 1966); Afrotyphlops brevis (Scortecci, 1929): 190–250 vertebrae in total (Roux-Estève 1974a, 1974b); Afrotyphlops calabresii (Gans & Laurent, 1965): 130–153 trunk and cloacal vertebrae plus unknown number of caudal vertebrae (Gans and Laurent 1965; Alexander and Gans 1966); Afrotyphlops congestus (Duméril & Bibron, 1844): 180–182 trunk and cloacal vertebrae plus 7–12 caudal vertebrae (Alexander and Gans 1966); Afrotyphlops congestus: 185–201 vertebrae in total in males and 191–206 vertebrae in total in females (Roux-Estève 1974a); Afrotyphlops cuneirostris: 115–126 trunk and cloacal vertebrae plus unknown number of caudal vertebrae (Gans and Laurent 1965; Alexander and Gans 1966); Afrotyphlops cuneirostris: 115–153 vertebrae in total (Roux-Estève 1974a, 1974b); Afrotyphlops decorosus (Buchholz & Peters in Peters, 1875): 307 vertebrae in total in males and 313–320 vertebrae in total in females (Roux-Estève 1974a); Afrotyphlops elegans (Peters, 1868): 210–218 vertebrae in total in males and 214–220 vertebrae in total in females (Roux-Estève 1974a); Afrotyphlops fornasinii (Bianconi, 1849): 144–159 vertebrae in total in males and 155–167 vertebrae in total in females (Roux-Estève 1974a); Afrotyphlops gierrai (Mocquard, 1897): 219–231 vertebrae in total in males and 225–242 vertebrae in total in females (Roux-Estève 1974a); Afrotyphlops liberiensis (Hallowell, 1848): 196–213 vertebrae in total in males and 201–218 vertebrae in total in females (Roux-Estève 1974a); Afrotyphlops lineolatus (Jan, 1864): 184–209 vertebrae in total in males and 187–214 vertebrae in total in females (Roux-Estève 1974a); Afrotyphlops manni (Loveridge, 1941): 301–332 vertebrae in total (Roux-Estève 1974a, 1974b); Afrotyphlops mucruso (Peters, 1854): 197 trunk vertebrae plus 5 cloacal vertebrae plus 8 caudal vertebrae (posteriormost 3 caudal vertebrae are fused) (List 1966); Afrotyphlops mucruso: 166–202 vertebrae in total in males and 182–224 vertebrae in total in females (Rhinotyphlops schlegelii dinga of Roux-Estève 1974a); Afrotyphlops nigrocandidus (Broadley & Wallach, 2000): 242 vertebrae in total (Broadley and Wallach 2007); Afrotyphlops obtusus (Peters, 1865): 251–261 vertebrae in total in males and 252–271 vertebrae in total in females (Roux-Estève 1974a); Afrotyphlops punctatus: 191–219 vertebrae in total in males and 201–224 vertebrae in total in females (Roux-Estève 1974a); Afrotyphlops punctatus: 181 trunk vertebrae plus 11 cloacal and caudal vertebrae (Nopcsa 1923); Afrotyphlops rondoensis (Loveridge, 1942): 197–216 vertebrae in total (Roux-Estève 1974a, 1974b); Afrotyphlops schlegelii (Bianconi, 1849): 184 trunk vertebrae plus 5 cloacal vertebrae plus 9 caudal vertebrae (posteriormost 4 caudal vertebrae are fused) (List 1966); Afrotyphlops schlegelii: 175–187 vertebrae in total in males and 194–206 vertebrae in total in females (Roux-Estève 1974a); Afrotyphlops schmidti (Laurent, 1956): 188–212 vertebrae in total (Broadley and Wallach 2007); Afrotyphlops steinhausi: 235–245 vertebrae in total in males and 241–255 vertebrae in total in females (Roux-Estève 1974a); Afrotyphlops tanganicanus (Laurent, 1964): 227–247 vertebrae in total (Roux-Estève 1974a, 1974b); Afrotyphlops usambaricus (Laurent, 1964): 196–211 vertebrae in total (Broadley and Wallach 2007); Amerotyphlops reticulatus (Linnaeus, 1758): 140 trunk vertebrae plus 3 cloacal vertebrae plus 9 caudal vertebrae (posteriormost 3 caudal vertebrae are fused) (List 1966); Anilios ammodytes (Montague, 1914): 193–231 trunk vertebrae plus unknown number of cloacal and caudal vertebrae (Sarin Tiatragul, unpublished data, personal communication to GLG); Anilios australis Gray, 1845: 145–164 trunk vertebrae plus unknown number of cloacal and caudal vertebrae (Sarin Tiatragul, unpublished data, personal communication to GLG); Anilios bicolor (Schmidt in Peters, 1858): 145–173 trunk vertebrae plus unknown number of cloacal and caudal vertebrae (Sarin Tiatragul, unpublished data, personal communication to GLG); Anilios bituberculatus (Peters, 1863): 219–300 trunk and cloacal vertebrae plus 13–15 caudal vertebrae (Alexander and Gans 1966); Anilios diversus (Waite, 1894): 186–230 trunk vertebrae plus unknown number of cloacal and caudal vertebrae (Sarin Tiatragul, unpublished data, personal communication to GLG); Anilios endoterus (Waite, 1918): 203–245 trunk vertebrae plus unknown number of cloacal and caudal vertebrae (Sarin Tiatragul, unpublished data, personal communication to GLG); Anilios fossor Shea, 2015: 250 trunk vertebrae plus unknown number of cloacal and caudal vertebrae (Sarin Tiatragul, unpublished data, personal communication to GLG); Anilios ganei (Aplin, 1998): 195–223 trunk vertebrae plus unknown number of cloacal and caudal vertebrae (Sarin Tiatragul, unpublished data, personal communication to GLG); Anilios guentheri (Peters, 1865): 249–293 trunk vertebrae plus unknown number of cloacal and caudal vertebrae (Sarin Tiatragul, unpublished data, personal communication to GLG); Anilios hamatus (Storr, 1981): 170–192 trunk vertebrae plus unknown number of cloacal and caudal vertebrae (Sarin Tiatragul, unpublished data, personal communication to GLG); Anilios kimberleyensis (Storr, 1981): 188–250 trunk vertebrae plus unknown number of cloacal and caudal vertebrae (Sarin Tiatragul, unpublished data, personal communication to GLG); Anilios leptosomus (Robb, 1972): 288–334 trunk vertebrae plus unknown number of cloacal and caudal vertebrae (Sarin Tiatragul, unpublished data, personal communication to GLG); Anilios ligatus (Peters, 1879): 140–166 trunk vertebrae plus unknown number of cloacal and caudal vertebrae (Sarin Tiatragul, unpublished data, personal communication to GLG); Anilios nigrescens Gray, 1845: 183–218 trunk vertebrae plus unknown number of cloacal and caudal vertebrae (Sarin Tiatragul, unpublished data, personal communication to GLG); Anilios pilbarensis (Aplin & Donnellan, 1993): 185–210 trunk vertebrae plus unknown number of cloacal and caudal vertebrae (Sarin Tiatragul, unpublished data, personal communication to GLG); Anilios pinguis (Waite, 1897): 144–151 trunk vertebrae plus unknown number of cloacal and caudal vertebrae (Sarin Tiatragul, unpublished data, personal communication to GLG); Anilios proximus (Waite, 1893): 160–173 trunk and cloacal vertebrae plus 7–12 caudal vertebrae (Alexander and Gans 1966); Anilios torresianus: 196 trunk and cloacal vertebrae plus 10–16 caudal vertebrae (Alexander and Gans 1966); Anilios waitii (Boulenger, 1895): 214–310 trunk vertebrae plus unknown number of cloacal and caudal vertebrae (Sarin Tiatragul, unpublished data, personal communication to GLG); Cyclotyphlops deharvengi Bosch & Ineich, 1994: 154 trunk vertebrae plus ~12 cloacal and caudal vertebrae (posteriormost caudal vertebrae are fused) (counted from the radiograph of the holotype in Bosch and Ineich 1994: fig. 7); Indotyphlops braminus: 170–180 trunk vertebrae plus 3 cloacal vertebrae plus 10–11 caudal vertebrae (posteriormost 2–3 caudal vertebrae are fused) (List 1966); Indotyphlops braminus: 154 trunk vertebrae plus 3 cloacal vertebrae plus 7 caudal vertebrae (Rochebrune 1881); Indotyphlops braminus: 179–196 vertebrae in total (Roux-Estève 1974b); Indotyphlops braminus: 178–206 vertebrae in total (Roux-Estève 1974a); Letheobia caeca (Duméril, 1856): 322–347 vertebrae in total in males and 336–371 vertebrae in total in females (Roux-Estève 1974a); Letheobia caeca: 349 vertebrae in total (Trape and Roux-Estève 1990); Letheobia caeca: 310–336 vertebrae in total (Roux-Estève 1975); Letheobia coecata (Jan, 1863): 200–228 vertebrae in total (Roux-Estève 1969, 1974a, 1974b); Letheobia crossii (Boulenger, 1893): 307–317 vertebrae in total (Roux-Estève 1974a, 1974b); Letheobia debilis (Joger, 1990): 443 vertebrae in total (Broadley and Wallach 2007); Letheobia decorosa (Buchholz & Peters in Peters, 1875): 285–320 vertebrae in total (Broadley and Wallach 2007); Letheobia feae (Boulenger, 1906): 311 vertebrae in total (Roux-Estève 1974a, 1974b); Letheobia gracilis (Sternfeld, 1910): 377–426 vertebrae in total (Roux-Estève 1974a, 1974b); Letheobia graueri (Sternfeld, 1913): 316–382 vertebrae in total (Roux-Estève 1974a, 1974b); Letheobia kibarae (Witte, 1953): 345–370 vertebrae in total (Roux-Estève 1974a, 1974b); Letheobia leucosticta (Boulenger, 1898): 236 vertebrae in total (Roux-Estève 1974a, 1974b); Letheobia lumbriciformis (Peters, 1874): 362–364 vertebrae in total in males and 374–382 vertebrae in total in females (Roux-Estève 1974a); Letheobia newtoni (Bocage, 1890): 290–321 vertebrae in total (Roux-Estève 1974a, 1974b); Letheobia obtusa (Peters, 1865): 251–271 vertebrae in total (Broadley and Wallach 2007); Letheobia pallida Cope, 1869: 235–303 vertebrae in total (Roux-Estève 1974a, 1974b); Letheobia praeocularis (Stejneger, 1894): 333–358 vertebrae in total (Roux-Estève 1974a, 1974b); Letheobia rufescens (Chabanaud, 1917): 442–448 vertebrae in total (Roux-Estève 1974a, 1974b); Letheobia somalica (Boulenger, 1895): 301–391 vertebrae in total (Roux-Estève 1974a, 1974b); Letheobia stejnegeri (Loveridge, 1931): 307–367 vertebrae in total (Roux-Estève 1974a, 1974b); Letheobia sudanensis (Schmidt, 1923): 366 vertebrae in total in males and 379–408 vertebrae in total in females (Roux-Estève 1974a); Letheobia swahilica Broadley & Wallach, 2007: 235–260 vertebrae in total (Broadley and Wallach 2007); Letheobia toritensis Broadley & Wallach, 2007: 280–303 vertebrae in total (Broadley and Wallach 2007); Letheobia uluguruensis (Barbour & Loveridge, 1928): 263–269 vertebrae in total (Gower et al. 2004); Letheobia uluguruensis: 263 vertebrae in total (Roux-Estève 1974a, 1974b); Letheobia wittei (Roux-Estève, 1974): 368–374 vertebrae in total (Roux-Estève 1974a, 1974b); Letheobia zenkeri (Sternfeld, 1908): 193–199 vertebrae in total (Roux-Estève 1974a, 1974b); Madatyphlops andasibensis (Wallach & Glaw, 2009): 180–186 trunk vertebrae plus 5 cloacal vertebrae plus 7–9 caudal vertebrae (posteriormost caudal vertebrae are fused) (Wallach and Glaw 2009); Madatyphlops boettgeri (Boulenger, 1893): 196 trunk vertebrae plus 4 cloacal vertebrae plus 7 caudal vertebrae (posteriormost 3 caudal vertebrae are fused) (List 1966); Madatyphlops domerguei (Roux-Estève, 1980): 170 vertebrae in total (Roux-Estève 1980); Madatyphlops platyrhynchus (Sternfeld, 1910): 218–227 vertebrae in total (Roux-Estève 1974a, 1974b); Malayotyphlops luzonensis (Taylor, 1919): 208 trunk and cloacal vertebrae plus 10 caudal vertebrae (Alexander and Gans 1966); Ramphotyphlops depressus Peters, 1880: ?175–181 trunk and cloacal vertebrae plus ?11–15 caudal vertebrae (Alexander and Gans 1966); Ramphotyphlops flaviventer (Peters, 1864): 203 trunk vertebrae plus 4 cloacal vertebrae plus 13 caudal vertebrae (posteriormost 3 caudal vertebrae are fused) (List 1966); Ramphotyphlops lineatus (Schlegel, 1839 in Schlegel 1837–1844): 251 trunk vertebrae plus 3 cloacal vertebrae plus 8 caudal vertebrae (posteriormost 3 caudal vertebrae are fused) (List 1966); Rhinotyphlops ataeniatus (Boulenger, 1912): 297–307 trunk and cloacal vertebrae plus unknown number of caudal vertebrae (Gans and Laurent 1965; Alexander and Gans 1966); Rhinotyphlops boylei (FitzSimons, 1932): 211–212 vertebrae in total (Roux-Estève 1974a, 1974b); Rhinotyphlops lalandei: 195–211 vertebrae in total in males and 195–222 vertebrae in total in females (Roux-Estève 1974a); Rhinotyphlops leucocephalus (Parker, 1930): 193 vertebrae in total (Roux-Estève 1974a); Rhinotyphlops schinzi (Boettger, 1887): 219–237 vertebrae in total in males and 223–244 vertebrae in total in females (Roux-Estève 1974a); Rhinotyphlops scorteccii (Gans & Laurent, 1965): ?226–255 trunk and cloacal vertebrae plus unknown number of caudal vertebrae (Gans and Laurent 1965; Alexander and Gans 1966); Rhinotyphlops scorteccii: 235–237 vertebrae in total in males and 241–255 vertebrae in total in females (Roux-Estève 1974a); Rhinotyphlops unitaeniatus (Peters, 1878): 315–334 trunk and cloacal vertebrae plus unknown number of caudal vertebrae (Gans and Laurent 1965; Alexander and Gans 1966); Rhinotyphlops unitaeniatus: 299–332 vertebrae in total (Roux-Estève 1974a, 1974b); Sundatyphlops polygrammicus (Schlegel, 1839 in Schlegel 1837–1844): 190–216 trunk vertebrae plus 3–5 cloacal vertebrae plus 14–18 caudal vertebrae (posteriormost 3–4 caudal vertebrae are fused) (List 1966); Typhlops jamaicensis (Shaw, 1802): 200 trunk vertebrae plus 15 cloacal and caudal vertebrae (Polly et al. 2001); Typhlops jamaicensis: 200–205 vertebrae in total, of which at least 10 are caudal ones (Evans 1955); Typhlops lumbricalis: 166 trunk vertebrae plus 3 cloacal vertebrae plus 10 caudal vertebrae (posteriormost 2 caudal vertebrae are fused) (List 1966); Typhlops lumbricalis: 176 trunk vertebrae plus 10 cloacal and caudal vertebrae (Janensch 1906); Typhlops lumbricalis: 175 trunk vertebrae plus 3 cloacal vertebrae plus 10 caudal vertebrae (Rochebrune 1881); Typhlops platycephalus Duméril & Bibron, 1844: 217 trunk vertebrae plus 5 cloacal vertebrae plus 11 caudal vertebrae (posteriormost 4 caudal vertebrae are fused) (List 1966); Typhlops pusillus Barbour, 1914: 155 trunk vertebrae plus 3 cloacal vertebrae plus 12 caudal vertebrae (posteriormost 4 caudal vertebrae are fused) (List 1966); Typhlops richardii Duméril & Bibron, 1844: 201 trunk vertebrae plus 3 cloacal vertebrae plus 13 caudal vertebrae (posteriormost 4 caudal vertebrae are fused) (List 1966); Typhlops rostellatus Stejneger, 1904: 179 trunk vertebrae plus 4 cloacal vertebrae plus 10 caudal vertebrae (posteriormost 2 caudal vertebrae are fused) (List 1966); Xerotyphlops socotranus (Boulenger, 1889): 226 vertebrae in total (Roux-Estève 1974a, 1974b); Xerotyphlops vermicularis: 189 trunk vertebrae plus 4 cloacal vertebrae plus 12 caudal vertebrae (posteriormost 4 caudal vertebrae are fused) (List 1966).

In general, there is plenty of information on the vertebral counts for many typhlopid genera and species. The largest amount of these data was provided in the monumental work of Roux-Estève (1974a), which, however, revealed information only about the total vertebral counts and not for particular regions of the column. One of the most striking features observed in typhlopids (like in many other scolecophidians) is the great intraspecific variation in the count of vertebrae of several taxa, particularly the trunk vertebrae. Such huge intraspecific vertebral number variation is observed in many genera and is more prominent in some species of Letheobia, where sometimes this difference exceeds the 100 vertebrae in the same species (see Roux-Estève 1974a). The average vertebral number of males is lower than that of females (Roux-Estève 1974a). In addition, that number is also correlated to elevation of the animal’s habitat, with the number decreasing towards the sea-level (Roux-Estève 1974a). Wallach (2009) reported that the number of vertebrae for the widespread (and in many areas invasive; see Wallach 2009; Ineich et al. 2017) species Indotyphlops braminus was geographically variable – we also observed a large variation in our studied sample and literature data for the same species. Notably, some species of Letheobia possess among the highest numbers of total vertebrae among all snakes, surpassing the 400, reaching even 448 in one species; in addition, in many species of that genus the total number of vertebrae is above 300, while the minimum recorded for the genus is 193 in one species. The genus Rhinotyphlops, also achieves high vertebral counts, reaching up to 334 in one species. The same applies to the genus Anilios Gray, 1845, two species of which have more than 300 trunk vertebrae although other species of this genus possess as few as less than 150 trunk vertebrae. More or less, the same is the case with the genus Amerotyphlops Hedges et al., 2014, where one species has more than 300 vertebrae in total, while in other two studied congeners, this numbers is only around 150. High numbers (more than 260) of total counts of vertebrae are also observed in the genera Cubatyphlops Hedges et al., 2014, and Grypotyphlops Peters, 1881. In contrast, the lowest numbers of total vertebral counts in typhlopids (and all snakes in general) are observed in Afrotyphlops Broadley & Wallach, 2009, where in one species this reaches the minimum of 115, though in other congeners this number can still go as high as 332. The number of caudal vertebrae in typhlopids is generally lower than that of leptotyphlopids, with most species (for which such rare available data do exist) possessing only around ~10 caudal vertebrae (with 5 being the lowest recorded number and 22 the highest one). Unfortunately, caudal vertebral counts have not been ever recorded for Letheobia, the typhlopid with (by far) the highest total number of vertebrae.

Gerrhopilidae Vidal et al., 2010

General information

Gerrhopilidae consists a small group of typhlopoids, with a little more than 20 known species, pertaining to two genera, i.e., Gerrhopilus Fitzinger, 1843, and Cathetorhinus Duméril & Bibron, 1844, distributed in southern Asia and certain islands of the Indian and western Pacific Oceans (Vidal et al. 2010; Pyron and Wallach 2014; Kraus 2023). Divergence date estimates suggest that gerrhopilids split from other scolecophidians already during the Late Cretaceous (Zheng and Wiens 2016; Miralles et al. 2018; Sidharthan and Karanth 2021).

The sole so far published figure of gerrhopilid vertebrae has been presented by Kraus (2017: fig. 5) for Gerrhopilus persephone Kraus, 2017, and shows only the atlas, axis, and the next two articulated vertebrae in dorsal, ventral, and lateral views. Unfortunately, these vertebrae originate from the anteriormost trunk region of the column and so are not very informative.

Material examined

Gerrhopilus mirus (Jan, 1860 in Jan & Sordelli 1860–1866) (FMNH 178534 [Morphosource.org: Media 000413003, ark:/87602/m4/413003 and Media 000413000, ark:/87602/m4/413000]).

Description (Fig. 14)

Trunk vertebrae. The morphology of the trunk vertebrae is very similar to other scolecophidians. See the respective part in Leptotyphlopidae above.

Figure 14.

Gerrhopilidae: Gerrhopilus mirus (FMNH 178534), trunk, cloacal, and caudal vertebrae.

Trunk/caudal transition. The morphology of these vertebrae is very similar to other scolecophidians. See the respective part in Leptotyphlopidae above.

Number of vertebrae. Gerrhopilus mirus (FMNH 178534): 202 (195+3+4).

Data from literature: Gerrhopilus persephone: 286 vertebrae in total (Kraus 2017).

Xenotyphlopidae Vidal et al., 2010

General information

Xenotyphlopidae is a recently established family of typhlopoids, comprising a single genus, Xenotyphlops Wallach & Ineich, 1996, with solely one valid species, Xenotyphlops grandidieri (Mocquard, 1905) from Madagascar (Wallach and Ineich 1996; Vidal et al. 2010; Wegener et al. 2013; Pyron and Wallach 2014). Divergence date estimates suggest that xenotyphlopids split from other scolecophidians already during the Late Cretaceous (Zheng and Wiens 2016; Miralles et al. 2018; Sidharthan and Karanth 2021).

No xenotyphlopid specimen was available for study. Although the cranial anatomy of this species has been recently described in detail (Chretien et al. 2019), almost nothing is known about its vertebrae. In fact, the only information on its vertebrae is the X-ray photographs of the anterior trunk portions of the column of the lectotype and paralectotype of Xenotyphlops grandidieri provided by Wallach and Ineich (1996: fig. 2). Unfortunately, that figure is not very informative as the few shown articulated anterior trunk vertebrae are depicted only in lateral view.

Number of vertebrae. Data from literature: Xenotyphlops grandidieri: 264 vertebrae in total (Wallach and Ineich 1996).

Anomalepididae Taylor, 1939

General information

Anomalepididae have been considered to represent the basalmost scolecophidians, mainly due to their peculiar cranial anatomy (Rieppel et al. 2009; Scanlon and Lee 2011; Marra Santos and Reis 2019; Linares-Vargas et al. 2021). Nevertheless, recent molecular evidence suggests that Scolecophidia is a paraphyletic assemblage, with anomalepidids instead lying either as the sister group of Alethinophidia or either as more basal to leptotyphlopids and typhlopoids (Vidal et al. 2010; Pyron and Burbrink 2012; Pyron et al. 2013; Zheng and Wiens 2016; Mirrales et al. 2018; Burbrink et al. 2020; Zaher et al. 2023), with considerably robust evidence for the former topology (see Miralles et al. 2018). Anomalepidids represent a very old lineage, with divergence date estimates suggesting that they appeared already during the Early Cretaceous (Miralles et al. 2018).

Previous figures of vertebrae of extant Anomalepididae have been so far presented only by List (1966), Palci et al. (2020), and Herrel et al. (2021). Among these, vertebrae from the cloacal and caudal series were presented by Palci et al. (2020). Besides these figures, important observations on the vertebral morphology of anomalepidids were made by Dunn (1941) and Dunn and Tihen (1944).

Material examined

Anomalepis mexicana Jan, 1860 in Jan & Sordelli 1860–1866 (FMNH 22853 [Morphosource.org: Media 000383763, ark:/87602/m4/383763]; MCZ Herp R-29220 [Morphosource.org: Media 000415858, ark:/87602/m4/415858]); Helminthophis frontalis (Peters, 1860) (MCZ Herp R-55117 [Morphosource.org: Media 000415384, ark:/87602/m4/415384); Liotyphlops albirostris (Peters, 1857) (UF Herp 43324 [Morphosource.org: Media 000493244, ark:/87602/m4/493244]); Liotyphlops beui (Amaral, 1924) (SAMA R40142); Liotyphlops bondensis Griffin, 1916 (X-rays of CPZ-UV 7290; CPZ-UV 7291; CPZ-UV 7292); Typhlophis squamosus (Schlegel, 1839 in Schlegel 1837–1844) (MNHN-RA-1999.8306; KUBI 69819 [Morphosource.org: Media 000075052, ark:/87602/m4/M75052]).

Description (Figs 15-18>)

Trunk vertebrae. The morphology of the trunk vertebrae is strikingly similar to other scolecophidians. See the respective part in Leptotyphlopidae above.

Figure 15.

Anomalepididae: Liotyphlops beui (SAMA R40142), posterior trunk and caudal vertebrae.

Figure 16.

Anomalepididae: Typhlophis squamosus (MNHN-RA-1999.8306), anterior trunk vertebrae.

Figure 17.

Anomalepididae: Anomalepis mexicana (MCZ Herp R-29220), trunk, cloacal, and caudal vertebrae.

Figure 18.

Anomalepididae: Anomalepis mexicana (FMNH 22853), trunk, cloacal, and caudal vertebrae.

Trunk/caudal transition. The morphology of these vertebrae is very similar to other scolecophidians. See the respective part in Leptotyphlopidae above.

Number of vertebrae. Anomalepis mexicana (FMNH 22853): 180 (170+4+6 [posteriormost caudal vertebrae are fused]); Anomalepis mexicana (MCZ Herp R-29220): 175 (167+4+6 [posteriormost caudal vertebrae are fused]); Helminthophis frontalis (MCZ Herp R-55117): 311 (296+5+10, including a final fusion); Liotyphlops albirostris (UF Herp 43324): 239 (226+3+10, including a final fusion); Liotyphlops bondensis (CPZ-UV 7290): 201 trunk and cloacal vertebrae plus 15 caudal vertebrae (posteriomost caudal vertebrae are fused); Liotyphlops bondensis (CPZ-UV 7291): 203 trunk and cloacal vertebrae plus 13 caudal vertebrae (posteriomost caudal vertebrae are fused); Liotyphlops bondensis (CPZ-UV 7292): 228 trunk and cloacal vertebrae plus 15 caudal vertebrae (posteriomost caudal vertebrae are fused); Typhlophis squamosus (KUBI 69819): 210 (201+3+6, including a final fusion).

Data from literature: Anomalepis aspinosus Taylor, 1939: 170 trunk vertebrae plus 3 cloacal vertebrae plus 5 caudal vertebrae (but some could be missing, especially from the caudal series) (Dunn 1941); Liotyphlops albirostris: 204–247 trunk vertebrae plus 3–5 cloacal vertebrae plus 8–16 caudal vertebrae (posteriormost 2–3 caudal vertebrae are fused) (List 1966); Liotyphlops albirostris: 229–247 trunk vertebrae plus 5 cloacal vertebrae (i.e., bearing forked ribs) plus 8–10 caudal vertebrae (“without ribs”) (Dunn and Tihen 1944); Liotyphlops ternetzii (Boulenger, 1896): 242 trunk and cloacal vertebrae plus 11 caudal vertebrae (Alexander and Gans 1966).

In general, it seems that the total vertebral counts of anomalepidids are considerably low (although the available data are limited and should therefore be handled with cautiousness), with the notable exception of Helminthophis Peters, 1860, where this number surpasses the 300. Number of trunk vertebrae ranges between 170 and 296. Interestingly, the very low number (around 10 or less) of caudal vertebrae approaches that observed in many typhlopids, compared to most leptotyphlopids, where this number is higher. Of note is that species of Anomalepis and Typhlophis Fitzinger, 1843, seem to possess much lower (5–6) number of caudal vertebrae compared to species of Liotyphlops (8–16) and Helminthophis (10).

Alethinophidia Nopcsa, 1923

General information

The “true” (“ἀληθινά”) “snakes” (“ὀφίδια”), as their name in Greek readily suggests, Alethinophidia was originally established by Nopcsa (1923) to distinguish them from scolecophidians and from a, now considered, paraphyletic assemblage of various Cretaceous and Paleogene forms (Cholophidia Nopcsa, 1923). The monophyly of this group has never been challenged. Vidal et al. (2007) suggested that Alethinophidia could be divided into two new major clades, Amerophidia (comprising aniliids and tropidophiids) and Afrophidia (comprising the remaining taxa, i.e., uropeltoids, booids, pythonoids, bolyeriids, xenophidiids, and caenophidians).

Amerophidia Vidal, Delmas & Hedges, 2007

General information

Amerophidia constitute a recently established group based on molecular phylogenies (Vidal et al. 2007). It consists of Aniliidae and Tropidophiidae and is thought to represent the most basal group of alethinophidians (Scanlon and Lee 2011). As their name attests, amerophidians are currently distributed in the Americas. Despite the strong molecular evidence supporting its monophyly (e.g., Vidal et al. 2007, 2010; Wiens et al. 2008; Pyron and Burbrink 2012; Streicher and Wiens 2016; Zheng and Wiens 2016; Burbrink et al. 2020; Ortega-Andrade et al. 2022), amerophidian apomorphic morphological characters are mostly lacking, with the exception of a few cranial features shared among aniliids and tropidophiids, primarily in the shape of their palate, but also a soft anatomical apomorphy, an oviduct connecting to the diverticuli of the cloaca, instead of connecting directly to the cloaca (Maisano and Rieppel 2007; Scanlon and Lee 2011; Siegel et al. 2011; Hsiang et al. 2015). Indeed, morphology-based phylogenies have failed to recover such close relationship among Aniliidae and Tropidophiidae (e.g., Gauthier et al. 2012; Scanferla et al. 2016; Smith and Scanferla 2021). This is also true for their vertebral morphology, which is rather different between aniliids and tropidophiids.

Aniliidae Stejneger, 1907

General information

The taxonomic content and exact affinities of the group that is commonly known as “pipesnakes”, i.e., the American Anilius Oken, 1816, and the Asian Anomochilus Berg, 1901, and Cylindrophis Wagler, 1828, has been variously altered throughout decades of systematic studies of snakes. They were once known during the 19^th century under the names Ilysiidae (or Ilysioidea) (e.g., Fitzinger 1826; Bonaparte 1852; Boulenger 1890b, 1893; Cope 1894, 1898; Gadow 1901; Janensch 1906) or Tortricidae (or Tortricina or Tortriciens) (e.g., Müller 1831; Duméril and Bibron 1844; Jan 1857, 1862, 1863; Peters 1861; Cope 1864, 1887, 1893, 1895, 1898; Günther 1864; Jan 1865; Carus 1868; Müller 1880; Rochebrune 1884; Zittel 1887–1890; Palacký 1898). Cope (1887) also included the uropeltids in anilioids, while Romer (1956), Kuhn (1961), Smith et al. (1977), and McDowell (1975, 1987) further added Xenopeltis Reinwardt in Boié, 1827, and Loxocemus Cope, 1861. Guibé (1970) placed in Aniliidae only Anilius, Cylindrophis, and Anomochilus. In a similar manner, Anilioidea was later confined to include these three genera, all pertaining to their own families or subfamilies (e.g., Cundall et al. 1993). That traditional concept of Anilioidea was considered as a paraphyletic assemblage by Gower et al. (2005), a view that has been subsequently followed by others (Smith 2013; Head 2021; Smith and Georgalis 2022). Indeed, recent molecular studies treat Anilius as the sister group of Tropidophiidae, united in a group termed Amerophidia, whereas Uropeltis Cuvier, 1829, and Cylindrophis are united in a more distantly related group termed Uropeltoidea (e.g., Miralles et al. 2018; Burbrink et al. 2020; Zaher et al. 2023; see the respective entry below). As such, Aniliidae is currently conceived to include among extant snakes solely Anilius and its single species, Anilius scytale (Linnaeus, 1758), distributed only in northern South America. Fossorial habits for these snakes are already clearly indicated by their etymology, i.e., from the Greek “ἀν-” (privative affix for “no”, “without”) and “ἥλιος” (“sun”).

Although a number of fossil remains and taxa has been referred to aniliids in the past few decades (e.g., Rage 1974, 1984, 1998; Szyndlar 1994, 2009; Szyndlar and Alférez 2005; Augé and Rage 2006; Syromyatnikova et al. 2019), taking also into consideration the paraphyly of the traditional concept of “Anilioidea”, it is not possible to determine where exactly most of these lie within Alethinophidia (see Head 2021; Smith and Georgalis 2022). Accordingly, the sole few definite fossil occurrences of Aniliidae are known from the Late Cretaceous and Cenozoic of the Americas (Head 2021; Head et al. 2022; Smith and Georgalis 2022), including also the only known fossil record of the extant Anilius scytale, from the Pliocene of Venezuela (Carrillo-Briceño et al. 2021).

Vertebral morphology of Aniliidae is characterized by being relatively heavily built, an elongate centrum, depressed cotyle and condyle, depressed neural arch, neural spine with short anterior lamina that is strongly reduced dorsoventrally but crosses most of the anteroposterior length of the neural arch, shallow (but not absent) median notch of the neural arch, elongate prezygapophyses elevated to just shorter than zygosphene and angled at around 20°–25°, prominent (very thick and plate-like in shape) hypapophysis in anterior trunk vertebrae and a distinct haemal keel in succeeding trunk vertebrae, lack of haemapophyses or hypapophyses in caudal vertebrae, and a very low number of caudal vertebrae (for more details see Description and figures of Anilius below).

Previous figures of vertebrae of extant Aniliidae have been so far presented by Rochebrune (1881), Hoffstetter and Gasc (1969), Gasc (1974), Hoffstetter and Rage (1977), Dowling and Duellman (1978), Rieppel (1979), Ikeda (2007), Palci et al. (2013a, 2018), Garberoglio et al. (2019), Fachini et al. (2020), Head (2021), and Alfonso-Rojas et al. (2023). Among these, vertebrae from the cloacal and/or caudal series have only been figured so far by Gasc (1974) and Alfonso-Rojas et al. (2023). Documentation of the embryonic development of aniliid vertebrae was provided by Guerra-Fuentes et al. (2023). Quantitative studies on the intracolumnar variability of aniliid vertebrae have been conducted by Gasc (1974) and Head (2021). Besides, important observations on the vertebrae of Anilius were made by Smith (2013), while Head (2021) recently provided a comprehensive study of the vertebral morphology of Aniliidae coupled with an emended diagnosis for the family, recognizing diagnostic features on the vertebrae that could differentiate them from uropeltoids.

Anilius Oken, 1816

Material examined

Anilius scytale (Linnaeus, 1758) MNHN-AC-1869.0772; MNHN-AC-1880.1892; MNHN-AC-1887.0901; MNHN-AC-1888.0186.

Description (Figs 19-25>)

Trunk vertebrae. Centrum elongate; cotyle and condyle strongly depressed; neural arch depressed; posterior median notch of the neural arch shallow (sometimes absent in posterior trunk vertebrae); neural spine with short anterior lamina that is very low, slightly shifted (but nor restricted to) the posterior portion of the neural arch; prezygapophyses elongate, reaching just shorter than the zygosphene and angled at around 20°–25° in anterior view; prezygapophyseal accessory processes short to moderate in length; hypapophyses plate-like (but elongate in the very anteriormost trunk vertebrae), restricted to the anterior trunk vertebrae, prominent until around V 30 and then diminishing in size until around V 40 to V 50; haemal keel in succeeding trunk vertebrae flattened; paracotylar foramina absent.

Figure 19.

Aniliidae: Anilius scytale (NHMUK 56.10.16), anterior trunk vertebrae.

Figure 20.

Aniliidae: Anilius scytale (NHMUK 56.10.16), trunk vertebrae.

Figure 21.

Aniliidae: Anilius scytale (NHMUK 56.10.16), trunk vertebrae.

Figure 22.

Aniliidae: Anilius scytale (NHMUK 56.10.16), cloacal and caudal vertebrae.

Figure 23.

Aniliidae: Anilius scytale (MNHN-AC-1869.0772), trunk vertebrae.

Figure 24.

Aniliidae: Anilius scytale (MNHN-AC-1869.0772), trunk vertebrae.

Figure 25.

Aniliidae: Anilius scytale (MNHN-AC-1869.0772), cloacal and caudal vertebrae.

Smith (2013: 161) further highlighted the presence of “weak swellings in the position of subcotylar tubercles” that appear obvious at approximately V 120 and become small but distinct processes at around V 130.

Trunk/caudal transition. All last trunk, cloacal, and caudal vertebrae retain a relatively flattened haemal keel. Smith (2013: 161), who had access to one of our specimens (MNHN-AC-1869.0772) mentioned that haemapophyses are represented in this taxon “only by weak, bilateral bumps near the posterior margin of the first two or three postcloacals”. Of course, this is simply a matter of terminology but we interpret this approach as “bumps” or tiny protrusions of a flattened haemal keel, instead of haemapophyses.

Number of vertebrae (all for Anilius scytale): NHMUK 56.10.16: 253 (234+4+15, including three fused posteriormost caudal vertebrae); MNHN-AC-1869.0772: 239+ (223+4+12+).

Data from literature and unpublished data from personal communications (all for Anilius scytale): 220 trunk vertebrae plus 4 cloacal vertebrae plus 14 caudal vertebrae (NHMUK 1855.5.28.23; Jason Head, unpublished data, personal communication to GLG); 217 trunk and cloacal vertebrae plus 16+ caudal vertebrae (Alexander and Gans 1966); 217 trunk vertebrae plus 22 cloacal and caudal vertebrae (Polly et al. 2001); 226 trunk vertebrae plus 37 cloacal and caudal vertebrae [the latter value seems erroneous] (Nopcsa 1923); 231 trunk vertebrae plus 5 cloacal vertebrae (possibly erroneous) plus 32 caudal vertebrae (Rochebrune 1881); 213 trunk vertebrae plus unknown number of cloacal and caudal vertebrae (Gasc 1974); 209 trunk vertebrae plus unknown number of cloacal and caudal vertebrae (Tsuihiji et al. 2012).

In general, it is interesting to note the very low number of caudal vertebrae in Anilius, where usually this number does not exceed 20. However, in older literature (if the respective counts of Rochebrune [1881] and Nopcsa [1923] were correct) there are reports of caudal vertebral counts higher than 30. Unfortunately, the specimens on which these counts are based are unknown.

Tropidophiidae Brongersma, 1951

General information

Commonly known as “dwarf boas”, they were for long lumped into an expansive “Boidae” (e.g., Cope 1887; Stull 1935; Romer 1956; Dowling 1959; Guibé 1970; Rage 1984). They were initially assembled together under the name Ungaliidae by Cope (1894) (subsequently spelled as Ungaliidae by the same author; Cope 1898), however, the typifying genus Ungalia Gray, 1842, is a junior synonym of Tropidophis Bibron in Ramón de la Sagra, 1838–1843, and Brongersma (1951) coined the name Tropidophiidae (see also Smith 1969). McDowell (1987) placed Tropidophiidae into their own superfamily, Tropidopheoidea, and divided them into two subfamilies, Tropidophinae (for Tropidophis and Trachyboa Peters, 1860) and Ungaliopheinae (for Exiliboa Bogert, 1968, and Ungaliophis Müller, 1880). More recently, though, it was suggested, based on external morphology and myology (Zaher 1994), that they represented a paraphyletic assemblage. This polyphyly was later further strongly demonstrated by molecular evidence, with phylogenetic analyses recovering Tropidophis and Trachyboa nested instead closer to aniliids, while Ungaliophis and Exiliboa were nested within Booidea (Wilcox et al. 2002; Lawson et al. 2004; Vidal et al. 2007, 2009; Wiens et al. 2008, 2012; Reynolds et al. 2014; Streicher and Wiens 2016; Zheng and Wiens 2016; Burbrink et al. 2020; Ortega-Andrade et al. 2022). This has resulted in a radically updated topology for Tropidophiidae, where Tropidophis and Trachyboa constitute its only extant members, confined to Tropical Americas, representing the sister group of Aniliidae (Smith and Georgalis 2022). Ungaliophiidae in turn is placed into “true” boas, being close to Charinaidae (Pyron et al. 2014; Reynolds and Henderson 2018; Georgalis and Smith 2020). Such distinction between Tropidophiidae and Ungaliophiidae is further supported by important differences in their cranial anatomy (Bogert 1968a; Smith and Georgalis 2022) as well as internal anatomy (Brongersma 1951).

A number of fossil forms from the Paleogene and Neogene of Europe, the Paleogene of Africa, the Paleogene and Neogene of southwestern Asia, and the Quaternary of the Americas have been referred to tropidophiids (Szyndlar and Rage 2003; Szyndlar et al. 2008; Rage and Augé 2010; McCartney and Seiffert 2016; Syromyatnikova et al. 2019, 2021; Georgalis et al. 2021c; see Smith and Georgalis 2022). Considering our current understanding of the Tropidophiidae concept, among the Paleogene and Neogene taxa, only an unnamed form from the Eocene of Egypt (McCartney and Seiffert 2016) could be more reliably assigned to tropidophiids, although certain European genera, such as the Eocene Szyndlaria Rage and Augé, 2010, might also be true members of that group (see Smith and Georgalis 2022). In any case, the extant genus Tropidophis is also known by few Pleistocene remains from the Caribbean Islands (Aranda et al. 2017; Syromyatnikova et al. 2021).

Vertebral morphology further corroborates such distinction between Tropidophiidae and Ungaliophiidae, though admittedly it does not provide any support on the suggested sister group relationship of Tropidophiidae with Aniliidae. The most distinctive feature of the vertebral morphology of Tropidophiidae is the presence of a broad hypapophysis in lateral view throughout their trunk vertebrae, which in all mid-trunk vertebrae has a distinct anteroventral corner forming approximately a right angle. In other portions of the vertebral column, this anteroventral corner of the hypapophysis can either be either of a right angle as well or occasionally show a different degree of inclination that protrudes much anteriorly (see examples in Dowling and Duellman 1978: fig. 104.2; Lee and Scanlon 2002: fig. 10G; McCartney and Seiffert 2016: fig. 6A). Besides, it is also the morphology in the cloacal and caudal series, possessing a longitudinal sequence of subcentral structures, that seems unique among snakes: cloacal and anterior caudal vertebrae of Tropidophiidae are provided with hypapophyses (differently shaped than the preceding trunk vertebrae), followed by distally forked (grooved) hypapophyses or small haemapophyses (or haemapophyses-like structures) in middle caudal vertebrae, and ultimately followed by keels retaining a medially located groove or a pit in posteriormost caudal vertebrae (for more details see Description and figures of Trachyboa and Tropidophis below).

Previous figures of vertebrae of extant Tropidophiidae have been so far presented by Holman (1967), Bogert (1968a, 1968b), Dowling and Duellman (1978), Lee and Scanlon (2002), Szyndlar and Rage (2003), Gauthier et al. (2012), Head (2021), Syromyatnikova et al. (2021), and Frýdlová et al. (2023). Among these, vertebrae from the cloacal and/or caudal series had never been figured so far, with the exception of a caudal vertebra (of unknown exact position in the tail) figured in Alfonso-Rojas et al. (2023) and a single μCT image of an articulated skeleton in Frýdlová et al. (2023); nevertheless, Szyndlar and Böhme (1996), Szyndlar and Rage (2003), and Szyndlar et al. (2008) emphasized considerably on the pattern of subcentral structures in the cloacal and caudal series of tropidophiids. Other authors (e.g., Underwood 1976; Dowling and Duellman 1978; Rage 1984; McDowell 1987; Holman 2000; Smith and Georgalis 2022) observed and highlighted the presence and/or shape of hypapophyses throughout the trunk portion of the column in tropidophiids. Smith and Georgalis (2022) took it a step further and suggested that the presence of the broad hypapophysis with a strong anteroventral corner in all mid-trunk vertebrae of all tropidophiids represents a diagnostic trait for the group. The presence of paracotylar foramina was recorded in Tropidophis haetianus (Cope, 1879) by Underwood (1976); based on the latter observation, McDowell (1987) erroneously mentioned the occurrence of paracotylar foramina in all tropidophiids, whereas Kluge (1991) erroneously mentioned the absence of these foramina in all of them.

Trachyboa Peters, 1860

Material examined

Trachyboa boulengeri Peracca, 1910 (NHMUK 1901.3.29.77; NHMUK 1907.3.29.26.77; UMMZ 190753); Trachyboa gularis Peters, 1860 (AMNH R28982; UMMZ 239492 [Morphosource.org: Media 000070188, ark:/87602/m4/M70188]).

Description (Figs 26-30)

Trunk vertebrae. The description is based on Trachyboa boulengeri. The vertebrae display a number of peculiarities unparalleled with other extant snakes: they are relatively very dorsoventrally tall and anteroposteriorly short in lateral view, with strongly laterally expanded zygapophyses in dorsal and ventral view; centrum much shorter than wide; cotyle and condyle orbicular; neural arch depressed, provided with distinct tubercles (or minute pterapophyses) above the postzygapophyseal areas; posterior median notch of the neural arch deep; neural spine twice as high as long, relatively very thick, surmounted by a broad plate, the latter produced anteriorly and posteriorly into distinct paired spurs; prezygapophyseal accessory processes not projecting laterally beyond the articular facets, but expanded posteriorly in dorsal view; short plate-like hypapophyses with a distinct anteroventral corner present throughout the trunk portion of the column; paracotylar foramina absent.

Figure 26.

Tropidophiidae: Trachyboa boulengeri (NHMUK 1901.3.29.77), trunk vertebrae. Note that this specimen consists of only these two vertebrae.

Figure 27.

Tropidophiidae: Trachyboa boulengeri (NHMUK 1907.3.29.26.77), trunk vertebrae and cloacal vertebrae. Note that there are only two cloacal vertebrae in this specimen.

Figure 28.

Tropidophiidae: Trachyboa boulengeri (NHMUK 1907.3.29.26.77), caudal vertebrae.

Figure 29.

Tropidophiidae: Trachyboa boulengeri (NHMUK 1907.3.29.26.77), caudal vertebrae.

Figure 30.

Tropidophiidae: Trachyboa boulengeri (UMMZ 190753), trunk vertebra.

The trunk vertebrae of the type species of the genus, Trachyboa gularis, generally resemble those of Trachyboa boulengeri, but differ from the latter in the absence of additional structures on the neural spine and neural arch, and the ?absent (or at least vestigial) prezygapophyseal accessory processes in the former species (see also Bogert 1968b: fig. 7C).

Trunk/caudal transition. The peculiarities observed in the trunk vertebrae of Trachyboa boulengeri are retained also in those from the cloacal and caudal portions of the column (except for the posteriormost caudal vertebrae). Notably, in the most complete available specimen (NHMUK 1907.3.29.26.77), skeletonized personally by one of us (ZS; see also above section “Parts of the vertebral column”), there are only two cloacal vertebrae (Fig. 27) – it is not clear though if this is the standard case for this particular species, or if it simply represents a variation (or even pathology) of this single particular specimen – in Trachyboa gularis (UMMZ 239492), nevertheless, there are four cloacal vertebrae. The hypapophysis of the last trunk vertebra is distinctly larger than in mid-trunk vertebrae; that structure (slightly shorter but thicker) is retained in the cloacal and nine anterior caudal vertebrae (Fig. 28). Paired haemapophyses first appear on the 10^th caudal vertebra, although they display the shape of grooved hypapophyses or keels rather than standard haemapophyses typical for most snakes; subcentral structures observed on the 16^th and 21^st caudal vertebrae are not grooved (i.e., these vertebrae possess a non-grooved haemal keel instead of haemapophyses; Fig. 28). Distinct tubercles (or minute pterapophyses) above the postzygapophyseal areas continue also to be present in the cloacal and caudal vertebrae.

In Trachyboa gularis (UMMZ 239492), the tiny paired haemapophyses (resembling grooved hypapophyses) appear already from the last cloacal (S 4) and continue until around C 12. The remaining few posterior caudal vertebrae possess a small keel, which is (in a random pattern) either ungrooved or slightly grooved.

Number of vertebrae. Trachyboa boulengeri (NHMUK 1907.3.29.26.77): ? (?+2+25); Trachyboa gularis (UMMZ 239492): 179 (154+4+21, plus a final fusion).

Data from literature: Trachyboa boulengeri: 131–134 trunk and cloacal vertebrae plus 24 caudal vertebrae (Alexander and Gans 1966).

Tropidophis Bibron in Ramón de la Sagra, 1838–1843

Material examined

Tropidophis canus (Cope, 1868) (AMNH R73066; UF Herp [Morphosource.org: Media 000445131, ark:/87602/m4/445131]; Tropidophis feicki Schwartz, 1957 (AMNH R81127); Tropidophis greenwayi Barbour & Shreve, 1936 (UF Herp 42392 [Morphosource.org: Media 000445141, ark:/87602/m4/445141]); Tropidophis haetianus (UF Herp 59679 [Morphosource.org: Media 000372277, ark:/87602/m4/372277); Tropidophis jamaicensis Stull, 1928 (NHMUK 1964.1239); Tropidophis melanurus (Schlegel, 1837) (AMNH R46690); Tropidophis semicinctus (Gundlach & Peters in Peters, 1864) (AMNH R7386); Tropidophis taczanowskyi (Steindachner, 1880) (NHMUK 44.2.27.8).

Description (Figs 31-35)

Trunk vertebrae. The description is primarily based on the complete skeleton of Tropidophis jamaicensis (NHMUK 1964.1239). Centrum as long as wide; cotyle and condyle orbicular; neural arch moderately vaulted; posterior median notch of the neural arch shallow to moderately deep; neural spine as high as long; prezygapophyseal accessory processes vestigial; hypapophyses plate-like and with a distinct anteroventral corner, present throughout the trunk portion of the column; paracotylar foramina present. Note that a vertebra from the same specimen of T. jamaicensis was previously figured by Szyndlar and Rage (2003: fig. 2O) as Tropidophis haetianus, as the former species was back then considered a subspecies of the latter.

Figure 31.

Tropidophiidae: Tropidophis jamaicensis (NHMUK 1964.1239), trunk vertebrae.

Figure 32.

Tropidophiidae: Tropidophis jamaicensis (NHMUK 1964.1239), posteriormost trunk, cloacal, and caudal vertebrae.

Figure 33.

Tropidophiidae: Tropidophis jamaicensis (NHMUK 1964.1239), caudal vertebrae.

Figure 34.

Tropidophiidae: Tropidophis jamaicensis (NHMUK 1964.1239), caudal vertebrae.

Figure 35.

Tropidophiidae: Tropidophis taczanowskyi (NHMUK 44.2.27.8), trunk vertebrae.

Mid-trunk vertebrae of Tropidophis taczanowskyi (four vertebrae studied) morphologically closely resemble those of T. jamaicensis; the main difference is that their hypapophyses are shorter, though still these possess the typical anteroventral corner. A similar situation with T. jamaicensis is also observed in the skeleton of Tropidophis greenwayi: here the hypapophysis of the mid-trunk vertebrae is also plate-like with a distinct anteroventral corner.

As for other species of the genus, published figures of trunk vertebrae of Tropidophis canus (Bogert 1968b: fig. 7B), Tropidophis haetianus (Head 2021: fig. 4B), and Tropidophis melanurus (Syromyatnikova et al. 2021: fig. 2C), demonstrate that these three species are also provided with a plate-like hypapophysis, possessing a distinct right-angled anteroventral corner produced anteriorly. Also for Tropidophis cacuangoae Ortega-Andrade et al., 2022, the sole published X-ray image of the holotype (Ortega-Andrade et al. 2022: fig. 8), depicts the body almost continuously in lateral view and thus the prominent hypapophyses with right-angled anteroventral corner appear to be prominent especially in the mid- and posterior trunk portion of the column. Note though that for Tropidophis melanurus, other published figures (Bogert 1968a: fig. 9A; Dowling and Duellman 1978: fig. 104.2; Lee and Scanlon 2002: fig. 10G) do show a different degree of inclination of the anteroventral corner (possessing a distinct “spur” in the anteroventral corner), and therefore in this species, the distinct right-angled anteroventral corner of the hypapophysis is evident only in part of the trunk region. Tropidophis feicki (Bogert 1968a: fig. 8C) has a short hypapophysis similar to that of T. taczanowskyi; for Tropidophis semicinctus, Bogert (1968a) stated that it is even less strongly developed. Bogert (1968a) regarded the structures in T. feicki and T. semicinctus to represent a haemal keel instead, but of course, the “terminology boundaries” between these elements are somehow a philosophical question (see also Discussion below). We consider the structure of T. feicki (and apparently also of T. semicinctus) as a short hypapophysis, as was also previously suggested by Smith and Georgalis (2022) for that species, and in any case, we highlight that this taxon also possesses the characteristic right-angled anteroventral corner in mid-trunk vertebrae. As for other vertebral features, it is worth noting that except for Tropidophis jamaicensis and T. melanurus the remaining species of the genus (at least the studied or the already published ones) do not possess paracotylar foramina.

Trunk/caudal transition. The description is again mainly based on Tropidophis jamaicensis. The hypapophysis of the last trunk vertebra is slightly longer than in mid-trunk vertebrae; in the following cloacal and anterior caudal vertebrae it becomes gradually shorter, inclined posteriorly, and pointed distally. A shallow groove appears on the tip of the hypapophysis of the 7^th caudal vertebra (i.e., V 183) of T. jamaicensis (but in T. greenwayi, the slightly grooved hypapophysis already appears from the 3^d caudal vertebra); in more posterior caudal vertebrae the subcentral structures are distally forked (grooved) hypapophyses rather than true paired haemapophyses, with this pattern continuing up to the very end of the tail. Posteriormost caudal vertebrae are fused.

Number of vertebrae. Tropidophis canus (UF Herp 20220): 193 (162+3+28, including a final fusion); Tropidophis greenwayi (UF Herp 42392): 193 (156+3+34, including a final fusion); Tropidophis haetianus (UF Herp 59679): 216 (176+3+37, including a final fusion); Tropidophis jamaicensis (NHMUK 1964.1239): 207 (173+3+31, including a final fusion).

Data from the literature and unpublished data from personal communications: Tropidophis cacuangoae: 157–160 trunk vertebrae plus 30–39 cloacal and caudal vertebrae, including a final fusion (Ortega-Andrade et al. 2022); Tropidophis haetianus: 192 trunk vertebrae plus 4 cloacal vertebrae plus 38 caudal vertebrae (NHMUK 1897.12.31.6; Jason Head, unpublished data, personal communication to GLG); Tropidophis melanurus: ~196–198 trunk and cloacal vertebrae plus 41 caudal vertebrae (Alexander and Gans 1966); Tropidophis semicinctus: 211 trunk and cloacal vertebrae plus 37+ caudal vertebrae (Alexander and Gans 1966).

Afrophidia Vidal, Delmas & Hedges, 2007

General information

This is the sister group of amerophidians, comprising the rest of alethinophidians (i.e., uropeltoids, booids, pythonoids, bolyeriids, xenophidiids, and caenophidians).

Uropeltoidea Müller, 1831

General information

Once placed along with Anilius, into an expanded, paraphyletic, concept of Anilioidea. However, recent phylogenetic analyses have instead recovered Anilius to be lying much more distantly, closer to the base of alethinophidians (see above). Uropeltoidea thus includes Uropeltidae, Cylindrophiidae, and Anomochilidae, all fossorial snakes, currently confined to Southern Asia (Gower et al. 2005). A fossil record is so far totally absent for uropeltoids, a rather frustrating fact, especially when considering that recent divergence date estimates suggested that Uropeltoidea split off during the Late Cretaceous (Cyriac and Kodandaramaiah 2017; Burbrink et al. 2020). The spellings Uropeltacea, Uropelta, Uropeltana, Uropeltina, and Uropeltiens have also been applied for this grouping during the 19^th century (e.g., Müller 1831; Bonaparte 1845, 1852; Jan 1857, 1865; Peters 1861), while Cope (1898) applied the name Rhinophiidae for uropeltids. Interestingly, Gray (1845) treated uropeltids as lizards.

Generally, vertebrae of Uropeltoidea are characterized by high-angled prezygapophyses (an average of >24°), neural spine lamina absent or greatly reduced, spine restricted to posterior edge of neural arch resulting in a saddle-shaped dorsal margin of the neural arch, and depressed neural arch with a shallow concave posteromedian notch.

Anomochilidae Cundall, Wallach & Rossman, 1993

General information

A rather enigmatic lineage of snakes. Anomochilidae are known exclusively from a single genus, Anomochilus Berg, 1901, encompassing three species known from only a very few available specimens, distributed in Southeastern Asia and Indonesia (Wallach et al. 2014). They were once lumped into an expanded concept of Aniliidae (e.g., Boulenger 1893; Dowling 1959; Rage 1987), then to Cylindrophiidae (e.g., McDowell 1975), however, they were subsequently shown to possess unique cranial features and represent a distinct family (Cundall and Rossman 1993; Cundall et al. 1993). Indeed, morphological data suggested that Anomochilidae were lying at the base of alethinophidians (Cundall et al. 1993) but recent molecular evidence attests for a closer relationship with cylindrophiids (Gower et al. 2005; Vidal et al. 2009; Pyron and Burbrink 2012; Zheng and Wiens 2016).

The postcranial osteology of Anomochilus has never been studied. Indeed, so far, the only published source on the vertebrae of anomochilids is an X-ray image of a paratype of Anomochilus monticola Das et al., 2008, provided by Das et al. (2008: fig. 3) (which, however, is not very informative) plus some vertebral counts in the same paper. In a previous study, Smith (1940) also mentioned an X-ray image (which he never figured), but he confined his observation solely to the absence of pelvic bones or femur and said nothing on the vertebrae.

Material examined

We only had available an X-ray of the posterior trunk, cloacal, and caudal regions of a skeleton (NHMUK 1946.1.17.4) of Anomochilus leonardi Smith, 1940.

Number of vertebrae. Anomochilus leonardi (NHMUK 1946.1.17.4): ~15 cloacal and caudal vertebrae (number of trunk vertebrae unknown).

Data from literature and unpublished data from personal communications: Anomochilus leonardi: 265 trunk and cloacal vertebrae plus 17 caudal vertebrae (Das et al. 2008); Anomochilus monticola Das et al., 2008: 264 trunk and cloacal vertebrae plus 11 caudal vertebrae (Das et al. 2008); Anomochilus weberi (Lidth de Jeude, 1890): 246 trunk vertebrae (last with forked rib) plus 4 cloacal vertebrae plus 9 caudal vertebrae (RMNH RENA 4691; Agustin Scanferla, unpublished data, personal communication).

Cylindrophiidae Fitzinger, 1843

General information

Cylindrophiids consist a monotypic family of uropeltoids, with a single genus, Cylindrophis and more than a dozen species, distributed only in Sri Lanka, southeastern Asia, and Indonesia (Wallach et al. 2014; Boundy 2021).

The vertebral morphology of cylindrophiids is primarily characterized by an elongate centrum, depressed cotyle and condyle, depressed neural arch, absent or very shallow median notch of the neural arch, absent haemal keels in mid- and posterior trunk vertebrae, neural spine vestigial and restricted to the posterior portion of the neural arch (disappearing entirely in the posterior vertebrae), absence of any subcentral structures in the cloacal and caudal portion of the column (with the exception of a moderately developed ridge-like keel in the last cloacal and two succeeding caudal vertebrae in Cylindrophis ruffus), and a very low number of caudal vertebrae (for more details, see Description and figures of Cylindrophis below).

Previous figures of vertebrae of extant Cylindrophiidae have been so far presented by Rochebrune (1881), Williams (1954, 1959), Gasc (1974), Hoffstetter and Rage (1977), Rieppel (1979), Polly and Head (2004), Ikeda (2007), Xing et al. (2018), Fachini et al. (2020), Palci et al. (2020), Head (2021), and Alfonso-Rojas et al. (2023). Among these, vertebrae from the cloacal and caudal series were presented by Gasc (1974), Palci et al. (2020), and Alfonso-Rojas et al. (2023). Quantitative studies on the intracolumnar variability of cylindrophiid vertebrae were conducted by Gasc (1974), Polly and Head (2004), and Head (2021).

Cylindrophis Wagler, 1828

Material examined

Cylindrophis maculatus (Linnaeus, 1758) (ZFMK 16549); Cylindrophis ruffus (Laurenti, 1768) (NHMUK uncat.; UF Herp 143722 [Morphosource.org: Media 000445111, ark:/87602/m4/445111]).

Description (Figs 36-42)

Trunk vertebrae. Centrum elongate; cotyle and condyle strongly depressed; neural arch depressed; posterior median notch of the neural arch absent or very shallow; neural spine vestigial and restricted to the posterior portion of the neural arch, disappearing entirely in the posterior vertebrae; prezygapophyseal accessory processes short; relatively elongated hypapophyses restricted to the anterior trunk vertebrae (approximately 36–40 in Cylindrophis ruffus and 50 in Cylindrophis maculatus); in more posterior trunk vertebrae haemal keel poorly developed and flattened; paracotylar foramina absent.

Figure 36.

Cylindrophiidae: Cylindrophis maculatus (ZFMK 16549), anterior trunk vertebrae.

Figure 37.

Cylindrophiidae: Cylindrophis maculatus (ZFMK 16549), trunk vertebrae.

Figure 38.

Cylindrophiidae: Cylindrophis maculatus (ZFMK 16549), mid- and posterior trunk vertebrae.

Figure 39.

Cylindrophiidae: Cylindrophis maculatus (ZFMK 16549), posteriormost trunk, cloacal, and caudal vertebrae.

Figure 40.

Cylindrophiidae: Cylindrophis ruffus (NHMUK uncat.), trunk vertebrae.

Figure 41.

Cylindrophiidae: Cylindrophis ruffus (NHMUK uncat.), posteriormost trunk and cloacal vertebrae.

Figure 42.

Cylindrophiidae: Cylindrophis ruffus (NHMUK uncat.), cloacal and caudal vertebrae.

It is worth noting that Smith (2013) mentioned the presence in Cylindrophis ruffus of distinct anteriorly directed tubercles ventral to the parapophysis in anterior trunk vertebrae and of distinct subcotylar tubercles in the mid- and posterior (but not posteriormost) trunk vertebrae – we did not observe these in our sample and it seems that these features could be intraspecifically variable.

Trunk/caudal transition. No subcentral structures occur in the posterior trunk, cloacal, and caudal vertebrae except for a moderately developed ridge-like keel in the last cloacal and two succeeding caudal vertebrae in Cylindrophis ruffus (Fig. 42). Smith (2013) also observed a similar structure in the same species, which he termed “hypapophysis” – observing a photograph of the S 1 of that uncatalogued MNHN specimen of Cylindrophis ruffus that was kindly shared to us by Krister Smith, we consider that this is a very similar structure to the one we call moderately developed ridge-like keel in our material of this species.

Number of vertebrae. Cylindrophis maculatus (ZFMK 16549): 201 (191+3+7); Cylindrophis ruffus (NHMUK uncat.): 228 (212+4+12, including three fused posteriormost vertebrae Cylindrophis ruffus (UF Herp 143722): 199 (187+4+8, including a final fusion).

Data from literature and unpublished data from personal communications: Cylindrophis maculatus: 195–209 trunk and cloacal vertebrae plus 8–?9 caudal vertebrae (Alexander and Gans 1966); Cylindrophis ruffus: 208 trunk vertebrae plus 4 cloacal vertebrae plus 8 caudal vertebrae (UMZC R4.12-3; Jason Head, unpublished data, personal communication to GLG); Cylindrophis ruffus: 203 trunk vertebrae plus 20 cloacal and caudal vertebrae (Polly et al. 2001); Cylindrophis ruffus: 194 trunk vertebrae plus 3 cloacal vertebrae plus unknown number of caudal vertebrae (Gasc 1974); Cylindrophis ruffus: 191 trunk vertebrae plus unknown number of cloacal vertebrae plus 10 caudal vertebrae, including a final fusion (Smith 2013 and Krister Smith, unpublished data, personal communication); Cylindrophis ruffus: 188 trunk vertebrae plus 14 cloacal and caudal vertebrae (Nopcsa 1923); Cylindrophis ruffus: 186 trunk and cloacal vertebrae plus 7 caudal vertebrae (Alexander and Gans 1966); Cylindrophis ruffus: 184–187 trunk vertebrae plus unknown number of cloacal and caudal vertebrae (Polly and Head 2004); Cylindrophis ruffus: 180–183 trunk vertebrae plus unknown number of cloacal and caudal vertebrae (Tsuihiji et al. 2012); Cylindrophis ruffus: 163 trunk vertebrae plus 9 cloacal vertebrae (apparently erroneous) plus 15 caudal vertebrae (Rochebrune 1881).

It should be noted that recently Cylindrophis ruffus has been recognized as a species complex, with several new cryptic species established (e.g., Amarasinghe et al. 2015; Kieckbusch et al. 2016, 2018; Bernstein et al. 2020); therefore, it is not certain whether all these dry skeletons (in particular specimens that were collected during the 18^th, 19^th, and/or early 20^th centuries) that were studied by us or were mentioned in older literature, belong indeed to Cylindrophis ruffus or some other species.

Uropeltidae Müller, 1831

General information

Uropeltids are a moderately diverse family of fossorial snakes, with more than 60 species, currently endemic to India and Sri Lanka (Wallach et al. 2014; Pyron et al. 2016; Boundy 2021; Sampaio et al. 2023). They are characterized by a large external keratinous shield on their tails (Pyron et al. 2016; Huntley et al. 2021), hence their common name as “shield-tailed” snakes (or simply “shieldtails”) and their etymology (from the Greek “οὐρά” meaning “tail” and “πέλτη” meaning “shield”). This is a very old lineage, with divergence dates suggesting that uropeltids split from other uropeltoids already by around the Paleocene–Eocene boundary (Cyriac and Kodandaramaiah 2017).

In terms of their vertebral morphology, Uropeltidae primarily differ from all other snakes (and actually all other amniotes) by their unique peculiar morphology of the atlas-axis complex, i.e., the axis articulates directly with the occipital condyle (for details see Baumeister 1908; Williams 1959; Hoffstetter and Gasc 1969; Rieppel 1979; Pyron et al. 2016; and references therein). As for the succeeding vertebrae, all uropeltid genera for which complete (or almost complete) axial skeletons were available display highly homogenous morphology of individual vertebrae as well as exactly the same pattern of intracolumnar variation. This is described in detail below for Brachyophidium. In general, vertebrae of uropeltids are characterized by an elongate centrum, depressed cotyle and condyle, a strongly concave zygosphene in dorsal view, high-angled prezygapophyses (an average of >24°), depressed neural arch, absent or very shallow concave posterior median notch of the neural arch, absent haemal keels in mid- and posterior trunk vertebrae, vestigial (or absent) neural spine posteriorly restricted resulting in a saddle-shaped dorsal margin of the neural arch, presence of prominent hypapophyses throughout the cloacal and caudal portions, and (most usually) a very low number of caudal vertebrae (for more details see Description and figures below).

Moreover, exactly beneath the scales of the characteristic external tail shield of uropeltids, there lies a peculiar bone structure which is fused to the termination of the posteriormost few fused caudal vertebrae (Baumeister 1908; Huntley et al. 2021). This internal osteological structure was referred by Huntley et al. (2021) as the “bony tail-shield”.

Previous figures of vertebrae of extant Uropeltidae have been so far presented by Baumeister (1908), Williams (1959), Hoffstetter and Gasc (1969), Dowling and Duellman (1978), Garberoglio et al. (2019), Palci et al. (2020), Head (2021), Ganesh et al. (2022), and Alfonso-Rojas et al. (2023). Among these, vertebrae from the cloacal and/or caudal series were presented by Baumeister (1908), Hoffstetter and Gasc (1969), Palci et al. (2020), and in a μCT image of a whole skeleton in Ganesh et al. (2022). Quantitative studies on the intracolumnar variability of uropeltid vertebrae were conducted by Hoffstetter and Gasc (1969), Gasc (1974), and Head (2021). Beyond these, brief descriptions and observations of uropeltid vertebrae had been done already by the 19^th century (Peters 1861). Hoffstetter (1968) observed that hypapophyses in Rhinophis blythii reappear before the cloaca. Before, Underwood (1967) reported erroneously the absence of hypapophyses behind the cervical portion of the column in Uropeltis and supposed that this condition may be characteristic for the whole family. More recent descriptions (without figures) of vertebral features of uropeltids were provided by Smith (2013).

We here studied individuals of multiple species of Brachyophidium Wall, 1921, Melanophidium Günther, 1864, Platyplectrurus Günther, 1868, Plectrurus Duméril & Duméril, 1851, Rhinophis Hemprich, 1820, Teretrurus Beddome, 1886, and Uropeltis Cuvier, 1829, which correspond to all extant genera (note that in some recent taxonomic schemes, Teretrurus is considered a senior synonym of Brachyophidium; Ganesh and Murthy 2022; Sampaio et al. 2023).

Brachyophidium Wall, 1921

Material examined

Brachyophidium rhodogaster Wall, 1921 (MCZ Herp R-18073).

Description (Figs 43-44)

Trunk vertebrae. Centrum elongate; neural arch depressed; posterior median notch of the neural arch absent or very shallow; neural spine vestigial and restricted to the posteriormost part of the neural arch, disappearing in mid- and posterior trunk vertebrae; high-angled prezygapophyses; prezygapophyseal accessory processes moderate in length; zygosphene narrow with strongly concave margins in dorsal view; hypapophyses spine-like, disappearing at the level of V 40 to V 50; moderately developed flattened haemal keel appears on posterior trunk vertebrae; paracotylar foramina absent; an asymmetrical subcentral foramen can be occasionally present.

Figure 43.

Uropeltidae: Brachyophidium rhodogaster (MCZ Herp R-18073), trunk vertebrae.

Figure 44.

Uropeltidae: Brachyophidium rhodogaster (MCZ Herp R-18073), posterior trunk, cloacal, and caudal vertebrae.

Trunk/caudal transition. The haemal keel becomes progressively larger in the last trunk to anterior cloacal portion of the column, eventually developing into a prominent hypapophysis in posterior cloacal and all following caudal vertebrae.

Number of vertebrae. Brachyophidium rhodogaster (MCZ Herp R-18073): ?145 (?133+3+9+; a few anterior trunk vertebrae lacking).

Data from literature: Brachyophidium rhodogaster: 138–143 trunk and cloacal vertebrae plus 9–12 caudal vertebrae (Alexander and Gans 1966).

Melanophidium Günther, 1864

Material examined

Melanophidium wynaudense (Beddome, 1863) (MCZ Herp R-24739).

Description (Fig. 45)

Trunk vertebrae. An axis and seven following vertebrae only were available for direct study; they do not differ from the anterior trunk vertebrae of other uropeltid genera.

Figure 45.

Uropeltidae: Melanophidium wynaudense (MCZ Herp R-24739), anterior trunk vertebrae.

Number of vertebrae. Data from literature and personal communications (all for Melanophidium wynaudense): 219+3+14+fusion (MCZ Herp R-24739; Agustin Scanferla, unpublished data, personal communication); 168 trunk and cloacal vertebrae plus 14+ caudal vertebrae (Alexander and Gans 1966).

Platyplectrurus Günther, 1868

Material examined

Platyplectrurus madurensis Beddome, 1877 (MCZ Herp R-18046); Platyplectrurus trilineatus (Beddome, 1867) (CAS Herp 244772 [Morphosource.org: Media 000074713, ark:/87602/m4/M74713]).

Description (Figs 46-47)

Trunk vertebrae. More elongate than trunk vertebrae of Brachyophidium; otherwise, the morphology is similar to that of other uropeltids.

Figure 46.

Uropeltidae: Platyplectrurus madurensis (MCZ Herp R-18046), trunk vertebrae.

Figure 47.

Uropeltidae: Platyplectrurus madurensis (MCZ Herp R-18046), posteriormost trunk, cloacal, and caudal vertebrae.

Trunk/caudal transition. The same morphology as in other uropeltids. In the posteriormost caudal vertebrae, the hypapophysis becomes more prominent (i.e., blade-like and dorsoventrally tall); the same applies to the neural spine, which is diminutive and vestigial in preceding caudal vertebrae but is taller in the posteriormost preserved caudal vertebra (V 173; Fig. 47).

Number of vertebrae. Platyplectrurus madurensis (MCZ Herp R-18046): ?173+ (?160+3+10+; a few anterior trunk vertebrae lacking); Platyplectrurus trilineatus (CAS Herp 244772): 174 (157+3+14).

Data from literature: Platyplectrurus madurensis: 166–167 trunk and cloacal vertebrae plus ?12–14+ caudal vertebrae (Alexander and Gans 1966).

Plectrurus Duméril & Duméril, 1851

Material examined

Plectrurus perroteti Duméril & Bibron in Duméril & Duméril, 1851 (MCZ Herp R-3875).

Description (Figs 48-49)

Trunk vertebrae. More elongate than trunk vertebrae of Brachyophidium; otherwise, the morphology is similar to that of other uropeltids.

Figure 48.

Uropeltidae: Plectrurus perroteti (MCZ Herp R-3875), trunk vertebrae.

Figure 49.

Uropeltidae: Plectrurus perroteti (MCZ Herp R-3875), posteriormost trunk, cloacal, and caudal vertebrae.

Trunk/caudal transition. The morphology is similar to that of other uropeltids.

Number of vertebrae. Plectrurus perroteti (MCZ Herp R-3875): ?147 (?141+3+3+; a few anterior trunk vertebrae lacking).

Data from literature: Plectrurus perroteti: 154 trunk and cloacal vertebrae plus 12+ caudal vertebrae (Alexander and Gans 1966).

Rhinophis Hemprich, 1820

Material examined

Rhinophis blythii Kelaart, 1853 (MCZ Herp R-4233; MNHN-AC uncat.); Rhinophis sanguineus Beddome, 1863 (UF Herp 78397 [Morphosource.org: Media 000072226, ark:/87602/m4/M72226]).

Description (Figs 50-52)

Trunk vertebrae. The morphology is relatively similar to that of other uropeltids. Hypapophyses are present in anterior trunk vertebrae up to V 40–V 45. Notably also, in most trunk vertebrae of Rhinophis blythii, there are peculiar parasagittal posterior projections of the neural arch (Figs 51–52), though still not present in all trunk vertebrae (Fig. 50).

Figure 50.

Uropeltidae: Rhinophis blythii (MCZ Herp R-4233), trunk, cloacal, and caudal vertebrae.

Figure 51.

Uropeltidae: Rhinophis blythii (MNHN-AC uncat.), trunk vertebrae.

Figure 52.

Uropeltidae: Rhinophis blythii (MNHN-AC uncat.), trunk, cloacal, and caudal vertebrae.

Trunk/caudal transition. The morphology is relatively similar to that of other uropeltids. The hypapophysis is rather prominent, dorsoventrally tall, and almost straight in the posteriormost caudal vertebra.

Number of vertebrae. Rhinophis blythii (MNHN-AC uncat.): ?160+ (?153+3+4+); several vertebrae belonging to this examined skeleton, at least those formerly illustrated by Hoffstetter and Gasc (1969), i.e., atlas and four posteriormost caudal vertebrae are apparently lacking (the specimen was labelled ibidem as belonging to the “pers. coll. Hoffstetter”); Rhinophis sanguineus (UF Herp 78397): 217 (204+4+9, including a final fusion and the shield).

Data from literature and unpublished data from personal communications: Rhinophis blythii: 152–157 trunk and cloacal vertebrae plus 7–11 caudal vertebrae (Alexander and Gans 1966); Rhinophis blythii: 151 trunk vertebrae plus unknown number of cloacal and caudal vertebrae (Hoffstetter 1968); Rhinophis philippinus (Cuvier, 1829): 150–166 trunk and cloacal vertebrae plus 5–9 caudal vertebrae (Alexander and Gans 1966); Rhinophis philippinus: 149 trunk vertebrae plus 4 cloacal vertebrae plus 6 caudal vertebrae (excluding shield) (UMZC R.5.7-1; Jason Head, unpublished data, personal communication to GLG); Rhinophis philippinus: 143 trunk plus 13 cloacal and caudal vertebrae (Rhinophis planiceps of Baumeister 1908).

Teretrurus Beddome, 1886

Material examined

Teretrurus sanguineus (Beddome, 1867) (CAS Herp 244362 [Morphosource.org: Media 000074710, ark:/87602/m4/M74710]).

Description

Trunk vertebrae. A relatively similar morphology as in other uropeltids.

Trunk/caudal transition. A similar morphology as in other uropeltids.

Number of vertebrae. Teretrurus sanguineus (CAS Herp 244362): 130 trunk vertebrae plus 13 cloacal and caudal vertebrae.

Uropeltis Cuvier, 1829

Material examined

Uropeltis arcticeps (Günther, 1875) (MCZ Herp R-22389 [Morphosource.org: Media 000483006, ark:/87602/m4/483006]); Uropeltis ceylanica Cuvier, 1829 (MCZ Herp R-3852); Uropeltis melanogaster (Gray, 1858) (NHMUK uncat.); Uropeltis woodmasoni (Theobald, 1876) (NHMUK 1930.5.8.75).

Description (Figs 53-56)

Trunk vertebrae. The morphology similar to other uropeltids except for the hypapophysis disappearing at the level of V 20 to V 30, as well as the neural spine that (although vestigial) is still visible in the posterior trunk vertebrae. The above observations are based on Uropeltis melanogaster (the only complete dry skeleton). In Uropeltis ceylanica (anteriormost and posteriormost vertebrae lacking) the vestiges of the neural spine are more reduced. It is further worth noting that, somehow similarly to the case of Rhinophis blythii above, there are peculiar parasagittal posterior projections of the neural arch in trunk vertebrae of Uropeltis melanogaster (though less well developed than those of Rhinophis blythii), while possibly these appear to be absent in Uropeltis ceylanica.

Figure 53.

Uropeltidae: Uropeltis ceylanica (MCZ Herp R-3852), trunk, cloacal, and caudal vertebrae.

Figure 54.

Uropeltidae: Uropeltis melanogaster (NHMUK uncat.), trunk vertebrae.

Figure 55.

Uropeltidae: Uropeltis melanogaster (NHMUK uncat.), posteriormost trunk, cloacal, and caudal vertebrae.

Figure 56.

Uropeltidae: Uropeltis woodmasoni (NHMUK 1930.5.8.75), trunk vertebrae.

Trunk/caudal transition. The same morphology as in other uropeltids. A vestigial (but clearly visible) neural spine in caudal vertebrae in Uropeltis melanogaster.

Number of vertebrae. Uropeltis arcticeps (MCZ Herp R-22389): 167 (149+3+15, including a final fusion); Uropeltis melanogaster (NHMUK uncat.): 177 (163+3+11).

Data from the literature: Uropeltis dindigalensis (Beddome, 1877): 164 trunk and cloacal vertebrae and 10 caudal vertebrae, including a final fusion (counted from the skeleton in Ganesh et al. 2022: fig. 7); Uropeltis melanogaster: 166 trunk vertebrae plus unknown number of cloacal and caudal vertebrae (Tsuihiji et al. 2012); Uropeltis ocellata (Beddome, 1863): 191–?198 trunk and cloacal vertebrae plus 14–15 caudal vertebrae (Alexander and Gans 1966); Uropeltis pulneyensis (Beddome, 1863): 167–173 trunk and cloacal vertebrae plus 9 caudal vertebrae (Alexander and Gans 1966); Uropeltis rubrolineata (Günther, 1875): 136–169 trunk and cloacal vertebrae plus 8–11 caudal vertebrae (Alexander and Gans 1966); Uropeltis woodmasoni: 166–179 trunk and cloacal vertebrae plus 8–15 caudal vertebrae (Alexander and Gans 1966).

The lowest vertebral counts in the entire family Uropeltidae were reported by Nopcsa (1923) for Silybura brevis Günther, 1862 (a junior synonym of Uropeltis ceylanica): 141 vertebrae altogether (127 trunk vertebrae plus 14 cloacal and caudal vertebrae) – this value only is only slightly lower than the one documented above for Teretrurus sanguineus (143 vertebrae altogether).

Constrictores Oppel, 1811 (sensu Georgalis and Smith 2020)

General information

Constrictores represents a recently defined group encompassing Booidea and Pythonoidea, plus tentatively also Bolyeriidae and Xenophidiidae (Georgalis and Smith 2020). They comprise the largest known snakes, both extant and extinct (see Georgalis and Smith 2020). They have a widespread current distribution (Georgalis and Smith 2020), coupled with an extensive and rich fossil record, especially during the Paleogene (Szyndlar and Rage 2003; Georgalis et al. 2021a; Smith and Georgalis 2022). The group has been recovered as monophyletic in several recent phylogenies (e.g., Slowinski and Lawson 2002; Lee et al. 2007; Pyron et al. 2013; Streicher and Wiens 2016; Zheng and Wiens 2016; Burbrink et al. 2020; Onary et al. 2022; Zaher et al. 2023), although other studies have instead proposed a closer relationship of pythonoids to uropeltoids instead (Lawson et al. 2004; Reynolds et al. 2014).

The generalized vertebral morphology of Constrictores is defined by massively built vertebrae, with a generally low ratio of their centrum length / neural arch width (<1.1), high neural spines, and a thick zygosphene (Georgalis and Smith 2020). However, significant deviations exist to this generalized rule, such as the elongated vertebrae of ungaliophiids, or the much thinner zygosphene of several taxa, or the complex caudal morphology of erycids and charinaids.

Bolyeriidae Hoffstetter, 1946

General information

Bolyeriidae includes only the two monotypic genera Bolyeria and Casarea from the Mascarene Islands, among which the former is now extinct since a few decades ago (Wallach et al. 2014; Georgalis and Smith 2020). They were long considered as members of an “expanded” Boidae (e.g., Cope 1887; Stull 1935; Hoffstetter 1946, 1960; Anthony and Guibé 1952; Dowling 1959; Guibé 1970; Rage 1987). A sister group relationship with Tropidophiidae has also been proposed (Underwood 1976; Wallach and Günther 1998), while other even directly included them within tropidophiids (e.g., Arnold 1980). According to recent phylogenies, the Asian Xenophidiidae are the closest relatives of bolyeriids (e.g., Lawson et al. 2004; Pyron and Burbrink 2012; Pyron et al. 2013; Figueroa et al. 2016), the two in turn forming the superfamily Bolyerioidea (Georgalis and Smith 2020). Indeed, Bolyeriidae and Xenophidiidae possess a synapomorphy, a maxilla divided into separate anterior and posterior parts (e.g., Anthony and Guibé 1952; Maisano and Rieppel 2007), that is a unique feature among all tetrapods (Georgalis and Smith 2020). Their exact affinities with other snakes are not established with certainty, with other phylogenies placing them close to Booidea (Streicher and Wiens 2016; Zheng and Wiens 2016; Harrington and Reeder 2017; Burbrink et al. 2020) or Pythonoidea (Lawson et al. 2004; species-tree analysis of Burbrink et al. 2020). Only subfossil remains from the Mascarenes exist (Hoffstetter 1960; Arnold 1980), with no pre-Quaternary fossil record.

Vertebral morphology of Bolyeriidae is principally characterized by the presence of prominent hypapophyses throughout the trunk column (for more details see Description and figures of Bolyeria and Casarea below).

Previous figures of vertebrae of extant Bolyeriidae have been so far presented by Underwood (1976), Hecht and LaDuke (1988), and Palci et al. (2020). Among these, vertebrae from the cloacal and caudal series of bolyeriids have never been figured so far. Quantitative study on the intracolumnar variability of bolyeriid vertebrae was conducted by Hoffstetter (1960). Besides the published figures, several authors observed the presence of hypapophyses throughout the trunk portion of the column in bolyeriids (e.g., Romer 1956; Hoffstetter 1946, 1960, 1968; McDowell 1975; Rage 1984; Szyndlar and Rage 2003).

Bolyeria Gray, 1842

Material examined

Bolyeria multocarinata (Boié H in Boié F, 1827) (UMMZ 190727).

Description (Fig. 57)

Trunk vertebrae. Only two mid-trunk vertebrae were examined. Centrum shorter than wide; cotyle and condyle orbicular; distinct subcotylar tubercles present; neural arch moderately vaulted; posterior median notch of the neural arch deep; neural spine as high as long, with distinct anterodorsal and posterodorsal projections; prezygapophyseal accessory processes long; hypapophyses sigmoidal, present throughout the column; paracotylar foramina present.

Figure 57.

Bolyeriidae: Bolyeria multocarinata (UMMZ 190727), two trunk vertebrae.

Trunk/caudal transition. Unknown.

Number of vertebrae. Data from literature: Bolyeria multocarinata: 198 trunk vertebrae plus 87 cloacal and caudal vertebrae (Hoffstetter 1960).

Casarea Gray, 1842

Material examined

Casarea dussumieri (Schlegel, 1837) (MNHN-AC-1993.3382; NHMUK 1992.996; UMMZ 190732).

Description (Figs 58-62)

Trunk vertebrae. Centrum as short as wide or somewhat longer; cotyle and condyle orbicular; neural arch moderately vaulted; posterior median notch of the neural arch deep; neural spine as high as long or slightly lower; prezygapophyseal accessory processes vestigial or very short; hypapophyses present throughout the trunk portion of the column, spine-like (more anterior vertebrae) to sigmoidal (more posterior vertebrae); paracotylar foramina present (occasionally even doubled from one side; see e.g., V 40 in Fig. 58).

Figure 58.

Bolyeriidae: Casarea dussumieri (MNHN-AC-1993.3382), trunk vertebrae.

Figure 59.

Bolyeriidae: Casarea dussumieri (MNHN-AC-1993.3382), trunk vertebrae.

Figure 60.

Bolyeriidae: Casarea dussumieri (MNHN-AC-1993.3382), trunk and cloacal vertebrae.

Figure 61.

Bolyeriidae: Casarea dussumieri (MNHN-AC-1993.3382), cloacal and caudal vertebrae.

Figure 62.

Bolyeriidae: Casarea dussumieri (UMMZ 190732), trunk and caudal vertebrae.

Trunk/caudal transition. The last trunk vertebrae are provided with a prominent hypapophysis, relatively larger and thicker than those on the more anterior vertebrae. The hypapophysis is retained on the cloacal vertebrae, but becomes smaller. Paired haemapophyses appear on the first caudal vertebra.

Number of vertebrae. Casarea dussumieri (MNHN-AC-1993.3382): 341 (228+3+110). Counts for the same specimen were previously published by Hoffstetter (1960).

Data from literature (all for Casarea dussumieri): 225 trunk and cloacal vertebrae (a few anterior vertebrae missing) plus 126 caudal vertebrae (Alexander and Gans 1966); 228 trunk vertebrae plus 113 cloacal and caudal vertebrae (McDowell 1975).

Xenophidiidae Wallach & Günther, 1998

General information

Xenophidiidae represents a rather enigmatic group of snakes that were only recently discovered, comprising a single genus, Xenophidion Günther & Manthey, 1995, with two species, confined in the Malay Peninsula, Borneo, and Sumatra (Günther and Manthey 1995; Wallach and Günther 1998; Quah et al. 2018). Frustratingly, both species are collectively known only by less than 10 specimens (Günther and Manthey 1995; Wallach and Günther 1998; Quah et al. 2018; Fukuyama et al. 2020). Xenophidion was originally regarded to be a caenophidian (Günther and Manthey 1995) and subsequently placed in its own family, Xenophidiidae as the sister group of tropidophiids (Wallach and Günther 1998). Xenophidiidae possess a unique set of external morphology, soft anatomy, and skeletal features that complicate understanding of their exact interrelationships with other snakes (see Wallach and Günther 1998). Nevertheless, their joint possession with Bolyeriidae of a unique cranial synapomorphy – an intramaxillary joint – suggests a close relationship of xenophidiids with that group (Gauthier et al. 2012; Georgalis and Smith 2020), a conclusion also coupled with molecular evidence (see entry of Bolyeriidae above). There is as yet no fossil record of xenophidiids.

No xenophidiid skeleton was available for study. In fact, the only so far known information on the vertebrae of Xenophidion is a single X-ray image of the holotype of the type species of the genus, Xenophidion acanthognathus Günther & Manthey, 1995, published by Wallach and Günther (1998: fig. 1). The resolution of that image is low and does not permit many conclusions, but Wallach and Günther (1998) provided a description of some interesting vertebral features, among which they highlighted the presence of “expanded blade-like haemapophyses (larger than the neural spines) in the caudal region”, which they considered as unique among snakes. The same authors further mentioned the presence of blade-like hypapophyses in posterior trunk vertebrae and the absence of prezygapophyseal accessory processes (Wallach and Günther 1998).

Number of vertebrae. Xenophidion acanthognathus (FMNH 235170 [holotype]): 183+4+53 (Agustin Scanferla, unpublished data, personal communication).

This number of vertebrae of Xenophidion acanthognathus is in accordance with the published number of ventrals and subcaudals described in the existing literature for different individuals of the same species: 181–185 ventrals and 51–55 subcaudals (Günther and Manthey 1995; Fukuyama et al. 2020). Similar data for Xenophidion schaefferi Günther & Manthey, 1995, indicate 176–178 ventrals and 43–45 subcaudals (Günther and Manthey 1995; Quah et al. 2018). Accordingly, the respective number of trunk and cloacal vertebrae for Xenophidion spp. ranges around 175–185 and that of the caudal vertebrae around 40–55. Therefore, these numbers are slightly lower than in Bolyeria and much lower than in Casarea.

Booidea Gray, 1825 (sensu Burbrink et al. 2020)

General information

Until relatively recently Booidea included both boas and pythons, but it is currently restricted only to boas and their closest relatives, i.e., the families Calabariidae, Sanziniidae, Boidae, Candoiidae, Erycidae, Charinaidae, and Ungaliophiidae (Pyron et al. 2014; Reynolds and Henderson 2018; Burbrink et al. 2020; Georgalis and Smith 2020; Scanferla and Smith 2020b; see above Table 1). They have a rather disjunct geographic distribution, scattered across the Americas, Africa, Madagascar, Eastern Europe, Asia, and certain Pacific Islands (Pyron et al. 2014; Reynolds and Henderson 2018). Their fossil record attests an even wider distribution, achieving a remarkable diversity during the Paleogene (Rage 1984; Szyndlar and Rage 2003; Head et al. 2009; Georgalis et al. 2021a; Smith and Georgalis 2022), including also astonishingly preserved skeletons (Szyndlar and Böhme 1996; Baszio 2004; Scanferla et al. 2016; Smith and Scanferla 2016, 2021; Scanferla and Smith 2020a, 2020b; Georgalis et al. 2021a; Chuliver et al. 2022), spectacular mummified trunk portions (Rochebrune 1884; Georgalis et al. 2021a), as well as the largest, so far, known snake of all time, Titanoboa Head et al., 2009, from the Paleocene of Colombia (Head et al. 2009).

Calabariidae Gray, 1858

General information

This group comprises only a single species, Calabaria reinhardtii, distributed in Central and Western Africa (Wallach et al. 2014). The species was originally described by Schlegel (1848) as a species of Eryx; however, its distinctiveness from other booids was already highlighted by Gray (1858a), who created for it, not only his new genus Calabaria, but also the new tribe Calabariina in order to accommodate this bizarre taxon. It was placed in its own monotypic (sub)family of Boidae, the Calabariinae (Underwood 1976; Rage 1987), or in Pythonidae (Bocage 1895; Schmidt 1923; Roux-Estève 1965; Underwood 1967; Guibé 1970; McDowell 1987), or joined with “erycines” (Kluge 1993b). Nevertheless, recent phylogenies, usually place Calabariidae within Booidea (e.g., Wiens et al. 2008; Pyron et al. 2013; Reynolds et al. 2014; Scanferla et al. 2016; Ruane and Austin 2017; Smith and Scanferla 2021; Onary et al. 2022; Zaher et al. 2023). No fossil record of calabariids exists so far.

Vertebrae of Calabariidae are indicative of the general constrictor morphology, but they primarily differ on the pattern of the subcentral structures in the trunk/caudal transition (see Description and figures of Calabaria below).

The only existing figures of vertebrae of Calabariidae in the literature so far were provided by Kluge (1993b) and Frýdlová et al. (2023), both of which presented also vertebrae from the cloacal and/or caudal portion of the column. Quantitative study on the intracolumnar variability of calabariid vertebrae was conducted by Head (2021). Besides, interesting observations (without figures) on calabariid vertebral structures have been mentioned by Rage (2001), Szyndlar and Böhme (1996), and Smith (2013).

Calabaria Gray, 1858

Material examined

Calabaria reinhardtii (Schlegel, 1848) (NHMUK 1911.10.28.17; SMF PH 68; UMMZ 190728; ZFMK 39543).

Description (Figs 63-70)

Trunk vertebrae. Centrum much shorter than wide; cotyle and condyle orbicular; neural arch moderately vaulted; posterior median notch of the neural arch deep; neural spine as high as long; prezygapophyseal accessory processes very short; hypapophyses disappear after the 40^th vertebra; haemal keel flattened, weakly developed; subcentral grooves deep; paracotylar foramina absent; asymmetrical subcentral foramina occasionally present in some vertebrae.

Figure 63.

Calabariidae: Calabaria reinhardtii (ZFMK 39543), anterior trunk vertebrae.

Figure 64.

Calabariidae: Calabaria reinhardtii (ZFMK 39543), trunk vertebrae.

Figure 65.

Calabariidae: Calabaria reinhardtii (ZFMK 39543), mid-trunk vertebra.

Figure 66.

Calabariidae: Calabaria reinhardtii (ZFMK 39543), trunk vertebrae.

Figure 67.

Calabariidae: Calabaria reinhardtii (ZFMK 39543), posterior trunk vertebrae.

Figure 68.

Calabariidae: Calabaria reinhardtii (ZFMK 39543), cloacal and anterior caudal vertebrae.

Figure 69.

Calabariidae: Calabaria reinhardtii (ZFMK 39543), caudal vertebrae.

Figure 70.

Calabariidae: Calabaria reinhardtii (UMMZ 190728), trunk vertebrae.

Trunk/caudal transition. A prominent plate-like hypapophysis present in last (around 15 or more) trunk vertebrae diminishing gradually in following cloacal vertebrae, taking the shape of a broad haemal keel around the cloacal/caudal transition; this keel diminishes entirely in more posterior caudal vertebrae. Beginning from the 5^th or 6^th caudal vertebra, indistinct outlines (or vestiges) of paired structures / tubercles (vestigial haemapophyses), can be visible until the end of the tail. Neural spine is considerably thickened and broad near the tip of the tail. The posteriormost caudal vertebrae are fused.

Number of vertebrae (all for Calabaria reinhardtii): ZFMK 39543: 256 (227+4+25), including a final fusion; SMF PH 68: 254 (225+4+25, including a final fusion).

Data from literature: Calabaria reinhardtii: 236 trunk and cloacal vertebrae plus 26+ caudal vertebrae (Alexander and Gans 1966).

Sanziniidae Romer, 1956

General information

Sanziniidae represents a small lineage of booids, pertaining to two genera (Acrantophis and Sanzinia) and four species, that are endemic to Madagascar (Pyron et al. 2014). Originally established as a subfamily of boids by Romer (1956), who also included in it the bolyeriids from the Mascarene Islands. Recent taxonomic schemes have elevated them to the family level (Pyron et al. 2014; Burbrink et al. 2020; Georgalis and Smith 2020).

Vertebral morphology of Sanziniidae is reminiscent of other constrictors. However, a principal difference lies within their caudal vertebrae, where they possess keels (that are partly grooved or bifurcated in Sanzinia) instead of haemapophyses. Actually, there appears to be a contradiction in the existing literature as far as it regards the subcentral structures of sanziniid vertebrae: Romer (1956) erroneously mentioned the presence of hypapophyses throughout the trunk vertebrae with McDowell (1975) also using the term “strongly projecting crest” for these structures. However, this observation was refuted by Hoffstetter (1960) and Szyndlar and Böhme (1996), who specified that there are no hypapophyses (by any definition of this term) in mid- and posterior trunk vertebrae. In an attempt to clarify this controversy, Smith (2013) suggested that ontogenetic variation may be the source of this confusion, as he highlighted that in a very small individual of Sanzinia madagascariensis there were distinct hypapophyses-like structures throughout the trunk vertebrae but these were progressively shifted to “regular” haemal keels in larger individuals. As for the cloacal and caudal region, Szyndlar and Böhme (1996) and Szyndlar et al. (2008) noted that sanziniids possess haemal keels and no haemapophyses in their caudal vertebrae, with some keels of Sanzinia bifurcated distally into two short spurs that may be interpreted as short haemapophyses. Smith (2013) nevertheless considered that this could also be ontogenetically variable, as he reported that in a very small individual, delicate haemapophyses were present on all caudal vertebrae except for the anteriormost approximately two ones, with the same pattern also generally present in mid-sized individuals.

Previous figures of vertebrae of extant Sanziniidae were so far only presented by Auffenberg (1958), Dowling (1959), and Gasc (1974). Among these, vertebrae from the cloacal and/or caudal series of sanziniids have never been figured so far. Figures of the microanatomy and histology of sanziniid vertebrae were presented by Houssaye et al. (2013). Quantitative study on the intracolumnar variability of sanziniid vertebrae was conducted by Hoffstetter (1960).

Acrantophis Jan, 1860 in Jan & Sordelli 1860–1866

Material examined

Acrantophis dumerili Jan, 1860 in Jan & Sordelli 1860–1866 (UF Herp 175572 [Morphosource.org: Media 000070007, ark:/87602/m4/M70007]; UMMZ 190701; UMMZ 190725).

Description (Figs 71-73)

Trunk vertebrae. Centrum much shorter than wide; cotyle and condyle slightly flattened; neural arch vaulted; posterior median notch of the neural arch deep; neural spine distinctly higher than long; prezygapophyseal accessory processes short; hypapophyses disappear at the level of the 80^th vertebra approximately; haemal keel in following vertebrae well-developed, ridge-like; paracotylar foramina absent.

Figure 71.

Sanziniidae: Acrantophis dumerili (UMMZ 190701), trunk vertebrae.

Figure 72.

Sanziniidae: Acrantophis dumerili (UMMZ 190701), trunk, cloacal, and caudal vertebrae.

Figure 73.

Sanziniidae: Acrantophis dumerili (UMMZ 190725), trunk vertebra.

Trunk/caudal transition. The haemal keel in the last trunk vertebra is not larger than those observed in more anterior posterior trunk vertebrae. In the cloacal vertebrae, the keel is enlarged into a strongly built hypapophysis. In the entire caudal portion of the column, it is reduced to a bulb-like (or tubercle-like) haemal keel.

Number of vertebrae. Acrantophis dumerili UMMZ 190701): 270 (230+4+36, including a final fusion); Acrantophis dumerili (UF Herp 175572): 269 (232+4+33, including a final fusion).

Data from literature: Acrantophis dumerili: less than 300 (in this approximately 230 trunk) vertebrae (based on Hoffstetter 1960: fig. 2); Acrantophis madagascariensis (Duméril & Bibron, 1844): ?246±10 trunk and cloacal vertebrae plus 34+ caudal vertebrae (Alexander and Gans 1966).

Sanzinia Gray, 1849

Material examined

Sanzinia madagascariensis (Duméril & Bibron, 1844) (MNHN-ZA-AC-1900.0122A; NHMUK 1967.77; SMF PH 55; SMF PH 56; UMMZ 190752; UMMZ 131713).

Description (Figs 74-79)

Trunk vertebrae. Centrum much shorter than wide; cotyle and condyle slightly depressed; neural arch vaulted; posterior median notch of the neural arch deep; neural spine higher than long; prezygapophyseal accessory processes vestigial; hypapophyses disappearing posteriorly to the 60^th vertebra; haemal keel in more posterior vertebrae well-developed, ridge-like; paracotylar foramina absent in most vertebrae – in Sanzinia examined by Kluge (1991) only one out of 10 sampled vertebrae possessed unilaterally a tiny foramen.

Figure 74.

Sanziniidae: Sanzinia madagascariensis (UMMZ 131713), trunk vertebrae.

Figure 75.

Sanziniidae: Sanzinia madagascariensis (UMMZ 131713), posteriormost trunk, cloacal, and anterior caudal vertebrae.

Figure 76.

Sanziniidae: Sanzinia madagascariensis (UMMZ 131713), caudal vertebrae.

Figure 77.

Sanziniidae: Sanzinia madagascariensis (UMMZ 131713), caudal vertebrae.

Figure 78.

Sanziniidae: Sanzinia madagascariensis (NHMUK 1967.77), trunk vertebra.

Figure 79.

Sanziniidae: Sanzinia madagascariensis (UMMZ 190752), trunk vertebrae.

Hoffstetter (1961:157) remarked the constant presence of a distinct foramen situated next to the dorsolateral corner of each side of the zygantrum that he regarded as somehow distantly analogue of the parazygantral foramina of madtsoiids. We confirm the presence of these foramina in Sanzinia but we note that they are less prominent than those in certain specimens of Candoia (and certainly less prominent than those of madtsoiids; see also Discussion below). Furthermore, Smith (2013) highlighted that the shape of the subcentral structures of Sanzinia could be subjected to ontogenetic variation, with very small individuals possessing distinct hypapophyses (or hypapophyses-like structures) throughout the trunk vertebrae, medium-sized individuals possessing sharp, projecting keels on their mid-trunk vertebrae and blunt haemal keels on their posterior trunk vertebrae, while large individuals possessed acute haemal keels in their mid-trunk vertebrae.

Trunk/caudal transition. The last trunk vertebrae possess a short hypapophysis, diminished in size in the following first cloacal vertebrae and then reduced entirely in the remaining cloacal vertebrae and three anteriormost caudal vertebrae. The remaining caudal vertebrae are provided with a distinct haemal keel; some of these keels, however, are bifurcated distally into two short spurs and thus may be interpreted as (distinctly reduced) haemapophyses. These “quasi-haemapophyses” together with normally developed keels are distributed at random (without any clear constant pattern) along the caudal portion of the column. In one specimen (SMF PH 56), however, all caudal vertebrae except for the four anteriormost ones, possess tiny haemapophyses. Moreover, the middle / posterior caudal vertebra of ZFMK 70428 (an individual slightly larger than UMMZ 131713) possesses distinct haemapophyses (Martin Ivanov and Krister Smith, personal communications). Posteriormost caudal vertebrae are fused.

Smith (2013) highlighted that in a very small individual (CM 145342), delicate haemapophyses were present on all caudal vertebrae except the anteriormost approximately two ones and the same pattern more or less observed also in mid-sized individuals he studied. Therefore, this reduction of haemapophyses in larger ontogenetic stages and adults could be potentially subjected to some degree of ontogenetic (or generally intraspecific) variation.

Number of vertebrae (all for Sanzinia madagascariensis): SMF PH 55: 286 (231+4+51 [including a final fusion]); SMF PH 56: 266 (220+4+42 [including a final fusion]); UMMZ 131713: 256 (217+4+35).

Data from literature (all for Sanzinia madagascariensis): ~300 vertebrae in total, of which ~225 trunk (based on Hoffstetter 1960: fig. 3); 261 vertebrae in total, of which 192 trunk (Jourdran 1904); 208 trunk and cloacal vertebrae plus 42+ caudal vertebrae (Alexander and Gans 1966).

Boidae Gray, 1825 (sensu Pyron et al. 2014)

General information

The concept of Boidae was once much enlarged, encompassing not only all Booidea but also most pythonoids, tropidophiids, bolyeriids, as well as the extinct madtsoiids (e.g., Bonaparte 1839, 1845, 1852; Boulenger 1893; Schmidt 1923; Stull 1935; Hoffstetter 1939a, 1960, 1961; Loveridge 1942; Smith 1943; Angel 1950; Witte 1953; Dowling 1959; Kuhn 1961; Stimson 1969; Guibé 1970; Hoffstetter and Rage 1972; Rage 1974, 1984, 1987; McDowell 1975; Underwood 1976; Underwood and Stimson 1990; Szyndlar 1991a; Kluge 1993a; Szyndlar and Böhme 1996; Harvey et al. 2000; Ivanov et al. 2000). However, in recent taxonomic schemes, following the advance of phylogenetic analyses, the content of Boidae has dramatically decreased to encompass only the five extant genera Boa, Chilabothrus, Corallus, Epicrates, and Eunectes, all currently distributed in the Americas (see Pyron et al. 2014; Reynolds and Henderson 2018; Scanferla and Smith 2020b). That taxonomic approach almost corresponds with the concept of the subfamily Boinae in previous decades (e.g., Stull 1935; Stimson 1969; Rage 1984; Szyndlar 1991a; Ivanov et al. 2000; Szyndlar and Rage 2003). They have a rich fossil record, including also remains of all the extant genera (Pregill 1981; Rage 2001; Head et al. 2006, 2012; Albino and Carlini 2008; Hsiou and Albino 2009; Camolez and Zaher 2010; Sánchez-Villagra 2012; Albino and Brizuela 2014; Bochaton et al. 2015; Aranda et al. 2017; Mead and Steadman 2017; Onary et al. 2017, 2018; Bochaton and Bailon 2018; Onary and Hsiou 2018; Carrillo-Briceño et al. 2021).

Trunk vertebrae of Boidae closely resemble those of other booids (except for candoiids and ungaliophiids) as well as other constrictors and more particularly, Pythonidae. However, some (but not all) boas possess paracotylar foramina that are totally absent in pythons; this difference has been particularly applied as a potentially useful tool in palaeontological research (e.g., Rage 1984; Szyndlar 1991a; Szyndlar and Rage 2003).

Vertebrae of Boidae have regularly and extensively appeared in the literature, including some of the first studies of snake vertebral morphology (e.g., D’Alton 1836). As such, a large number of studies have so far presented figures of vertebrae of Boidae, including Grant (1841), Rochebrune (1881), Holman (1967), Gasc (1974), Lee and Scanlon (2002), Szyndlar and Rage (2003), Head et al. (2009, 2022), Albino (2011), Albino et al. (2018), Onary and Hsiou (2018), Palci et al. (2018), Georgalis and Scheyer (2019), Machado-Filho (2020), and Georgalis et al. (2021a), including even vertebrae of early ontogenetic stages, such as embryos (Chuliver et al. 2022). Among these, vertebrae from the cloacal and/or caudal series were presented by Szyndlar and Rage (2003), Machado-Filho (2020), and Alfonso-Rojas et al. (2023). Figures of the microanatomy and histology / transverse sections of boid vertebrae were presented by Buffrénil and Rage (1993) and Houssaye et al. (2010). Quantitative studies on the intracolumnar variability of boid vertebrae were also conducted by Hoffstetter (1960) and Gasc (1974).

Boa Linnaeus, 1758

Material examined

Boa constrictor Linnaeus, 1758 (HNHM 2004.77.1; ISEZ R/102; ISEZ R/104; ISEZ R/106; ISEZ R/110; ISEZ R/457; MGPT-MDHC 175 (juvenile); MGPT-MDHC 500; MNCN 15988; SMF PH 35; SMF PH 36; SMF PH 37; SMF PH 40; SMF PH 41; SMF PH 42; SMF PH 43; SMF PH 44; SMF PH 45 (juvenile); SMF PH 46; SMF PH 57; SMF PH 220).

Description (Figs 80-85)

Trunk vertebrae. Centrum much shorter than wide; cotyle and condyle orbicular; neural arch vaulted; posterior median notch of the neural arch deep; neural spine much higher than long in most trunk vertebrae; prezygapophyseal accessory processes short; hypapophyses disappearing between the 60^th and 70^th vertebrae; haemal keel in more posterior vertebrae well-developed, ridge-like; paracotylar foramina present.

Figure 80.

Boidae: Boa constrictor (ISEZ R/457), trunk vertebrae.

Figure 81.

Boidae: Boa constrictor (ISEZ R/457), trunk vertebrae.

Figure 82.

Boidae: Boa constrictor (ISEZ R/457), posteriormost trunk and cloacal vertebrae.

Figure 83.

Boidae: Boa constrictor (ISEZ R/457), cloacal and caudal vertebrae.

Figure 84.

Boidae: Boa constrictor (ISEZ R/457), caudal vertebrae.

Figure 85.

Boidae: Boa constrictor (ISEZ R/106), trunk vertebra.

Georgalis and Scheyer (2019) highlighted the substantial modification of the shape and thickness of the zygosphene during ontogeny in Boa.

Trunk/caudal transition. Distinct hypapophyses that appear in the two last trunk vertebrae (in SMF PH 40, these distinct hypapophyses appear even earlier in more preceding trunk vertebrae), diminish gradually in size in succeeding cloacal vertebrae; it is reduced to an indistinct haemal keel in the last (or two last) cloacal vertebrae and the anteriormost caudal vertebrae. Haemapophyses first appear on the third caudal vertebra; sometimes they are unpaired (unilateral), followed by normally developed (paired) structures on the following caudal vertebrae.

Number of vertebrae (all for Boa constrictor): SMF PH 44: 346 (271+4+71); SMF PH 37: 321 (254+5+62); SMF PH 57: 312 (246+4+62 [posteriormost caudal vertebrae fused]); MGPT-MDHC 500: 310 (255+4+51, including a final fusion); SMF PH 36: 309 (249+4+56); SMF PH 40: 309 (248+4+57); SMF PH 45 (juvenile): 305 (246+4+55); MGPT-MDHC 175 (juvenile): 298 (245+5+48); ISEZ R/457: 294+ (258+5+31+); SMF PH 35: 252 trunk vertebrae (cloacal and caudal vertebrae missing); SMF PH 46: 245 trunk vertebrae (cloacal and caudal vertebrae missing).

Data from the literature and unpublished data from personal communications: Boa constrictor: 302–310 vertebrae in total (Parmley and Reed 2003); Boa constrictor: 305 vertebrae in total, among which 60 cloacal and caudal vertebrae (Owen 1850, 1877); Boa constrictor: 228 trunk and cloacal vertebrae plus 56 caudal vertebrae (Jingsong Shi, unpublished data, personal communication to GLG); Boa constrictor: 250 trunk vertebrae plus 8 cloacal vertebrae (apparently erroneous) plus 44 caudal vertebrae (Rochebrune 1881); Boa constrictor: 226–248 trunk vertebrae plus 4–5 cloacal vertebrae plus 31–50 caudal vertebrae (Machado-Filho 2020); Boa constrictor: 246 trunk vertebrae plus 59 cloacal and caudal vertebrae (Nopcsa 1923); Boa constrictor: 237 trunk vertebrae plus 5 cloacal vertebrae plus 42 caudal vertebrae (Albino 2011); Boa imperator Daudin, 1803: 237–244 trunk vertebrae plus 4–5 cloacal vertebrae plus 47–60 caudal vertebrae (Machado-Filho 2020).

It should be noted that Boa constrictor has recently been recognized as a species complex, with other cryptic species recognized (Reynolds et al. 2014; Card et al. 2016; Reynolds and Henderson 2018); therefore, it is not certain whether all these dry skeletons (in particular specimens that were collected during the 18^th, 19^th, and/or early 20^th centuries) that were studied by us or were mentioned in older literature, belong indeed to Boa constrictor or some other species.

Chilabothrus Duméril & Bibron, 1844

Material examined

Chilabothrus angulifer (Bibron, 1840 in Ramón de la Sagra, 1838–1843) (MNHN-ZA-AC-1892.0089; SMF PH 61); Chilabothrus subflavus (Stejneger, 1901) (SMF PH 32).

Description

Trunk vertebrae. Centrum shorter than wide; cotyle and condyle orbicular; posterior median notch of the neural arch deep; neural spine as high as long in lateral view; in Chilabothrus angulifer, neural spine is very thick in dorsal view; prezygapophyses slightly dorsally inclined in anterior view; prezygapophyseal accessory processes vestigial; zygosphene with a prominent median lobe in dorsal view; in C. angulifer, zygosphene possesses a distinct median ridge in anterior view, while it is very thin in Chilabothrus subflavus; hypapophyses diminish strongly in size after V 20 and disappear at around V 40 in C. angulifer, while in C. subflavus hypapophyses are prominent at least until V 60 and start to disappear at around the level of V 70; haemal keel in more posterior vertebrae prominent in C. angulifer but rather indistinct in C. subflavus; paracotylar foramina absent.

Trunk/caudal transition (for Chilabothrus angulifer). A prominent haemal keel is present in posterior trunk vertebrae and it becomes a short hypapophysis in posteriormost trunk vertebrae. Cloacal and anteriormost caudal vertebrae too possess a short hypapophysis. In succeeding caudal vertebrae, this hypapophysis diminishes in size gradually and grooved keels reminiscent of tiny haemapophyses appear in the 5^th caudal vertebra. Normally paired haemapophyses first appear on the 6^th caudal vertebra.

Number of vertebrae. Chilabothrus angulifer (SMF PH 61): 308 (249+5+53); Chilabothrus subflavus (SMF PH 32): 329 (282+5+42).

Data from literature: Chilabothrus angulifer: 290 trunk and cloacal vertebrae plus ~79+2 caudal vertebrae (Alexander and Gans 1966); Chilabothrus angulifer: 284–288 trunk vertebrae plus 50–51 cloacal and caudal vertebrae (Holman 1967); Chilabothrus angulifer: 275–283 trunk vertebrae plus 4–5 cloacal vertebrae plus 44–54 caudal vertebrae (Machado-Filho 2020); Chilabothrus striatus (Fischer, 1856): 291–296 trunk and cloacal vertebrae plus 91–92 caudal vertebrae (Alexander and Gans 1966); Chilabothrus striatus: 280–290 trunk vertebrae plus 3–5 cloacal vertebrae plus 55–81 caudal vertebrae (Machado-Filho 2020); Chilabothrus striatus: 348–367 vertebrae in total (Parmley and Reed 2003).

Corallus Daudin, 1803

Material examined

Corallus caninus (Linnaeus, 1758) (PIMUZ A/III 1024; SMF PH 47; SMF PH 48; SMF PH 54; SMF PH 182); Corallus cropanii (Hoge, 1954) (AMNH 92997); Corallus hortulana (Linnaeus, 1758) (USNM 306071; UMMZ 190735).

Description (Figs 86-88)

Trunk vertebrae. Centrum nearly as long as wide; cotyle and condyle depressed (especially ventrally); neural arch vaulted; posterior median notch of the neural arch deep; neural spine as high as long; prezygapophyses nearly horizontal (each inclined from horizontal plane almost 0°) in anterior view (but slightly more inclined [~13–14°] from horizontal plane in Corallus cropanii; Fig. 86); prezygapophyseal accessory processes vestigial; zygosphene with a prominent median lobe in dorsal view and possessing a distinct median ridge in anterior view; hypapophyses disappearing between V 70 and V 80 (in Corallus hortulana); haemal keel in more posterior vertebrae indistinct; paracotylar foramina present in Corallus annulatus (Cope, 1875) and Corallus cropanii (see also Kluge 1991).

Figure 86.

Boidae: Corallus cropanii (AMNH 92997), trunk vertebrae.

Figure 87.

Boidae: Corallus hortulana (UMMZ 190735), trunk vertebra.

Figure 88.

Boidae: Corallus hortulana (USNM 306071), trunk vertebra.

Onary et al. (2018) also documented the presence of small, pit-shaped foramina in Corallus, which they interpereted as parazygantral foramina.

Trunk/caudal transition (for Corallus hortulana). The last trunk and cloacal vertebrae are provided with prominent haemal keels (rather than short hypapophyses). Paired haemapophyses first appear on the second caudal vertebra.

Number of vertebrae. Corallus caninus (SMF PH 47): 286 (204+4+78); Corallus caninus (SMF PH 48): 276: 204+3+69); Corallus caninus (SMF PH 54): 291 (208+3+80); Corallus caninus (SMF PH 182): 283 (204+2+77); Corallus hortulana (USNM 306071): 360 (259+?2+99) (some vertebrae in this specimen were displaced; the loss of a possible additional cloacal vertebra cannot be excluded).

Data from literature and unpublished data from personal communications: Corallus annulatus: 286 trunk vertebrae plus 4 cloacal vertebrae plus 103 caudal vertebrae (Machado-Filho 2020); Corallus annulatus: 268 trunk and cloacal vertebrae plus 86+ caudal vertebrae (Alexander and Gans 1966); Corallus batesii (Gray, 1860): 201–223 trunk vertebrae plus 3–4 cloacal vertebrae plus 59–72 caudal vertebrae (Machado-Filho 2020); Corallus batesii: 214 trunk vertebrae plus unknown number of cloacal and caudal vertebrae (Teixeira 2013); Corallus caninus: 209–223 trunk vertebrae plus 3–5 cloacal vertebrae plus 72–80 caudal vertebrae (Machado-Filho 2020); Corallus caninus: 203 trunk and cloacal vertebrae plus 73 caudal vertebrae (Alexander and Gans 1966); Corallus caninus: 202 trunk vertebrae plus unknown number of cloacal and caudal vertebrae (Tsuihiji et al. 2012); Corallus caninus: 265 trunk and cloacal vertebrae plus 81 caudal vertebrae (Jingsong Shi, unpublished data, personal communication to GLG); Corallus caninus: 197 trunk vertebrae plus 65 cloacal and caudal vertebrae (Holman 1967); Corallus cookii Gray, 1842: 259–270 trunk vertebrae plus 3 cloacal vertebrae plus 110–114 caudal vertebrae (Machado-Filho 2020); Corallus cookii: 268 trunk vertebrae plus 112 cloacal and caudal vertebrae (Nopcsa 1923); Corallus cropanii: 189 trunk vertebrae plus 3 cloacal vertebrae plus 60+ caudal vertebrae (Machado-Filho 2020); Corallus hortulana: 283 trunk vertebrae plus 3 cloacal vertebrae plus 107 caudal vertebrae (Machado-Filho 2020); Corallus hortulana: 280 trunk vertebrae plus more than 124 cloacal and caudal vertebrae (some caudal vertebrae were missing) (Nopcsa 1923); Corallus hortulana: 261 trunk and cloacal vertebrae plus 112 caudal vertebrae (Boa enydris of Alexander and Gans 1966); Corallus hortulana: 277–291 trunk vertebrae plus unknown number of cloacal and caudal vertebrae (Teixeira 2013); Corallus ruschenbergerii (Cope, 1875): 264–271 trunk vertebrae plus 3–4 cloacal vertebrae plus 110–144 caudal vertebrae (Machado-Filho 2020).

What is of interest is the remarkably high number of caudal vertebrae (reaching up to 144), apparently correlated with the arboreal lifestyle of this genus.

Epicrates Wagler, 1830

Material examined

Epicrates cenchria (Linnaeus, 1758) (ISEZ R/437; ISEZ R/458 [juvenile]; ISEZ R/459; SMF PH 25; SMF PH 26).

Description (Figs 89-91)

Trunk vertebrae. Centrum much shorter than wide; cotyle and condyle orbicular; neural arch moderately vaulted; posterior median notch of the neural arch deep (but not as deep as in Corallus or Sanziniidae); neural spine distinctly taller than long in lateral view and rather thick in dorsal view, occasionally with a distinct bifurcation in its anterior and/or posterior edges; prezygapophyseal accessory processes vestigial; hypapophyses disappearing between around the 50^th and 60^th vertebrae; haemal keel well developed, ridge-like; subcentral grooves deep; paracotylar foramina absent.

Figure 89.

Boidae: Epicrates cenchria (ISEZ R/437), trunk vertebrae.

Figure 90.

Boidae: Epicrates cenchria (ISEZ R/437), trunk and cloacal vertebrae.

Figure 91.

Boidae: Epicrates cenchria (ISEZ R/437), caudal vertebrae.

Trunk/caudal transition. The last trunk vertebrae possess a moderately developed hypapophysis; the cloacal vertebrae are provided with a prominent haemal keel produced caudally into a distinct spur. Paired haemapophyses first appear on the second caudal vertebra (two specimens examined) or the first caudal vertebra (one specimen examined), or the haemapophyses even appear on the last cloacal vertebra (two specimens examined).

Number of vertebrae (all for Epicrates cenchria): SMF PH 25: 310 (254+5+51); SMF PH 26: 295 (243+4+48); ISEZ R/437: 289 (235+4+50, including a final fusion); ISEZ R/458: 285 (235+3+47, including a final fusion); ISEZ R/459: 291 (234+4+53).

Data from literature: Epicrates alvarezi Abalos, Baez & Nader, 1964: 249 trunk vertebrae plus 5 cloacal vertebrae plus 49 caudal vertebrae (Machado-Filho 2020); Epicrates assissi Machado, 1944: 270 trunk vertebrae plus 5 cloacal vertebrae plus 54 caudal vertebrae (Machado-Filho 2020); Epicrates cenchria: 235–283 trunk vertebrae plus 4–5 cloacal vertebrae plus 41–60 caudal vertebrae (Machado-Filho 2020); Epicrates cenchria: 272 trunk vertebrae plus 6 cloacal vertebrae plus unknown number of caudal vertebrae (Gasc 1974); Epicrates cenchria: 265 trunk vertebrae plus 64 cloacal and caudal vertebrae (Polly et al. 2001); Epicrates cenchria: 261 trunk vertebrae plus unknown number of cloacal and caudal vertebrae (Teixeira 2013); Epicrates crassus Cope, 1862: 231–236 trunk vertebrae plus 4 cloacal vertebrae plus 40–49+ caudal vertebrae (Machado-Filho 2020); Epicrates crassus: 230 trunk vertebrae plus unknown number of cloacal and caudal vertebrae (Teixeira 2013); Epicrates maurus Gray, 1849: 235 trunk vertebrae plus 4 cloacal vertebrae plus 52 caudal vertebrae (Machado-Filho 2020).

Eunectes Wagler, 1830

Material examined

Eunectes murinus (Linnaeus, 1758) (ISEZ R/103; ISEZ R/265; ISEZ R/266; MNCN 15385; ZFMK 5179); Eunectes notaeus Cope, 1862 (SMF PH 60; SMF PH 70).

Description (Figs 92-95)

Trunk vertebrae. Vertebrae very large, massive, and robust; centrum much shorter than wide; cotyle and condyle slightly depressed; neural arch moderately vaulted; posterior median notch of the neural arch deep; neural spine much higher than long; prezygapophyseal accessory processes short; hypapophyses disappearing between V 60 and V 70; haemal keel in more posterior vertebrae well-developed, ridge-like, produced posteriorly; paracotylar foramina absent.

Figure 92.

Boidae: Eunectes murinus (ZFMK 5179), anterior trunk vertebrae.

Figure 93.

Boidae: Eunectes murinus (ZFMK 5179), trunk vertebrae.

Figure 94.

Boidae: Eunectes murinus (ZFMK 5179), posteriormost trunk, cloacal, and anteriormost caudal vertebrae.

Figure 95.

Boidae: Eunectes murinus (ZFMK 5179), caudal vertebrae.

Lee and Scanlon (2002) treated the foramina at the edges of the postzygapophyses in posterior view as parazygantral foramina. Such small foramina are also present in our here presented material, though they are smaller than the ones presented by Lee and Scanlon (2002: fig. 10K). See Discussion below for more details on this feature.

Trunk/caudal transition. The last trunk vertebrae bear a (plate-like) hypapophysis that is retained (spur-like) in all cloacal vertebrae and first caudal vertebra. Paired haemapophyses first appear on the second caudal vertebra.

Number of vertebrae. Eunectes murinus (ZFMK 5179): 313 (252+4+57) including a final fusion; Eunectes notaeus (SMF PH 60): 323 (271+5+47); Eunectes notaeus (SMF PH 70): 295 (235+3+57).

Data from literature: Eunectes deschauenseei Dunn & Conant, 1936: 238 trunk vertebrae plus 3 cloacal vertebrae plus 45 caudal vertebrae (Machado-Filho 2020); Eunectes murinus: 252 trunk vertebrae plus 59 cloacal and caudal vertebrae (Polly et al. 2001); Eunectes murinus: 249 trunk vertebrae plus unknown number of cloacal and caudal vertebrae (Tsuihiji et al. 2012); Eunectes murinus: 233–248 trunk vertebrae plus 3–4 cloacal vertebrae plus 68 caudal vertebrae (Machado-Filho 2020); Eunectes murinus: 247–259 trunk vertebrae plus 68–72 cloacal and caudal vertebrae (Nopcsa 1923); Eunectes notaeus: 235 trunk vertebrae plus 4 cloacal vertebrae plus 52 caudal vertebrae (Machado-Filho 2020); Eunectes notaeus: 238 trunk and cloacal vertebrae plus 65+ caudal vertebrae (Alexander and Gans 1966).

Candoiidae Pyron, Reynolds & Burbrink, 2014

General information

This group of island boas comprises a single genus, Candoia, with five known species, distributed across several islands of the Pacific Ocean (Pyron et al. 2014). No fossil record is as yet known; only a few archaeozoological remains from New Caledonia exist (Daza et al. 2021).

Vertebral morphology of Candoiidae is generally similar to that of other booids, but they can be readily differentiated from other booids by the continuous presence of hypapophyses throughout the trunk (for more details, see Description and figures of Candoia below).

Previous figures of vertebrae of extant Candoiidae have been so far presented only by Gasc (1974), Underwood (1976), and Frýdlová et al. (2023). Among these, vertebrae from the cloacal and/or caudal series of candoiids have never been figured so far, with the exception of a single μCT image of an articulated skeleton in Frýdlová et al. (2023). Besides this previous figuring, several authors observed the presence of hypapophyses throughout the trunk portion of the column in Candoia (Hoffstetter and Gasc 1969; Malnate 1972; McDowell 1975; Rage 1984; Szyndlar and Böhme 1996; Szyndlar and Rage 2003).

Candoia Gray, 1842

Material examined

Candoia aspera (Günther, 1877) (UMMZ 190729); Candoia bibroni (Duméril & Bibron, 1844) (UMMZ 190730); Candoia carinata (Schneider, 1801) (NHMUK 83.6.28.51; UMMZ 190731; ZFMK uncat.).

Description (Figs 96-102)

Trunk vertebrae. Centrum shorter than wide or as short as wide; cotyle and condyle orbicular to slightly depressed; neural arch moderately vaulted; posterior median notch of the neural arch deep; neural spine height variable among different species: either of medium height (Candoia bibroni, one vertebra studied), or distinctly higher than long (Candoia carinata) or more than twice as high as long (Candoia aspera, two vertebrae studied) – in a very old individual of Candoia carinata (ZFMK uncat.; Fig. 102), the neural spine is very low; prezygapophyses usually not much inclined, with the exception of a vertebra of Candoia aspera (UMMZ 190729; Fig. 96), where these are strongly dorsally inclined; prezygapophyseal accessory processes vestigial; hypapophyses present throughout the trunk portion of the column, of varying shape, usually plate-like (two of three specimens of Candoia carinata; C. aspera); paracotylar foramina present; parazygantral foramina present in two specimens of Candoia carinata (UMMZ 190731; ZFMK uncat.) but absent in another specimen of the same species (NHMUK 83.6.28.51) and in Candoia aspera and C. bibroni.

Figure 96.

Candoiidae: Candoia aspera (UMMZ 190729), trunk vertebrae.

Figure 97.

Candoiidae: Candoia bibroni (UMMZ 190730), trunk vertebra.

Figure 98.

Candoiidae: Candoia carinata (NHMUK 83.6.28.51), trunk vertebrae.

Figure 99.

Candoiidae: Candoia carinata (NHMUK 83.6.28.51), posteriormost trunk and cloacal vertebrae.

Figure 100.

Candoiidae: Candoia carinata (NHMUK 83.6.28.51), caudal vertebrae.

Figure 101.

Candoiidae: Candoia carinata (UMMZ 190731), trunk vertebra.

Figure 102.

Candoiidae: Candoia carinata (ZFMK uncat.), trunk vertebrae (of a very old individual).

Trunk/caudal transition (for Candoia carinata). The plate-like hypapophysis is present in anterior cloacal vertebra. In the last cloacal vertebra, it changes into a haemal keel, gradually becoming larger and wider in the succeeding caudal vertebrae. Paired haemapophyses appear first on the 5^th caudal vertebra.

Number of vertebrae. Candoia carinata (NHMUK 83.6.28.51): 213+ (172+3+38+).

Data from literature: Candoia aspera: 144 trunk and cloacal vertebrae plus 21+ caudal vertebrae (Alexander and Gans 1966); Candoia carinata: 190 trunk vertebrae plus 4 cloacal vertebrae plus unknown number of caudal vertebrae (Gasc 1974); Candoia paulsoni (Stull, 1956): 190 trunk vertebrae plus unknown number of cloacal and caudal vertebrae (Tsuihiji et al. 2012).

Erycidae Bonaparte, 1831 (sensu Pyron et al. 2014)

General information

Fitzinger (1843) called them Gongylophes. Commonly known as the Sand Boas, the Old World genus Eryx (including also Gongylophis Wagler, 1830, as a junior synonym in current taxonomies) along with the New World genera Charina Gray, 1849, and Lichanura Cope, 1861, were until recently conceived to represent a distinct subfamily of boids, termed Erycinae (e.g., Hoffstetter 1955; Romer 1956; Hoffstetter and Rage 1972; Underwood 1976; Dowling and Duellman 1978; Rage 1984, 1987; McDowell 1987; Szyndlar 1991a, 1994; Kluge 1993b; Mead and Schubert 2013). However, an array of phylogenies, based primarily on molecular data or combined molecular+morphological evidence, have challenged this topology, and the traditional concept of erycines has been recently considered to be paraphyletic, i.e., with the Old World erycines pertaining to a different group (Erycidae) than the New World ones (Charinaidae) (Wilcox et al. 2002; Lawson et al. 2004; Pyron et al. 2013; Reynolds et al. 2014; Hsiang et al. 2015; Burbrink et al. 2020; Scanferla and Smith 2020b). Nevertheless, certain morphology-based or molecular+morphology combined phylogenies contrastingly still recover Erycidae and Charinaidae together as a monophyletic clade (e.g., Kluge 1993b; Gauthier et al. 2012; Scanferla et al. 2016; Smith and Scanferla 2021). It is of course, beyond the scope of this paper to assess the relationships of Erycidae with Charinaidae, but due to the lack of consensus, we treat them independently. However, we should note that vertebral morphology (i.e., the peculiar highly complex osteology of the caudal vertebrae, which is shared among Erycidae and Charinaidae and is totally absent in Ungaliophiidae) supports affinities between erycids and charinaids.

The current concept of Erycidae is confined to the genus Eryx, which comprises 13 species distributed in southeastern Europe, southwestern Asia, and Africa (Reynolds and Henderson 2018; Boundy 2021). Erycidae have a moderately rich fossil record, with remains of the extant genus Eryx already identified since the Miocene to the Quaternary of Europe, Asia, and northern Africa (e.g., Hoffstetter and Rage 1972; Rage 1976; Bailon 1989, 1991; Szyndlar 1991a; Szyndlar and Zerova 1992; Szyndlar and Schleich 1994; Ivanov 2000; Malakhov 2005; Szyndlar and Alférez 2005; Nadachowski et al. 2006; Delfino et al. 2011; Maul et al. 2015; Blain 2016; Smith et al. 2016; Villa et al. 2021; Serdyuk et al. 2023; Shi et al. 2023a). Apart from fossils of Eryx, as it is now known that the European Paleogene hosted also charinaids (i.e., Rageryx Smith & Scanferla, 2021), several other forms from the Paleogene and early Neogene of Europe that possess a complex caudal vertebral morphology (e.g., genus Cadurceryx Hoffstetter & Rage, 1972), cannot be confidently identified as Erycidae or Charinaidae, and are tentatively referred as “erycines” (e.g., Georgalis and Scheyer 2021; Smith and Georgalis 2022).

Trunk vertebrae of erycids generally resemble those of other booids (except for candoiids and ungaliophiids) although certain features, such as the (usual) absence of paracotylar foramina distinguish them from others (booids and [to a lesser degree] sanziniids and the charinaid Lichanura). Nevertheless, it is the highly complex morphology of the caudal vertebrae that characterizes erycids, a feature shared only with charinaids (for more details, see Description and figures of Eryx below). Notably also, erycids are the only snakes, in which (in certain species) osteoderms are present (Frýdlová et al. 2023). These osteoderms occur around the cloaca and the tail and their unique presence in erycids, together with their highly complex caudal vertebral morphology, have been suggested to be associated with their fossorial locomotion (Frýdlová et al. 2023).

Previous figures of vertebrae of extant Erycidae were so far presented by Owen (1850), Sood (1941, 1948), Bogert (1968a), Hoffstetter and Gasc (1969), Hoffstetter and Rage (1972), Gasc (1974), Tokar (1989), Szyndlar (1994), Szyndlar and Schleich (1994), Venczel (2000), Georgalis and Scheyer (2019), Frýdlová et al. (2023), and Shi et al. (2023a). Among these, vertebrae from the cloacal and/or caudal series were presented by Sood (1941), Bogert (1968a), Hoffstetter and Gasc (1969), Hoffstetter and Rage (1972), Tokar (1989), Szyndlar (1994), Szyndlar and Schleich (1994), Venczel (2000), and Frýdlová et al. (2023). Figures of the microanatomy and histology / transverse sections of erycid vertebrae were presented by Sood (1948) and Houssaye et al. (2013). Quantitative studies on the intracolumnar variability of erycid vertebrae were conducted by Gasc (1974) and Head (2021). Besides, an extensive analysis of the mode of articulation between neighbouring caudal vertebrae of erycids was provided by Szyndlar (1994).

Eryx Daudin, 1803

Material examined

Eryx colubrinus (Linnaeus, 1758) (ISEZ R/370; MGPT-MDHC 172; SMF PH 24; SMF PH 203 [juvenile]; SMF PH 204 [juvenile]; UF Herp 151340 [Morphosource.org: Media 000476408, ark:/87602/m4/476408]; UMMZ 190384; UMMZ 190339; ZFMK 50246); Eryx conicus (Schneider, 1801) (ISEZ R/360; NHMUK 1930.5.8.13; NHMUK 1964.1224; MNHN-ZA-AC-1907.0319; SMF PH 18; UMMZ 18380; UMMZ 190949; UMMZ 128037); Eryx elegans (Gray, 1849) (AMNH R143761 [formerly SIZNASU Oph. 1650/4151]); Eryx jaculus (Linnaeus, 1758) (ISEZ R/15; ISEZ R/250; ISEZ R/264 [juvenile]; ISEZ R/321; NHMW 21520; NHMUK 1920.1.20.1526); Eryx jayakari Boulenger, 1888 (NHMUK 1909.10.15.8); Eryx johnii (Russell, 1802) (ISEZ R/330; NHMUK 1930.5.8.26; NHMUK 1930.5.8.27; NHMUK 1930.5.8.28; NHMUK 1930.5.8.30; MNHN-AC-1912.0418; MNHN-ZA-AC-1872.0059; SMF PH 20); Eryx miliaris (Pallas, 1773) (SIZNASU uncat.; Tokar Coll. 1117; Tokar Coll. 1119; UMMZ 190697); Eryx muelleri (Boulenger, 1892) (CAS Herp 136230); Eryx somalicus Scortecci, 1939 (MSNS 5250); Eryx tataricus (Lichtenstein in Eversmann, 1823) (ISEZ R/482; SIZNASU uncat.; Tokar Coll. 1116).

Description (Figs 103-108)

Trunk vertebrae. Centrum wider than long; cotyle and condyle orbicular; neural arch depressed; posterior median notch of the neural arch deep; neural spine low or of medium height (the tallest neural spine is found in Eryx johnii, according to Underwood [1976]); prezygapophyses much dorsally inclined in anterior view, with prezygapophyseal articular facets situated above the base of the neural canal; prezygapophyseal accessory processes short except for Eryx jayakari in which they are long; hypapophyses disappearing usually at the level of ca. 50^th vertebra; haemal keel in Eryx conicus and Eryx colubrinus broad and well developed, in other species weakly developed or absent; paracotylar foramina absent.

Figure 103.

Erycidae: Eryx jaculus (NHMUK 1920.1.20.1526), trunk vertebrae.

Figure 104.

Erycidae: Eryx jaculus (NHMUK 1920.1.20.1526), posteriormost trunk, cloacal, and anteriormost caudal vertebrae.

Figure 105.

Erycidae: Eryx jaculus (NHMUK 1920.1.20.1526), anterior caudal vertebrae.

Figure 106.

Erycidae: Eryx jaculus (NHMUK 1920.1.20.1526), caudal vertebrae. Abbreviations: aepl anterior extension of pleurapophysis; pepl posterior extension of pleurapophysis; pepr posterior extension of prezygapophysis; pow postzygapophyseal wing; pt pterapophysis.

Figure 107.

Erycidae: Eryx jaculus (NHMUK 1920.1.20.1526), posterior caudal vertebrae.

Figure 108.

Erycidae: Eryx jaculus (NHMUK 1920.1.20.1526), whole caudal series.

Trunk/caudal transition. Last trunk and anterior cloacal vertebrae provided either with hypapophysis (Eryx colubrinus) or either with no distinct subcentral structures. Haemapophyses appear on the 1^st to 5^th caudal vertebrae. Posterior caudal vertebrae display highly complex morphology: they are provided with additional apophyses unknown in other snakes, serving for articulation. The most complex morphology is observed in Eryx miliaris and the simplest (in fact, extremely simple as for Eryx standards) is in Eryx elegans. In Eryx johnii, zygosphenes and zygantra are absent from the posterior caudal vertebrae. A detailed description of the caudal osteology of Eryx will be presented elsewhere.

Number of vertebrae. Eryx colubrinus (UF Herp 151340): 213 (185+3+25, including a final fusion); Eryx colubrinus (SMF PH 24): 212 (178+3+31); Eryx colubrinus (ZFMK 50246): 206+ (184+4+18+ [posteriormost caudal vertebrae missing]); Eryx colubrinus (UMMZ 190339): 204 (174+3+27, including a final fusion); Eryx colubrinus (MGPT-MDHC 172): 202 (171+3+28, including a final fusion); Eryx colubrinus (ISEZ R/370): 195 (170+4+21, including a final fusion); Eryx conicus (UMMZ 190949): 204 (176+3+25, including a final fusion); Eryx conicus (UMMZ 128037): 198 (171+4+23, including a final fusion); Eryx conicus (SMF PH 18): 191 (169+4+18 [plus a final fusion]); Eryx conicus (NHMUK 1930.5.8.13): 20 caudal vertebrae, including a final fusion (trunk and cloacal vertebrae missing); Eryx elegans (AMNH R143761): 207 (177+3+27, including a final fusion); Eryx jaculus (NHMUK 1920.1.20.1526): 193 (160+4+29, including a final fusion); Eryx jaculus (ISEZ R/15): ?3 cloacal vertebrae plus 25 caudal vertebrae, including a final fusion (trunk vertebrae missing); Eryx jayakari (NHMUK 1909.10.15.8): 3 cloacal vertebrae plus 25+ caudal vertebrae, including a final fusion (most trunk and some posteriormost caudal vertebrae missing); Eryx johnii (SMF PH 20): 246 (217+3+26, including a final fusion); Eryx johnii (ISEZ R/330): 240 (219+5+16, including a final fusion); Eryx johnii (NHMUK 1930.5.8.30): 4 cloacal vertebrae plus 34 caudal vertebrae, including a final fusion (number of trunk vertebrae unknown); Eryx johnii (MNHN-ZA-AC-1872.0059): 3 cloacal vertebrae plus 21 caudal vertebrae, including a final fusion (number of trunk vertebrae unknown); Eryx miliaris (UMMZ 190697): 198 (177+4+17, including a final fusion); Eryx miliaris (SIZNASU uncat.): 3 cloacal vertebrae plus 25 caudal vertebrae, including a final fusion (trunk vertebrae missing); Eryx miliaris (Tokar Coll. 1117): 2 cloacal vertebrae plus 25 caudal vertebrae, including a final fusion (trunk vertebrae missing); Eryx miliaris (Tokar Coll. 1119): 3 cloacal vertebrae plus 21 caudal vertebrae, including a final fusion (trunk vertebrae missing); Eryx muelleri (CAS Herp 136230): 210 (183+4+23, including a final fusion); Eryx somalicus (MSNS 5250): 3 cloacal vertebrae plus 18+ caudal vertebrae (number of trunk vertebrae unknown and posteriormost caudal vertebrae missing); Eryx tataricus (ISEZ R/482): ~191 (~168+4+19+ [posteriormost caudal vertebrae missing]); Eryx tataricus (Tokar Coll. 1116): 28+ caudal vertebrae (trunk, cloacal, and posteriormost caudal vertebrae missing).

Data from literature and unpublished data from personal communications: Eryx colubrinus: 187–190 trunk and cloacal vertebrae plus 28–29 caudal vertebrae (Alexander and Gans 1966); Eryx conicus: 190 trunk and cloacal vertebrae plus 18+ caudal vertebrae (Alexander and Gans 1966); Eryx jaculus: 174–185 trunk and cloacal vertebrae plus 19 caudal vertebrae (Alexander and Gans 1966); Eryx jaculus: 175–185 trunk vertebrae plus 27–33 cloacal and caudal vertebrae (Nopcsa 1923); Eryx jaculus: 193 trunk vertebrae plus 9 cloacal vertebrae (apparently erroneous) plus 20 caudal vertebrae (Rochebrune 1881): Eryx jaculus: 210 vertebrae in total (NHMC80.3.114.20; Petros Lymberakis, personal communication to GLG); Eryx johnii: 208 trunk vertebrae plus 3 cloacal vertebrae plus unknown number of caudal vertebrae (Gasc 1974); Eryx johnii: 4 cloacal vertebrae and 31 caudal vertebrae (Sood 1941); Eryx miliaris: 210 trunk and cloacal vertebrae plus 26 caudal vertebrae (IVPP OV 2728; Jingsong Shi, unpublished data, personal communication to GLG).

An interesting feature of Eryx is a very low variation in the total number of vertebrae within the genus.

Charinaidae Gray, 1849 (sensu Burbrink et al. 2020)

General information

Charinaidae currently comprise two genera, Charina and Lichanura, with four species, distributed in North and northern Central America (Boundy 2021). It is recovered as the sister group of Ungaliophiidae in most recent phylogenetic analyses (e.g., Pyron et al. 2013; Reynolds et al. 2014; Zheng and Wiens 2016; Burbrink et al. 2020; Zaher et al. 2023). This close, sister group relationship between charinaids and ungaliophiids inferred by molecular data has prompted their treatment in many recent phylogenies and taxonomic schemes as “subfamilies” (Charinainae and Ungaliophiinae respectively) of an expanded concept of Charinaidae (e.g., Pyron et al. 2013, 2014; Reynolds and Henderson 2018; Georgalis and Smith 2020; Boundy 2021), yet other works treat them both at the family level (e.g., Wilcox et al. 2002; Wallach et al. 2014; Burbrink et al. 2020; Zaher et al. 2023). We here follow the latter opinion and treat Charinaidae and Ungaliophiidae at the family level, taking into consideration the above mentioned fact that certain morphology-based or molecular+morphology combined phylogenies recover Erycidae and Charinaidae together as a clade (e.g., Kluge 1993b; Gauthier et al. 2012; Scanferla et al. 2016; Smith and Scanferla 2021), and most importantly the shared complex caudal vertebral morphology of charinaids and erycids, which is a key anatomical feature (e.g., Szyndlar 1994; Szyndlar and Schleich 1994; Smith 2013; Smith and Scanferla 2021).

The fossil record attests a higher diversity of charinaids in the past, including also extinct species of the extant genera (see Holman 2000; Wallach et al. 2014; Boundy 2021), as well as a significantly wider distribution, encompassing also the Eocene of Europe, where the genus Rageryx was confidently referred to that lineage (Smith and Scanferla 2021).

Note that the original name of the family, Charinidae, recently had its spelling emended to Charinaidae by a formal decision of ICZN (2020), in order to avoid homonymy with its junior synonym of the arachnid family Charinidae Quintero, 1986, which had, however, a widespread usage in arthropod literature. An alternative name, Lichanuridae, has also been applied to this family in the 19^th century (Cope 1868b).

Vertebral morphology of Charinaidae is closely similar to that of Erycidae, with the two groups sharing also the characteristic complex caudal vertebrae, though still differences occur (for more details see Description and Figures of Charina and Lichanura below). Parham et al. (2012) proposed that the presence of a laterally swollen, trilobate neural spine combined with incipient pterapophyses on the dorsolateral margin of the posterior neural arch in caudal vertebrae is unique in Charina and Lichanura among extant snakes and can be used as the primary diagnostic feature of Charinaidae.

Previous figures of vertebrae of extant Charinaidae have been so far presented by Holman (1967), Bogert (1968a), Hoffstetter and Rage (1972), Ruben (1977), Szyndlar (1987, 1994), Gauthier et al. (2012), Smith and Scanferla (2021), and Frýdlová et al. (2023). Among these, vertebrae from the cloacal and/or caudal series have been presented by Holman (1967), Bogert (1968a), Hoffstetter and Rage (1972), Szyndlar (1987, 1994), Gauthier et al. (2012), Smith and Scanferla (2021), and Frýdlová et al. (2023). Quantitative study on intracolumnar variation of charinaid vertebrae was conducted by Head (2021). Besides, an extensive analysis of the mode of articulation between neighbouring caudal vertebrae of charinaids was provided by Szyndlar (1994). In addition, Hoffstetter and Rage (1972) and Kluge (1991) noted the occurrence of paracotylar foramina in Lichanura (but absent in Charina).

Charina Gray, 1849

Material examined

Charina bottae (Blainville, 1835) (MNHN-RA-0730 [part of the skeleton of the holotype]; NHMUK 1969.2948, UMMZ 240637 [Morphosource.org: Media 000068484, ark:/87602/m4/M68484]).

Description (Figs 109-112)

Trunk vertebrae. Centrum longer than wide or occasionally as wide as long; cotyle and condyle slightly depressed; neural arch depressed; posterior median notch of the neural arch deep; neural spine low; prezygapophyseal accessory processes short to vestigial; hypapophyses disappearing at the level of ca. 50^th vertebra; haemal keel broad, weakly developed; paracotylar foramina absent.

Figure 109.

Charinaidae: Charina bottae (NHMUK 1969.2948), trunk vertebrae.

Figure 110.

Charinaidae: Charina bottae (NHMUK 1969.2948), posteriormost trunk, cloacal, and caudal vertebrae.

Figure 111.

Charinaidae: Charina bottae (NHMUK 1969.2948), caudal vertebrae.

Figure 112.

Charinaidae: Charina bottae (NHMUK 1969.2948), posteriormost caudal vertebrae.

Trunk/caudal transition. The last trunk and cloacal vertebrae are provided with a weakly developed haemal keel; no subcentral structures are present in the anteriormost caudal vertebrae. The cloacal vertebrae are abruptly shortened compared to preceding trunk vertebrae and succeeding caudal vertebrae. Paired haemapophyses appear in the 3^rd caudal vertebra. Posterior caudal vertebrae, provided with enlarged neural spine and distinct pterapophyses, are fused into a compact structure. A detailed description of the caudal osteology of Charina will be presented elsewhere.

Number of vertebrae. Charina bottae (UMMZ 240637): 256 (212+6+>38) (about 10 posteriormost caudal vertebrae are fused; precise number of caudal vertebrae cannot be estimated); Charina bottae (NHMUK 1969.2948): 241 (200+4+>37) (about 10 posteriormost caudal vertebrae are fused; precise number of caudal vertebrae cannot be estimated); Charina bottae (MNHN-RA 0730 [part of the skeleton of the holotype]): ~34 caudal vertebrae, including a final fusion (number of trunk and cloacal vertebrae unknown).

Data from literature (all for Charina bottae): 215 trunk and cloacal vertebrae plus 44 caudal vertebrae (Alexander and Gans 1966); 209 trunk vertebrae plus 38 cloacal and caudal vertebrae (Holman 1967).

Lichanura Cope, 1861

Material examined

Lichanura orcutti Stejneger, 1889 (CAS Herp 200860); Lichanura trivirgata Cope, 1861 (MNHN-AC uncat.; SMF PH 21; UMMZ 190748).

Description (Figs 113-117)

Trunk vertebrae. Centrum shorter than wide; cotyle and condyle orbicular; neural arch depressed; posterior median notch of the neural arch deep; neural spine low; prezygapophyseal accessory processes short; hypapophyses disappearing at the level of ca. 50^th vertebra; haemal keel weakly developed to absent; paracotylar foramina present in a minority of trunk vertebrae.

Figure 113.

Charinaidae: Lichanura orcutti (CAS Herp 200860), trunk and cloacal vertebrae.

Figure 114.

Charinaidae: Lichanura orcutti (CAS Herp 200860), cloacal and caudal vertebrae.

Figure 115.

Charinaidae: Lichanura orcutti (CAS Herp 200860), caudal vertebrae.

Figure 116.

Charinaidae: Lichanura orcutti (CAS Herp 200860), posterior caudal vertebrae.

Figure 117.

Charinaidae: Lichanura orcutti (CAS Herp 200860), whole caudal series.

Trunk/caudal transition. A short hypapophysis is present in the last trunk vertebrae; it disappears in cloacal vertebrae. No subcentral structure occurs in the first caudal vertebrae. Paired haemapophyses appear in the second caudal vertebra and gradually become longer in succeeding vertebrae. Only rudimentary additional processes are present in caudal vertebrae (unlike Charina and Eryx), particularly in anterior caudal vertebrae. Posterior caudal vertebrae are provided with an enlarged bifurcated neural spine and minute pterapophyses and they lack zygosphene-zygantrum articulations. A detailed description of the caudal osteology of Lichanura will be presented elsewhere.

Number of vertebrae : Lichanura orcutti (CAS Herp 200860): 294 (239+2+53), including a final fusion of about four vertebrae; Lichanura trivirgata (SMF PH 21): 285 (236+3+46); Lichanura trivirgata (MNHN-AC uncat.): 28+ caudal vertebrae (number of trunk and cloacal vertebrae unknown and posteriormost caudal vertebrae missing).

Data from literature (all for Lichanura trivirgata): 235±2 trunk and cloacal vertebrae plus 52+ caudal vertebrae (Lichanura roseofusca of Alexander and Gans 1966); 254 trunk vertebrae plus 44 cloacal and caudal vertebrae (Lichanura roseofusca of Holman 1967).

Ungaliophiidae McDowell, 1987 (sensu Burbrink et al. 2020)

General information

Long considered as a subfamily of Tropidophiidae (Ungaliopheinae of McDowell [1987] – spelling formally emended to Ungaliophiinae by ICZN [2020]), ungaliophiids were recently demonstrated as being distinct booids, on the basis of molecular data (Slowinski and Lawson 2002; Wilcox et al. 2002; Lawson et al. 2004; Wiens et al. 2008; Pyron et al. 2013; Burbrink et al. 2020), with this view supported also by their distinctive skulls (Bogert 1968a), vertebrae (Bogert 1968a; Smith 2013), and external morphology and myology (Zaher 1994) (see entry of Tropidophiidae above). Therefore, according to recent taxonomic schemes, Ungaliophiidae form the sister group of Charinaidae (Pyron et al. 2014; Georgalis and Smith 2020; Scanferla and Smith 2020b). More particularly, in such taxonomic schemes and phylogenies, they have been considered either as a subfamily (i.e., Ungaliophiinae) of (an expanded) Charinaidae (e.g., Pyron et al. 2014; Georgalis and Smith 2020) or either as their own family, Ungaliophiidae (Zaher 1994; Wilcox et al. 2002; Burbrink et al. 2020; Zaher et al. 2023). We select here the latter nomenclature, especially because caudal vertebral morphology of charinaids is strikingly most similar to erycids and not to ungaliophiids (see also the entry of Charinaidae above).

Ungaliophiidae currently comprise two genera, Exiliboa and Ungaliophis, with only three species in total, inhabiting continental Central and northern South America (Wallach et al. 2014; Boundy 2021). The fossil record though attests to a much larger past distribution including the Eocene of North America (Smith 2013) and Europe (Scanferla et al. 2016; Scanferla and Smith 2020b).

Vertebral morphology of Ungaliophiidae possesses striking differences compared to that of other booids and constrictors in general. More particularly, their trunk vertebrae are characterized by a distinctive elongation (with the CL/NAW ratio ≥1.1) and light construction, while their caudal vertebrae are characterized by the presence of a haemal keel (instead of haemapophyses) throughout the caudal series which only disappears near the tip of tail. Indeed, Smith (2013) has highlighted these two diagnostic features of ungaliophiids as synapomorphies within constrictors.

Ungaliophiid vertebrae have only been rarely figured. In fact, previous figures of vertebrae of extant Ungaliophiidae have been so far only presented by Bogert (1968a, 1968b) and Smith (2013). Among these, vertebrae from the cloacal and/or caudal series have been presented by Smith (2013). Besides this figuring, Szyndlar and Böhme (1996), Szyndlar and Rage (2003), Szyndlar et al. (2008), and Smith (2013) emphasized considerably on the pattern of subcentral structures in the cloacal and caudal series of ungaliophiids.

Exiliboa Bogert, 1968

Material examined

Exiliboa placata Bogert, 1968 (MVZ Herps 137126 [Morphosource.org: Media 000076130, ark:/87602/m4/M76130]; UTACV R 37871).

Description (Figs 118-123)

Trunk vertebrae. Centrum longer than wide; cotyle and condyle slightly depressed; neural arch slightly depressed; posterior median notch of the neural arch deep; neural spine dorsoventrally high with a distinct thickening and lateral widening of its dorsal margin, crossing around two thirds of the midline of the neural arch; cotyle and condyle orbicular; prezygapophyseal accessory processes absent to vestigial; hypapophyses present on anterior trunk vertebrae, being sigmoid until around V 25 and subsequently plate-like until around V 40, and gradually diminishing in size to be replaced by a prominent haemal keel in mid-trunk and posterior trunk vertebrae; paracotylar foramina absent.

Figure 118.

Ungaliophiidae: Exiliboa placata (UTACV R 37871), trunk vertebrae.

Figure 119.

Ungaliophiidae: Exiliboa placata (UTACV R 37871), trunk vertebrae.

Figure 120.

Ungaliophiidae: Exiliboa placata (UTACV R 37871), posteriormost trunk and cloacal vertebrae.

Figure 121.

Ungaliophiidae: Exiliboa placata (UTACV R 37871), cloacal and anterior caudal vertebrae.

Figure 122.

Ungaliophiidae: Exiliboa placata (UTACV R 37871), anterior caudal vertebrae.

Figure 123.

Ungaliophiidae: Exiliboa placata (UTACV R 37871), caudal vertebrae.

The mid-trunk vertebra of the same species, illustrated by Bogert (1968a: fig. 8) closely matches the same morphology. Note that Bogert (1968a) also claimed that hypapophyses were restricted to only the first 20 anterior trunk vertebrae, however, this was apparently because he preferred to use the term “haemal keel” for smaller, blade-like hypapophyses (Bogert 1968a).

Trunk/caudal transition. A blade-like and thick hypapophysis is present in the last trunk and in cloacal vertebrae. In caudal vertebrae, this is replaced by a distinct, moderately developed, and thick haemal keel; the keel is still present in the tip of the tail; pleurapophyses long, robust, and laterally directed throughout the caudal series; the posteriormost (approximately) two caudal vertebrae are fused.

Smith (2013) highlighted that the long, robust and laterally directed pleurapophyses throughout the tail represent an autapomorphy for Exiliboa. We figure this feature here for this taxon – we also note that this prominence and lateral direction is also the case for Ungaliophis though not to the same extent.

Number of vertebrae (all for Exiliboa placata). UTACV R 37871: 196 (159+4+33, including a final fusion); MVZ Herps 137126: 195 (165+4+26, including a final fusion).

Data from literature and unpublished data from personal communications (all for Exiliboa placata): 166 trunk vertebrae plus 3 cloacal vertebrae plus 28 caudal vertebrae (the posteriormost 4 or 5 are partially fused) (Bogert 1968a); 165 trunk vertebrae plus 27 cloacal and caudal vertebrae plus a final fusion (NMNH 209414B; Krister Smith, unpublished data, personal communication to GLG).

Ungaliophis Müller, 1880

Material examined

Ungaliophis continentalis Müller, 1880 (UMMZ 190698).

Description (Figs 124-127)

Trunk vertebrae. Centrum as long as wide (but probably due to the juvenile / subadult ontogenetic stage of the individual); cotyle and condyle slightly depressed; neural arch slightly depressed; posterior median notch of the neural arch deep; neural spine of medium height, located at the posterior half of the neural arch; prezygapophyseal accessory processes vestigial to short; hypapophyses disappearing posteriorly to V 50; haemal keel indistinct (but probably due to the juvenile stage of the specimen); paracotylar foramina absent.

Figure 124.

Ungaliophiidae: Ungaliophis continentalis (UMMZ 190698), trunk vertebrae.

Figure 125.

Ungaliophiidae: Ungaliophis continentalis (UMMZ 190698), trunk and cloacal vertebrae.

Figure 126.

Ungaliophiidae: Ungaliophis continentalis (UMMZ 190698), cloacal and anterior caudal vertebrae.

Figure 127.

Ungaliophiidae: Ungaliophis continentalis (UMMZ 190698), caudal vertebrae.

Note that the skeleton described above belonged (as indicated among other features by its very small absolute dimensions and relatively broad neural canal) to a subadult specimen and this is the reason of the relative shortness of its vertebrae. Nevertheless, the few published figures of Ungaliophis vertebrae already attest the dependence of the elongation of the centrum to ontogeny: the trunk vertebrae of Ungaliophis continentalis and Ungaliophis panamensis Schmidt, 1933, illustrated by Bogert (1968a: figs 9B and 8A, respectively), that are characterized by elongate centra, apparently represented adult snakes. Indeed, a juvenile female of U. panamensis figured by Bogert (1968b: fig. 7A) possesses a shorter centrum. Furthermore, according again to Bogert (1968a), the shape and prominence of the haemal keel of the trunk vertebrae of Ungaliophis spp. is also subjected to variation: it can be discernible in large (>600 mm in total length) individuals but almost invisible in smaller individuals of U. panamensis, and it was almost absent in one adult individual of U. continentalis. In any case, caution should be paid when interpreting these terminologies, as Bogert (1968a) could have a different sense of the term haemal keel, confined perhaps solely to dorsoventrally high structures, judging from a posterior trunk vertebra with a wide haemal keel (Bogert 1968a: fig. 11) that in the caption he described as showing the “absence of any trace of the haemal keel”.

Trunk/caudal transition. A short hypapophysis is present in the last trunk and (gradually diminishing in size) in cloacal vertebrae. In caudal vertebrae, this is replaced by a distinct (although moderately developed) haemal keel; the keel disappears in the last caudal vertebrae.

Number of vertebrae. Ungaliophis continentalis (UMMZ 190698): 275 (224+3+48) including a final fusion.

Data from literature and unpublished data from personal communications: Ungaliophis continentalis: 238 trunk vertebrae plus 50 cloacal and caudal vertebrae plus a final fusion (NMNH 344819; Krister Smith, unpublished data, personal communication to GLG); Ungaliophis panamensis: 255 trunk vertebrae plus 49 cloacal and caudal vertebrae plus a final fusion (NMNH 209215; Krister Smith, unpublished data, personal communication to GLG).

Pythonoidea Fitzinger, 1826 (sensu Wallach et al. 2014)

General information

Pythonoidea comprises the Old World Pythonidae and Xenopeltidae plus the American Loxocemidae (Wallach et al. 2014; Georgalis and Smith 2020).

Xenopeltidae Bonaparte, 1845

General information

Xenopeltidae represents a monotypic family, with a single genus, Xenopeltis, with only three species distributed in southeastern Asia (Orlov et al. 2022). Unlike other constrictors that were ubiquitously lumped into an expanded “Boidae”, Xenopeltis was instead already recognized as its own distinct family (or subfamily) for a long time (e.g., Bonaparte 1845, 1852; Cope 1864, 1887, 1895, 1898; Nopcsa 1923; Smith 1943; Romer 1956; Dowling 1959; Guibé 1970). Indeed, Bonaparte (1845) established Xenopeltina as a member of the broader family Herpetidae, which included also uropeltids (his Uropeltina), calamariids (his Calamarina), and the homalopsid Erpeton Lacépède, 1801 (his Herpetina). In any case, the exact affinities of Xenopeltis with other snakes were unresolved for a long time, and the genus was occasionally considered to pertain to (the traditional concept of) “aniliids” (e.g., Jan 1857, 1865; Romer 1956; Kuhn 1961; Smith et al. 1977; McDowell 1987) or even within caenophidians (Colubroidea in Cope 1898). Nevertheless, although a single analysis recovered Xenopeltis as close to Cylindrophis (Noonan and Chippindale 2006), the majority of recent phylogenetic analyses have instead recovered Xenopeltis as closely related to pythonids (e.g., Slowinski and Lawson 2002; Wiens et al. 2008; Pyron and Burbrink 2012; Pyron et al. 2013 Reynolds et al. 2014; Figueroa et al. 2016; Streicher and Wiens 2016; Zheng and Wiens 2016; Harrington and Reeder 2017; Burbrink et al. 2020; Onary et al. 2022; Zaher et al. 2023). Accordingly, Xenopeltidae has been placed into Pythonoidea in current taxonomic schemes (Wallach et al. 2014; Georgalis and Smith 2020). Affinities of Xenopeltis within Constrictores is also supported by its dorsal scale microstructure morphology (Pauwels et al. 2000). Despite the fact that recent divergence dates estimate that xenopeltids split from other snakes already by the Late Cretaceous (e.g., Pyron and Burbrink 2012), there is no existing fossil record for the group.

Vertebral morphology of Xenopeltidae is characterized by being heavily built with centra distinctly longer than wide, the anterior ventral projection of the axis fused to the bone (this is sutured in most other snakes except for uropeltids), the presence of longitudinal bilateral ridges on the zygantral mounds, and the unique shape of the neural spine. We further highlight here the distinct notch in the ventral edge (visible in lateral view) of the hypapophyses of the anterior (but not anteriormost) trunk vertebrae, as unique among known snakes. Another, almost unique feature among non-caenophidian snakes seems to be the first appearance of the haemapophyses already on the cloacal vertebrae, but this is intraspecifically variable (for more details see Description and figures of Xenopeltis below) – a similar case with haemapophyses already appearing in the cloacal vertebrae is observed in the boid Epicrates and the pythonid Morelia Gray, 1842 – also in those two taxa it is intraspecifically variable (see the respective parts above and below).

Previous figures of vertebrae of extant Xenopeltidae have been so far presented by Hoffstetter and Gasc (1969), Gasc (1974), Lee and Scanlon (2002), Ikeda (2007), Garberoglio et al. (2019), Orlov et al. (2022), and Frýdlová et al. (2023). Among these, vertebrae from the cloacal and/or caudal series have been presented by Gasc (1974), Garberoglio et al. (2019), Orlov et al. (2022), and Frýdlová et al. (2023). Quantitative studies on the intracolumnar variability of xenopeltid vertebrae has been also conducted by Gasc (1974) and Head (2021). Beyond these, important descriptions of xenopeltid vertebral features were made by Szyndlar and Böhme (1996), Rage (2001), and Smith (2013).

Xenopeltis Reinwardt in Boié, 1827

Material examined

Xenopeltis unicolor Reinwardt in Boié, 1827 (FMNH 178973; FMNH 191771; MGPT-MDHC 117; MNHN-AC-1909.0007; NCSM 84957 [Morphosource.org: Media 000070181, ark:/87602/m4/M70181]; NHMUK 66.7.10.6; NHMUK 1930.5.8.133-135).

Description (Figs 128-136)

Trunk vertebrae. Centrum distinctly longer than wide; cotyle and condyle orbicular; neural arch moderately vaulted; posterior median notch of the neural arch deep; neural spine low, not standing higher than the junction of the posterior margins of the neural arch and forming thus a continuous Y-shaped ridge, occupying the posterior half of the neural arch; prezygapophyseal accessory processes short; hypapophyses restricted to anterior vertebrae (they disappear after V 40–V 45), elongated and thin in the anteriormost ca. 10–15 vertebrae but in the succeeding ones, plate-like, with a distinct notch in its ventral edge, visible in lateral view (condition observed in all of our specimens examined [as well as in Gasc’s [1974: fig. 24] figured specimen; a feature unknown in other snakes); haemal keel well-developed, in mid-trunk vertebrae ridge-like, in posterior trunk vertebrae flattened, approaching or exceeding the width of the cotyle; subcentral grooves deep; well-developed subcotylar tubercles in posterior trunk vertebrae; paracotylar foramina absent.

Figure 128.

Xenopeltidae: Xenopeltis unicolor (MGPT-MDHC 117), trunk vertebrae.

Figure 129.

Xenopeltidae: Xenopeltis unicolor (MGPT-MDHC 117), trunk vertebrae.

Figure 130.

Xenopeltidae: Xenopeltis unicolor (MGPT-MDHC 117), cloacal vertebrae.

Figure 131.

Xenopeltidae: Xenopeltis unicolor (MGPT-MDHC 117), anterior caudal vertebrae.

Figure 132.

Xenopeltidae: Xenopeltis unicolor (MGPT-MDHC 117), caudal vertebrae.

Figure 133.

Xenopeltidae: Xenopeltis unicolor (MNHN-AC-1909.0007), trunk vertebrae.

Figure 134.

Xenopeltidae: Xenopeltis unicolor (MNHN-AC-1909.0007), trunk vertebrae.

Figure 135.

Xenopeltidae: Xenopeltis unicolor (MNHN-AC-1909.0007), posteriormost trunk, cloacal, and caudal vertebrae.

Figure 136.

Xenopeltidae: Xenopeltis unicolor (NHMUK 1930.5.8.133-135), trunk, cloacal, and caudal vertebrae. Note that the structure looking like a lymphapophysis in the last T is an isolated bifurcated rib.

Trunk/caudal transition. The haemal keel of the last trunk vertebra(e) is wider than in more anterior vertebrae. Its posterior end broadens gradually in succeeding vertebrae of the cloacal region and then bifurcates, giving rise to haemapophyses. In four of the examined specimens, the first haemapophyses appear on the first or second caudal vertebrae. In the remaining three, however, they appear yet on the last (or on the penultimate) cloacal vertebra, the condition almost not occurring in all other extant non-caenophidians (but see also Epicrates above and Morelia below, for other similar, but also intraspecifically variable, exceptions).

Smith (2013) highlighted the presence of longitudinal, bilateral ridges where there should be situated the zygantral mounds of other constrictors, with these ridges being particularly well developed on middle and caudal vertebrae, which he regarded, along with the prominent subcotylar tubercles and the shape of the neural spine, as autapomorphic features of Xenopeltis unicolor (they could potentially be autapomorphic of the genus as a whole, but X. unicolor was the only species available for study to both Smith 2013 and to us).

Number of vertebrae (all for Xenopeltis unicolor). MGPT-MDHC 117: 222 (188+3+31); NCSM 84957: 220 (187+4+29); MNHN-AC-1909.0007: 218 (186+3+29); FMNH 191771: 211 (178+3+30); FMNH 178973: 207 (173+4+31). The counts of caudal vertebrae include final fusions, present in all specimens.

Data from literature and unpublished data from personal communications: Xenopeltis hainanensis Hu & Zhao in Zhao, 1972: 165 trunk and cloacal vertebrae plus 20 caudal vertebrae (IVPP OV 2752; Jingsong Shi, unpublished data, personal communication to GLG); Xenopeltis intermedius Orlov et al., 2022: 172 trunk and 25 cloacal and caudal vertebrae (estimated from Orlov et al. 2022: fig. 11); Xenopeltis unicolor: 187–190 trunk and cloacal vertebrae plus 26–?30 caudal vertebrae (Alexander and Gans 1966); Xenopeltis unicolor: 188 trunk vertebrae plus 34 cloacal and caudal vertebrae (Nopcsa 1923); Xenopeltis unicolor: 188 trunk vertebrae plus 30 cloacal and caudal vertebrae (Polly et al. 2001); Xenopeltis unicolor: 187 trunk and cloacal vertebrae plus unknown number of caudal vertebrae (Gasc 1974); Xenopeltis unicolor: 183 trunk vertebrae plus 7 cloacal vertebrae (apparently erroneous) plus 20 caudal vertebrae (Rochebrune 1881); Xenopeltis unicolor: 188–210 vertebrae in total (Romer 1956); Xenopeltis unicolor: 181 trunk vertebrae plus unknown number of cloacal and caudal vertebrae (Tsuihiji et al. 2012).

Loxocemidae Cope, 1861

General information

Similar to Xenopeltis, the interrelationships of the Central American species Loxocemus bicolor have been convoluted for many decades, with the taxon placed either close to booids and pythonoids or either close to “anilioids” (Cope 1861a, 1861b, 1887; Boulenger 1893; Schmidt 1923; Stull 1935; Romer 1956; Kuhn 1961; Bogert 1968a; Smith et al. 1977; McDowell 1987; Rage 1987). Nevertheless, the advance of phylogenetics shed more light on its exact affinities, with most recent studies recovering Loxocemus as closely related to Xenopeltis and pythonids (e.g., Slowinski and Lawson 2002; Wiens et al. 2008; Pyron and Burbrink 2012; Pyron et al. 2013 Reynolds et al. 2014; Figueroa et al. 2016; Streicher and Wiens 2016; Zheng and Wiens 2016; Harrington and Reeder 2017; Burbrink et al. 2020; Onary et al. 2022) and accordingly Loxocemidae has been placed into Pythonoidea in recent taxonomic schemes (Wallach et al. 2014; Georgalis and Smith 2020; Smith and Georgalis 2022). Fossil record attests the presence of loxocemids already since the late Eocene (Smith 2013).

Vertebral morphology of Loxocemidae is reminiscent of other constrictors, but some important characteristic features can be indeed recognized (see Description and figures of Loxocemus below for details).

Previous figures of vertebrae of extant Loxocemidae have been so far presented only by McDowell and Bogert (1954) and Bogert (1968a). Among these, vertebrae from the cloacal and/or caudal series have never been figured so far. Besides, important descriptions and notes on loxocemid vertebrae have been made by Underwood and Kochva (1993), Szyndlar and Böhme (1996), Rage (2001), and Smith (2013).

Loxocemus Cope, 1861

Material examined

Loxocemus bicolor Cope, 1861 (MVZ Herps 143487 [Morphosource.org: Media 000076134, ark:/87602/m4/M76134]; NHMUK 82.8.17.16, UF Herp 24747 [Morphosource.org: Media 000071842, ark:/87602/m4/M71842]; UMMZ 128027; UMMZ 190749).

Description (Figs 137-140)

Trunk vertebrae. Centrum shorter than wide; cotyle and condyle moderately depressed to orbicular; neural arch moderately vaulted; posterior median notch of the neural arch deep; neural spine as high as long; prezygapophyseal accessory processes very short; hypapophyses disappearing at the level of V 60; haemal keel flattened, moderately developed, approaching (but not exceeding) the width of the cotyle; prominent subcentral ridges and deep subcentral grooves in posterior trunk vertebrae; paracotylar foramina absent.

Figure 137.

Loxocemidae: Loxocemus bicolor (UMMZ 128027), trunk vertebrae.

Figure 138.

Loxocemidae: Loxocemus bicolor (UMMZ 128027), trunk, cloacal, and caudal vertebrae.

Figure 139.

Loxocemidae: Loxocemus bicolor (UMMZ 190749), trunk vertebrae.

Figure 140.

Loxocemidae: Loxocemus bicolor (NHMUK 82.8.17.16), trunk vertebra.

Szyndlar and Böhme (1996) highlighted that the most characteristic feature of the trunk vertebrae is the fusion of the subcentral ridges with parapophyses into uniform and flat laminae, rendering the centrum looking as a sharply delimited triangle in ventral view. Smith (2013) too highlighted the subcentral ridges, considering that the “extremely well-developed subcentral grooves and ridges of posterior vertebrae” represented an apomorphy.

Trunk/caudal transition. No subcentral structures occur on the last trunk, cloacal, and anteriormost caudal vertebrae or, at most, an indistinct flattened haemal keel may be present. Paired haemapophyses first appear on the C 3 in UMMZ 128027 or even the C 6 in UF Herp 24747. The posteriormost caudal vertebrae are fused.

Smith (2013) reported that the posteriormost trunk and cloacal vertebrae are provided with a hypapophysis, but we did not observe this pattern in our studied material.

Number of vertebrae (all for Loxocemus bicolor). MVZ Herps 143487: 309 (262+4+43, including a final fusion); UMMZ 128027: 308 (256+4+48, including a final fusion); UF Herp 24747: 307 (258+4+45, including a final fusion).

Data from literature: Loxocemus bicolor: 267 trunk vertebrae plus unknown number of cloacal and caudal vertebrae (Tsuihiji et al. 2012).

Pythonidae Fitzinger, 1826

General information

Pythonids comprise some the largest and most impressive snakes of all time (Murphy and Henderson 1997). Their fascinating nature is aptly imprinted also in their etymology, with the namesake type genus Python Daudin, 1803, named after the mythical serpent (Πύθων) that was guarding the Oracle of Delphi in Greek mythology. Pythons were long lumped for many decades into an expansive Boidae (see the respective entry above), although still certain early workers treated them as a distinct family (e.g., Cope 1864, 1887, 1893, 1898; Zittel 1887–1890; Lydekker 1888; Hoffmann 1890; Bocage 1895). Nevertheless, in all recent taxonomic schemes they are ubiquitously placed in their own distinct family (Vidal and Hedges 2009; Schleip and O’Shea 2010; Pyron et al. 2013; Wallach et al. 2014; Barker et al. 2015; Zheng and Wiens 2016; Burbrink et al. 2020; Georgalis and Smith 2020; Boundy 2021; Ivanov 2022; Smith and Georgalis 2022).

Molecular data and fossil evidence support the origination of pythonids already by the Paleogene (Esquerré et al. 2020; Zaher and Smith 2020). As a matter of fact, pythonids have a relatively rich fossil record, including also remains of the extant genera Python (Hoffstetter 1964; Rage 1976, 2008b; Szyndlar 1991a; Ivanov 2000; Szyndlar and Rage 2003; Delfino et al. 2004, 2018; Head 2005; Ivanov and Böhme 2011; Georgalis et al. 2020a, 2020b; Head and Müller 2020; Ivanov et al. 2020; Singh et al. 2022a, 2022b), Morelia (Scanlon 2001), and Liasis Gray, 1842 (Scanlon and Mackness 2001).

Pythons comprise more than 40 extant species, distributed over large parts of Africa, Asia, and Australia (Schleip and O’Shea 2010; Wallach et al. 2014; Boundy 2021). This moderate diversity of species is reflected in a wide spectrum of sizes (ranging from approximately 0.5 m to 9 m in length) and large disparity of body forms and life habits. There is still no consensus on the number of valid genera, with the past few decades witnessing splitting or lumping of available genera based on molecular and/or morphological evidence (e.g., Stimson 1969; McDowell 1975; Cogger et al. 1983; Wells and Wellington 1984, 1985; Underwood and Stimson 1990; Kluge 1993a; Harvey et al. 2000; Rawlings et al. 2008; Pyron et al. 2013; Reynolds et al. 2014; Schleip 2014; Barker et al. 2015; Esquerré et al. 2020, 2021; Kaiser et al. 2020). This is particularly true for the Australo-Papuan species, which have frequently and radically changed generic allocations within at least 11 genera (Antaresia Wells and Wellington, 1984, Apodora Kluge, 1993, Bothrochilus Fitzinger, 1843, Chondropython Meyer, 1874, Leiopython Hubrecht, 1879, Liasis, Morelia, Nawaran Esquerré et al., 2020, Nyctophilopython Wells & Wellington, 1985, Python, and Simalia Gray, 1849) (see e.g., Stimson 1969; McDowell 1975; Cogger et al. 1983; Wells and Wellington 1984, 1985; Stimson and McDowell 1986; ICZN 1988; Underwood and Stimson 1990; Harvey et al. 2000; Rawlings at al. 2008; Pyron et al. 2013; Reynolds et al. 2014; Schleip 2014; Barker et al. 2015; Esquerré et al. 2020, 2021; Kaiser et al. 2020) – the only notable exception is the genus Aspidites Peters, 1877, whose validity and taxonomic content have remained stable.

In the present paper, we treat the vertebral description of the Australo-Papuan genera Antaresia, Bothrochilus, Leiopython, Liasis, and Simalia collectively, as the vertebral differences among these are not too important (see entry of Simalia below). This approach has been also applied in palaeontological literature, where only skeletal material was available and therefore an expansive concept of Liasis (sensu lato) was followed (e.g., Scanlon 2001; Scanlon and Mackness 2001). Moreover, these genera (Antaresia, Apodora, Bothrochilus, Leiopython, Liasis, and Simalia) form a monophyletic group when non-skeletal characters are excluded (Kluge 1993a; see also Scanlon 2001; Scanlon and Mackness 2001). A similar rationale applies to the case of the Asian Python reticulatus, which is placed in its own genus Malayopython Reynolds, Niemiller & Revell, 2014 (see Reynolds et al. 2014), but we treat its description collectively with Python spp. On the other hand, we treat Morelia and Aspidites on their own, because of some peculiarities observed in their vertebrae.

Vertebrae of most pythonids closely resemble one another. At the same time they are very similar to those of most boids, displaying the same generalized morphological pattern: they are usually relatively short, wide, and massive, provided with vaulted neural arches, high neural spine and reduced prezygapophyseal accessory processes. A principal characteristic feature of most pythons (except for Python curtus and Python brongersmai) is the very high amount of vertebrae, exceeding values observed in most other living snakes (with the exception of the leptotyphlopid Rhinoleptus and the typhlopid Letheobia; see the respective entries above); the total number of vertebrae in pythonids is higher than 300, in some species more than 400 (see also “Parts of the vertebral column” above for the case of Nyctophilopython oenpelliensis which could potentially have an even higher vertebral count). It is further worth noting that complete fossil skeletons of snakes from Konservat-Lagerstätten localities demonstrate that high counts of vertebrae occurred in fossil Booidea (notably Eoconstrictor Scanferla & Smith, 2020, and Messelophis Baszio, 2004) but, strangely, not in fossil Pythonidae (Baszio 2004; Scanferla and Smith 2020a; Zaher and Smith 2020). For detailed vertebral counts of extant pythonid taxa, see the respective entries of the genera below.

Similarly to boas, pythonid vertebrae were among the first to be presented in early snake anatomical works. This was apparently due to the fascination and general interest surrounding pythons as well as their large size, which therefore consequently rendered them easier to dissect. The first comprehensive documentation of pythonid vertebrae was conducted by D’Alton (1836), who provided extensive descriptions and figures of (both trunk and caudal) vertebrae of Python. Since then, numerous illustrations of vertebrae of different species of pythons were fairly often published by various authors. Principal examples of previous figures of vertebrae of extant Pythonidae were presented by Owen (1841, 1850, 1857, 1877), Rochebrune (1881), Albrecht (1883), Hoffmann (1890), Smith (1943), Romer (1956), Holman (1967), Hoffstetter and Gasc (1969), Gasc (1974), Underwood (1976), Holman (1982), Rage (1984), Palci et al. (2013a, 2013b, 2018, 2020), Xing et al. (2018), Garberoglio et al. (2019), Georgalis and Scheyer (2019), Fachini et al. (2020), Palci et al. (2020), and Shi et al. (2023b), including also from individuals of earlier ontogenetic stages (Xing et al. 2018). Among these, vertebrae from the cloacal and/or caudal series have received considerably less attention, being figured solely by Rage (1984) and Palci et al. (2020). Figures of the microanatomy and histology / transverse sections of pythonid vertebrae were presented by Hoffstetter and Gasc (1969) and Houssaye et al. (2013). Quantitative studies on the intracolumnar variability of pythonid vertebrae were conducted by Gasc (1974) and Scanlon and Mackness (2001).

Python Daudin, 1803

Material examined

Python bivittatus Kuhl, 1820 (ISEZ R/327; ISEZ R/461; NHMW 35675); Python curtus Schlegel, 1872 (MGPT-MDHC 106; MGPT-MDHC 107; NHMUK 1988.598); Python molurus (Linnaeus, 1758) (ISEZ R/460; MNCN 13816; SMF PH 184); Python regius (Shaw, 1802) (MGPT-MDHC 116; MGPT-MDHC 144; MGPT-MDHC 161; MGPT-MDHC 162; MGPT-MDHC 163; MGPT-MDHC 173; MGPT-MDHC 196; MGPT-MDHC 456; MNHW Reptilia-0326 [juvenile]; MNHW Reptilia-0327 [juvenile]; SMF PH 28; SMF PH 29; SMF PH 49; SMF PH 51; SMF PH 58 [juvenile]; SMF PH 222 [juvenile]; UMMZ 190750); Python sebae (Gmelin, 1789) (MGPT-MDHC 121; SMF PH 1 [baby]; ZFMK 5199).

Description (Figs 141-149)

Trunk vertebrae. Centrum much shorter than wide; cotyle and condyle orbicular; neural arch vaulted; posterior median notch of the neural arch deep; neural spine considerably higher than long (in Python curtus, two to almost three times higher than long); prezygapophyseal accessory processes vestigial or very short; shallow interzygapophyseal constriction; hypapophyses disappearing at the level of V 70 to V 80; haemal keel in more posterior vertebrae moderately or well developed, ridge-like (Python curtus, Python regius) or somewhat wider and less distinct (Python bivittatus, Python molurus); paracotylar foramina absent.

Figure 141.

Pythonidae: Python curtus (NHMUK 1988.598), trunk vertebrae.

Figure 142.

Pythonidae: Python curtus (NHMUK 1988.598), trunk vertebrae.

Figure 143.

Pythonidae: Python curtus (NHMUK 1988.598), cloacal vertebrae.

Figure 144.

Pythonidae: Python curtus (NHMUK 1988.598), caudal vertebrae.

Figure 145.

Pythonidae: Python regius (UMMZ 190750), trunk vertebrae.

Figure 146.

Pythonidae: Python sebae (ZFMK 5199), trunk vertebrae. The skeleton was disarticulated and therefore exact numbers of the figured vertebrae are not precisely known but the vertebrae are shown in a preceding-succeeding row.

Figure 147.

Figure 148.

Pythonidae: Python sebae (ZFMK 5199), trunk and cloacal vertebrae. The skeleton was disarticulated and therefore exact numbers of the figured vertebrae are not precisely known but the vertebrae are shown in a preceding-succeeding row.

Figure 149.

Pythonidae: Python sebae (ZFMK 5199), caudal vertebrae. The skeleton was disarticulated and therefore exact numbers of the figured vertebrae are not precisely known but the vertebrae are shown in a preceding-succeeding row.

Trunk/caudal transition. A bulb-like, more or less prominent, haemal keel (it can be considered a hypapophysis in Python curtus) appears in the last trunk vertebrae and then diminishes gradually and becomes flattened in succeeding cloacal vertebrae. In a specimen of Python molurus (ISEZ R/460), short haemapophyses appear on the second cloacal vertebra, then disappear, and reappear again on the last cloacal and first caudal vertebra; the two anteriormost cloacal vertebrae of this snake are fused and apparently pathologically affected. In other examined snakes, normally developed haemapophyses can appear on the first (Python sebae) or second to sixth caudal vertebrae (Python bivittatus, P. regius, and P. molurus); in the last case, traces of haemapophyses (keels bifurcated posteriorly into two minute spurs) can be seen on the preceding caudal vertebra. Posteriormost caudal vertebrae can be fused (e.g., in P. molurus).

Number of vertebrae. Python bivittatus (ISEZ R/461): 344 (273+3+68); Python curtus (MGPT-MDHC 107): 211 (177+4+30, including a fusion of posteriormost caudal vertebrae); Python curtus (NHMUK 1988.598): 207+ (174+4+29+); Python curtus (MGPT-MDHC 106): 203 (170+4+29); Python molurus (ISEZ R/327): 317+ (257+4+56+); Python molurus (SMF PH 184): 349 (272+4+73, including a fusion of posteriormost caudal vertebrae); Python molurus (ISEZ R/460): 342 (272+4+66); Python regius (SMF PH 28): 239 (205+4+30); Python regius (SMF PH 29): 239 (206+4+29); Python regius (SMF PH 49): 242 (206+4+32 [posteriormost caudal vertebrae are fused]); Python regius (MNHW Reptilia-0326 [juvenile]): 251+ (209+5+37, including a fusion of posteriormost caudal vertebrae); Python regius (MGPT-MDHC 456): 249 (211+4+34, including a final fusion of posteriormost caudal vertebrae); Python regius (MGPT-MDHC 162): 245 (208+5+32, including a final fusion of posteriormost caudal vertebrae); Python regius (SMF PH 58 [juvenile]): 245+ (207+4+34+ [several caudal vertebrae are missing]); Python regius (MGPT-MDHC 173 [juvenile]): 241+ (213+4+24+ [posteriormost caudal vertebrae missing]); Python regius (MGPT-MDHC 163): 237 (204+4+29, including a final fusion of posteriormost caudal vertebrae); Python regius (MNHW Reptilia-0327 [juvenile]): 230+ (213+17 cloacal and caudal vertebrae+ [several caudal vertebrae are missing]); Python sebae (MGPT-MDHC 121): 278+ (274+4+[caudal vertebrae missing]); Python sebae (SMF PH 1 [baby]): 332 (280+4+68 [posteriormost caudal vertebrae are fused]).

Data from literature and unpublished data from personal communications: Python bivittatus: 327 trunk and cloacal vertebrae plus 93 caudal vertebrae (Jingsong Shi, unpublished data, personal communication to GLG); Python brongersmai Stull, 1938: 178–179 trunk vertebrae plus unknown number of cloacal and caudal vertebrae (Tsuihiji et al. 2012); Python curtus: 162–180 trunk and cloacal vertebrae plus 31–36 caudal vertebrae (Jingsong Shi, unpublished data, personal communication to GLG); Python molurus: 372 trunk vertebrae plus 61 cloacal and caudal vertebrae (Janensch 1906); Python molurus: 291 vertebrae in total, of which 70 caudal vertebrae (Python tigris of Owen 1877); Python molurus: 274 trunk vertebrae plus 75 cloacal and caudal vertebrae (Nopcsa 1923); Python molurus: 324–328 vertebrae in total (Parmley and Reed 2003); Python molurus: 269 trunk vertebrae plus unknown number of cloacal and caudal vertebrae (Tsuihiji et al. 2012); Python molurus: 259 trunk vertebrae plus 68 cloacal and caudal vertebrae (Polly et al. 2001); Python molurus: 253 trunk vertebrae plus 72 cloacal and caudal vertebrae (Python tigris of Fischer 1857); Python molurus: 4 cloacal and 59 caudal vertebrae (Sood 1941); Python molurus: 366 trunk vertebrae plus 8 cloacal vertebrae (apparently erroneous) plus 61 caudal vertebrae (Rochebrune 1881); Python regius: 281 trunk vertebrae plus 67 cloacal and caudal vertebrae (Owen 1850); Python regius: 215 trunk vertebrae plus 33 cloacal and caudal vertebrae (Polly et al. 2001); Python regius: 207–211 trunk and cloacal vertebrae plus 32–35 caudal vertebrae (Jingsong Shi, unpublished data, personal communication to GLG); Python sebae: 306 trunk vertebrae plus 62 cloacal and caudal vertebrae (Janensch 1906); Python sebae: 283 trunk and cloacal vertebrae plus 75+ caudal vertebrae (Alexander and Gans 1966); Python sebae: 333 trunk vertebrae plus unknown number of cloacal and caudal vertebrae (Albrecht 1883); Python sebae: 287 trunk vertebrae plus unknown number of cloacal and caudal vertebrae (Gasc 1974); Python sebae: 282 trunk vertebrae plus 63 cloacal and caudal vertebrae (Nopcsa 1923); Python sebae: 302 trunk vertebrae plus 6 cloacal vertebrae (probably erroneous) plus 62 caudal vertebrae (Rochebrune 1881).

The total value of 435 vertebrae for Python molurus by Rochebrune (1881) has been repeated in several subsequent works (e.g., Hoffmann 1890; Baur 1897; Janensch 1906; Simpson 1933; Angel 1950) as well as in popular literature, purportedly, as the highest number of vertebrae ever observed in living snakes (although, in any case, it does not surpass the respective number of the leptotyphlopid Rhinoleptus and the typhlopid Letheobia). Subsequent observations made on the same material by Hoffstetter and Gasc (1969) did not confirm Rochebrune’s counting; consequently, the latter authors stated that the maximum value for recent snakes is “more than 400” (apparently unaware at that time of the respective numbers of Rhinoleptus and Letheobia); in any case, a specimen of Python bivittatus reported above reached a total of 420 vertebrae. Again in the work of Rochebrune (1881), mistakes also apparently apply to the given number of cloacal vertebrae of Python sebae (6) and P. molurus (8) (see also “Parts of the vertebral column” above for a discussion on the erroneous counts of cloacal vertebrae in Rochebrune’s [1881] work). On the other side of the vertebral count spectrum, the lowest count of vertebrae for Python is observed in P. curtus and P. brongersmai.

Malayopython Reynolds, Niemiller & Revell, 2014

Material examined

Malayopython reticulatus (Schneider, 1801) (ISEZ R/436 [juvenile]); Malayopython timoriensis (Peters, 1876) (SMF PH 27 [juvenile]).

Description (Figs 150-151)

Trunk vertebrae. The morphology is relatively similar to that of Python above. The haemal keel in more posterior vertebrae is somewhat wider and less distinct (like in Python bivittatus and Python molurus).

Figure 150.

Pythonidae: Malayopython reticulatus (ISEZ R/436), trunk vertebrae.

Figure 151.

Pythonidae: Malayopython reticulatus (ISEZ R/436), trunk, cloacal, and caudal vertebrae.

Trunk/caudal transition. The morphology is relatively similar to that of Python above, but normally developed haemapophyses can appear on the third (Malayopython reticulatus and Malayopython timoriensis) caudal vertebra.

Number of vertebrae. Malayopython reticulatus (ISEZ R/436) (juvenile): 413 (312+3+98); Malayopython timoriensis (SMF PH 27) (juvenile): 352 (287+5+60 [posteriormost caudal vertebrae are fused]).

Data from literature: Malayopython reticulatus: 269–316 trunk vertebrae plus 92–102 cloacal and caudal vertebrae (Nopcsa 1923).

Aspidites Peters, 1877

Material examined

Aspidites melanocephalus (Krefft, 1864) (NHMUK 1946.1.8.2 [part of the holotype]; QM J30786; UMMZ 190743).

Description (Figs 152-157)

Trunk vertebrae. Centrum much shorter than wide; cotyle and condyle orbicular; neural arch vaulted; posterior median notch of the neural arch deep; neural spine higher than long in most trunk vertebrae; prezygapophyseal accessory processes vestigial; hypapophyses restricted to about the first 70 vertebrae; haemal keel moderately developed, ridge-like (more anterior vertebrae) to flattened (more posterior vertebrae); paracotylar foramina absent.

Figure 152.

Pythonidae: Aspidites melanocephalus (QM J30786), anterior trunk vertebrae.

Figure 153.

Pythonidae: Aspidites melanocephalus (QM J30786), trunk vertebrae.

Figure 154.

Pythonidae: Aspidites melanocephalus (QM J30786), posteriormost trunk and cloacal vertebrae.

Figure 155.

Pythonidae: Aspidites melanocephalus (QM J30786), cloacal and caudal vertebrae.

Figure 156.

Pythonidae: Aspidites melanocephalus (NHMUK 1946.1.8.2 [part of the holotype]), mid-trunk vertebra.

Figure 157.

Pythonidae: Aspidites melanocephalus (UMMZ 190743), anterior trunk vertebrae.

Trunk/caudal transition. The last trunk and cloacal vertebrae provided with a bulb-like haemal keel, disappearing in the anteriormost caudal vertebrae. Paired haemapophyses appear on the third caudal vertebra. Shape and length of haemapophyses in lateral view vary across the succeeding vertebrae: they are smaller in their first appearance, they get larger in C 6, possessing also a distinct anteroventral projection, and they get longer towards V 20. Posteriormost caudal vertebrae are fused.

Number of vertebrae. Aspidites melanocephalus (QM J30786): 363 (307+5+51).

Data from literature (all for Aspidites melanocephalus): 337 trunk vertebrae plus 5 cloacal vertebrae plus 52 caudal vertebrae (Scanlon 1993); 385 vertebrae in total (Parmley and Reed 2003); 293 trunk and cloacal vertebrae plus 48+ caudal vertebrae (Alexander and Gans 1966).

Morelia Gray, 1842

Material examined

Morelia spilota (Lacépède, 1804) (MNHW Reptilia-0328; SMF PH 4; SMF PH 6; SMF PH 8; SMF PH 67); Morelia viridis (Schlegel, 1872) (QM J2108; SMF PH 12; SMF PH 22 [juvenile]; SMF PH 183; UF Herp 120539 [Morphosource.org: Media 000408449, ark:/87602/m4/408449]; ZFMK 5212).

Description (Figs 158-161)

Trunk vertebrae. Centrum shorter than wide; cotyle and condyle orbicular; neural arch vaulted; posterior median notch of the neural arch deep; neural spine as high as long. In the neural spine of Morelia viridis, its dorsal edge is slightly expanded laterally and produced anteriorly into a prominent bifurcated spur (these structures disappear in the trunk/caudal transition). Neural spine of posteriormost trunk vertebrae rather short; prezygapophyseal accessory processes vestigial; hypapophyses disappearing at the level of V 60 approximately in M. viridis and after V 70 in M. spilota; haemal keel in more posterior vertebrae flattened, moderately developed; paracotylar foramina absent. The vertebrae of Morelia spilota illustrated by Gasc (1974: fig. 7) possess relatively much shorter than wide centrum and distinctly higher than long neural spine.

Figure 158.

Pythonidae: Morelia viridis (ZFMK 5212), trunk vertebrae.

Figure 159.

Pythonidae: Morelia viridis (ZFMK 5212), trunk vertebrae.

Figure 160.

Pythonidae: Morelia viridis (ZFMK 5212), cloacal and caudal vertebrae.

Figure 161.

Pythonidae: Morelia viridis (QM J2108), posteriormost trunk, cloacal, and anterior caudal vertebrae.

Trunk/caudal transition. The last trunk and cloacal vertebrae provided with a ridge-like haemal keel (in one examined specimen of Morelia viridis expanded posteriorly into a widened spur). Paired haemapophyses first appear usually in the second caudal vertebra (in one specimen of M. viridis [QM J2108] the first haemapophysis is unpaired), although they can also appear in the first caudal (Morelia spilota [SMF PH 8]). In one specimen of Morelia viridis (SMF PH 183) clearly paired haemapophyses already appear in the last (4^th) cloacal vertebra. In one specimen of M. spilota, the first haemapophysis is unpaired and appears already in the last (5^th) cloacal vertebra. Posteriormost caudal vertebrae can be fused in M. spilota.

Number of vertebrae. Morelia spilota (SMF PH 67): 392 (305+5+82 [posteriormost caudal vertebrae are fused]); Morelia spilota (SMF PH 6): 366 (277+5+84); Morelia spilota (MNHW Reptilia-0328): 354 (287+5+62); Morelia spilota (SMF PH 8): 320+ (260+4+56+ [posteriormost caudal vertebrae are missing]); Morelia viridis (QM J2108): 339 (248+4+87); Morelia viridis (ZFMK 5212): 317 (240+4+73); Morelia viridis (SMF PH 22 [juvenile]): 326 (237+4+85); Morelia viridis (SMF PH 183): 345 (243+4+98); Morelia viridis (UF Herp 120539): 337 (242+4+91).

Data from literature and unpublished data from personal communications: Morelia spilota: 371 vertebrae in total (Parmley and Reed 2003); Morelia spilota: 286 trunk vertebrae plus 75 cloacal and caudal vertebrae (Polly et al. 2001); Morelia spilota: 276 trunk and cloacal vertebrae plus 86 caudal vertebrae (Python [Morelia] argus of Alexander and Gans 1966); Morelia spilota: 269 trunk vertebrae plus unknown number of cloacal and caudal vertebrae (Python argus of Gasc 1974); Morelia viridis: 248 trunk and cloacal vertebrae plus 89 caudal vertebrae (Jingsong Shi, unpublished data, personal communication to GLG); Morelia viridis: 240±2 trunk and cloacal vertebrae plus 83±6 caudal vertebrae (Alexander and Gans 1966); Morelia viridis: 232–237 trunk vertebrae plus 40–75 cloacal and caudal vertebrae (Holman 1967).

It is worth highlighting the remarkably high number of caudal vertebrae (reaching up to 98), apparently correlated with the arboreal lifestyle of this genus.

Simalia Gray, 1849

Material examined

Simalia amethistina (Schneider, 1801) (QM J51148); Simalia boeleni (Brongersma, 1953) (SMF PH 110).

Description (Figs 162-167)

Trunk vertebrae. Centrum much shorter than wide; cotyle and condyle orbicular or moderately (ventrally) depressed; neural arch vaulted or moderately vaulted; posterior median notch of the neural arch deep; neural spine of medium height; prezygapophyseal accessory processes vestigial or very short; hypapophyses disappearing after V 70 (Simalia boeleni) or between V 80 and V 90 (Simalia amethistina); haemal keel in more posterior vertebrae moderately developed and moderately broad; paracotylar foramina absent.

Figure 162.

Pythonidae: Simalia amethistina (QM J51148), anterior trunk vertebrae.

Figure 163.

Pythonidae: Simalia amethistina (QM J51148), trunk vertebrae.

Figure 164.

Pythonidae: Simalia amethistina (QM J51148), trunk vertebra.

Figure 165.

Pythonidae: Simalia amethistina (QM J51148), trunk vertebrae.

Figure 166.

Pythonidae: Simalia amethistina (QM J51148), cloacal and anterior caudal vertebrae.

Figure 167.

Pythonidae: Simalia amethistina (QM J51148), caudal vertebrae.

Trunk/caudal transition. A hypapophysis, that appears in the last trunk vertebra, diminishes gradually in size in more posterior vertebrae; in the last cloacal vertebra(e) or first caudal vertebra it can be considered a haemal keel, with flattened (slightly grooved in Simalia amethistina and Simalia boeleni) posterior end. Haemapophyses (unpaired unilateral in S. amethistina) first appear on the second caudal vertebra (Fig. 167).

Number of vertebrae. Simalia amethistina (QM J51148): 416 (355+4+57, including a final fusion); Simalia boeleni (SMF PH 110): 378 (305+5+68).

Data from literature: Simalia amethistina: 323 trunk vertebrae plus 106 cloacal and caudal vertebrae (Nopcsa 1923); Simalia amethistina: 318–322±5 trunk and cloacal vertebrae plus 118–119±1 caudal vertebrae (Alexander and Gans 1966); Simalia amethistina: 322 trunk vertebrae plus 10 cloacal vertebrae (apparently erroneous) plus 92 caudal vertebrae (Rochebrune 1881); Simalia boeleni: 360–363 vertebrae in total (Parmley and Reed 2003).

Antaresia Wells & Wellington, 1984

Material examined

Antaresia childreni (Gray, 1842) (UMMZ 190746).

Description (Fig. 168)

Trunk vertebrae. The morphology is relatively similar to that of Simalia above. Neural arch moderately vaulted.

Figure 168.

Pythonidae: Antaresia childreni (UMMZ 190746), trunk vertebra.

Trunk/caudal transition. The morphology is relatively similar to that of Simalia above.

Number of vertebrae. Data from literature: Antaresia childreni: 275 trunk and cloacal vertebrae plus 48+ caudal vertebrae (Alexander and Gans 1966).

Apodora Kluge, 1993

Material examined

Apodora papuana (Peters & Doria, 1878) (AMS R16488).

Description (Fig. 169)

Trunk vertebrae. The morphology is relatively similar to that of Simalia above.

Figure 169.

Pythonidae: Apodora papuana (AMS R16488), trunk vertebra.

Trunk/caudal transition. The morphology is relatively similar to that of Simalia above.

Number of vertebrae. Apodora papuana (AMS R16488): 350+ trunk vertebrae (several [at least 20] anterior trunk, cloacal, and caudal vertebrae missing; John Scanlon, unpublished data, personal communication to GLG).

Bothrochilus Fitzinger, 1843

Material examined

Bothrochilus boa (Schlegel, 1837) (ZFMK 5203; UMMZ 190744).

Description (Figs 170-172)

Trunk vertebrae. The morphology is relatively similar to that of Simalia above. Neural arch moderately vaulted.

Figure 170.

Pythonidae: Bothrochilus boa (ZFMK 5203), trunk vertebrae. The skeleton was disarticulated and therefore exact numbers of the figured vertebrae are not precisely known but the vertebrae are shown in a preceding-succeeding row.

Figure 171.

Pythonidae: Bothrochilus boa (ZFMK 5203), trunk, cloacal, and caudal vertebrae. The skeleton was disarticulated and therefore exact numbers of the figured vertebrae are not precisely known but the vertebrae are shown in a preceding-succeeding row.

Figure 172.

Pythonidae: Bothrochilus boa (UMMZ 190744), trunk vertebra.

Trunk/caudal transition. The morphology is relatively similar to that of Simalia above, but hypapophysis of last trunk vertebra is more prominent.

Number of vertebrae. Data from literature (all for Bothrochilus boa): 318–322 vertebrae in total (Parmley and Reed 2003); 266 trunk and cloacal vertebrae plus 57±1 caudal vertebrae (Alexander and Gans 1966).

Leiopython Hubrecht, 1879

Material examined

Leiopython albertisii (Peters & Doria, 1878) (SMF PH 50).

Description (Figs 173-174)

Trunk vertebrae. There is much similarity with Simalia described above. Centrum much shorter than wide; cotyle and condyle orbicular or moderately flattened; neural arch moderately vaulted; neural spine relatively short and its posterodorsal edge posteriorly inclined; hypapophyses disappear after V 50; haemal keel in succeeding vertebrae rather broad.

Figure 173.

Pythonidae: Leiopython albertisii (SMF PH 50), trunk vertebra.

Figure 174.

Pythonidae: Leiopython albertisii (SMF PH 50), posteriormost trunk, cloacal, and anterior caudal vertebrae.

Trunk/caudal transition. The subcentral structures of this vertebral region are more or less similar to that of Simalia described above. The posteriormost trunk vertebrae develop a very short hypapophysis. Cloacal vertebrae possess a grooved hypapophysis. This grooved hypapophysis develops even more in anterior caudal vertebrae, being reminiscent of “quasi-haemapophyses”, but normally shaped haemapophyses commence at the level of C 6, and subsequently continue throughout the tail. The posteriormost vertebrae are fused.

No vertebrae of Leiopython had so far been figured but additional observations can be gleaned from the short description of the caudal vertebral patterns by McDowell (1975), who personally studied a skeleton of Leiopython albertisii. According to that author, the anteriormost three caudal vertebrae (“3 vertebrae with simple transverse processes” in his writings) possessed a haemal keel (“free, triangular haemal bones on the ventral surface of the centra” in his writings), while the succeeding 65 caudal vertebrae possessed typically paired haemapophyses, followed by six fused posteriormost caudal vertebrae (McDowell 1975: 43).

Number of vertebrae. Leiopython albertisii (SMF PH 50): 350 (279+5+66 [including a final fusion]).

Data from literature (all for Leiopython albertisii): 273 trunk vertebrae plus 5 cloacal vertebrae plus 77 caudal vertebrae (posteriormost 6 caudal vertebrae are fused) (McDowell 1975); 347 vertebrae in total (Parmley and Reed 2003).

Liasis Gray, 1842

Material examined

Liasis mackloti Duméril & Bibron, 1844 (UMMZ 190747; LSUMZ Herps 099023 [Morphosource.org: Media 000474530, ark:/87602/m4/474530]).

Description (Fig. 175)

Trunk vertebrae. The morphology is relatively similar to that of Simalia above.

Figure 175.

Pythonidae: Liasis mackloti (UMMZ 190747), trunk vertebra.

Trunk/caudal transition. The morphology is relatively similar to that of Simalia above.

Number of vertebrae. Liasis mackloti (LSUMZ Herps 099023): 398 (305+5+88, including a final fusion).

Data from literature: Liasis fuscus Peters, 1873: 282 trunk and cloacal vertebrae plus ~76 caudal vertebrae (Alexander and Gans 1966).

Discussion

Phylogenetic implications

It is evident from the multiple figures and descriptions above that extant non-caenophidian snakes exhibit an astonishingly wide range of vertebral morphologies, structures, and intracolumnar patterns. Therefore, it can be ascertained that snake vertebrae can be informative for taxonomic purposes, allowing at many times identifications to the family or even the genus and the species level, assuming of course that intracolumnar variation can be properly assessed (Szyndlar and Böhme 1996; Head et al. 2022; Smith and Georgalis 2022). However, despite this taxonomic utility of snake vertebrae, there is little room for accurate phylogenetic considerations, as many features are strongly convergent, repeatedly acquired, and/or widespread across different lineages (Szyndlar and Böhme 1996; Smith and Georgalis 2022). Nevertheless, we discuss certain structures and features of the snake vertebral column and assess their potential value for phylogenetic consideration.

Number of vertebrae

The total number of vertebrae may substantially differ among closely related members of many ophidian lineages and on account of that, this feature seems of little use for phylogeny reconstruction. The genus Eryx is an important exception, where the total number of vertebrae in the column does not differ substantially among its congeneric members. Nevertheless, this feature has long been used as a diagnostic value for certain taxa, especially for scolecophidians (e.g., Gans and Laurent 1965; Roux-Estève 1974a; Broadley and Wallach 2007, 2009). Moreover, even within a single snake species, the number of vertebrae can vary drastically, being even subjected to sexual dimorphism in certain taxa (e.g., Roux-Estève 1974a, 1974b; Shine 2000; Pinto et al. 2015; Martins et al. 2023) or even to changes due to the different elevation of the animal’s habitat, with the number decreasing towards the sea-level (Roux-Estève 1974a, 1974b). In general, the total number of vertebrae is more dependent on locomotion and general life habits and not necessarily on the absolute size of a species (Lindell 1994; Müller et al. 2010). This is further witnessed by the fact that the highest numbers of vertebral counts among extant snakes are observed in the two opposite edges of their size spectrum, i.e., in some of the smallest taxa (the leptotyphlopid Rhinoleptus and the typhlopid Letheobia) and in some of the largest ones (pythonids and some boids).

The reduction in the number of caudal vertebrae seems of greater importance. A relatively long tail is reminiscent of lizards and has been considered a primitive feature (e.g., Lee 1997: character 112; Lee and Scanlon 2002: character 187). On the other hand, the tail size (and therefore also the number of vertebrae) relative to the body length is dependent on the ecology of the species, with arboreal snakes possessing long bodies with proportionally long tails, terrestrial snakes possessing long bodies with shorter tails, and fossorial snakes possessing short bodies with very short tails (Polly et al. 2001; Brandley et al. 2008). Interestingly, the lowest number of caudal vertebrae is observed in most “primitive” extant snakes, including scolecophidians. It is worth noting that some Cretaceous hind-limbed snakes have really low numbers of caudal vertebrae (e.g., only 24 in Eupodophis descouensi [Rage & Escuillié, 2000]; see Rage and Escuillié 2000). Besides the common intraspecific variability, however, this number too is also subjected to sexual variation (e.g., Pinto et al. 2015). Among more “advanced” non-caenophidians, aniliids, tropidophiids, uropeltids, sanziniids, and xenopeltids possess relatively low numbers of caudal vertebrae. It is worth mentioning that Smith (2013) devised one way to estimate the reduction in the number of caudal vertebrae (at least, the relative proportion of trunk and caudal vertebrae) in fossil snakes based on isolated bones – that approach expands the possibility of studying the ecology and evolution of this character beyond extant taxa and fossil snake assemblages from Konservat-Lagerstätten localities.

Centrum length / width ratio

Scolecophidians, aniliids, cylindrophiids, uropeltids, ungaliophiids, and xenopeltids are characterized by elongate (i.e., distinctly longer than wide) vertebral centra. The remaining non-Colubroides snake families generally display reverse proportions (i.e., distinctly wider than long); sometimes the centrum length and centrum width are roughly equal. In rare cases, single genera of the latter families (Charina, Casarea) may possess centrum longer than wide, i.e., the condition approaching that of snakes of the group Colubroides.

There is no agreement in the literature about the weight or polarity of the discussed feature. Hecht and LaDuke (1988) apparently overestimated the significance of the relative centrum length in bolyeriid snakes: they named “fundamentally different” vertebrae of Bolyeria (shorter) and Casarea (longer). In fact, however, the vertebral length does not differ greatly between both these snakes. On the other hand, differences in the relative length of centra of trunk vertebrae can be observed between the charinaids Charina and Lichanura, but nevertheless the complex morphology of the posterior caudal vertebrae of both snakes, as well as cranial and molecular evidence, clearly demonstrate their close relationships. Smith (2013) regarded the distinct elongation of ungaliophiid vertebral centra as a derived condition.

Hypapophyses

The term “hypapophysis” generally has been used in reference to prominent projections, while those of low configuration being referred to as haemal keels (or even simply keels). But as rightly pointed out by Bogert (1968a: 25), “in view of the variations in the shape and the degree of development of the hypapophyses, it seems futile to expect investigators to be wholly objective in their choice of terms”. Such view was also aptly highlighted by Malnate (1972), who considered the distinction among hypapophyses and haemal keels as subjective, arbitrary, and, presumably, as the basis for the confusion in the reported in the literature “presence or absence of hypapophyses”, especially on the posterior trunk vertebrae. In fact, among non-caenophidian snakes, only the structures observed in Bolyeriidae (provided with a backwardly directed spine beneath the condyle), Tropidophiidae (plate-like, provided with a distinctive anteroventral corner), and Candoiidae (smaller, blade-like shaped) may be interpreted as “true” hypapophyses. Among these three groups, the hypapophyses of bolyeriids are the most prominent, but we still here treat the (smaller) structures of tropidophiids and candoiids as hypapophyses. In that respect, we tend to disagree with Hoffstetter and Gasc (1969), who treated these structures in Candoia and tropidophiids as a secondary development of a simple haemal keel. But of course, as highlighted above, the distinction between true hypapophyses and haemal keels can be sometimes just a matter of philosophical question (or just personal taste).

The shape of the hypapophyses varies significantly throughout the column (and across the anterior portion of the column for the taxa that lack these posteriorly). However, occasionally, certain features of the hypapophyses maybe useful in identifications: for example, as highlighted above, in Xenopeltis there is a distinct notch in the ventral edge of the hypapophyses (in lateral view) of the anterior trunk vertebrae, with this feature being unique among snakes.

There is significant variability in regards to the number of vertebrae possessing hypapophyses among different snake taxa, with certain groups possessing them throughout the column, others possessing them only at the anterior part of the trunk, while a few extinct taxa (certain palaeophiids) even possessed doubled hypapophyses in their anterior trunk vertebrae (an anterior and a posterior hypapophysis). Moreover, in certain snakes, which lack hypapophyses in their posterior trunk vertebrae, hypapophyses are present in their posteriormost trunk vertebra(e) and/or the cloacal vertebrae and/or (part of) the caudal series, but these structures are different (see below “Subcentral structures around the cloaca”).

As mentioned in the Introduction, the presence or absence of hypapophyses in the posterior trunk vertebrae has been considered important by many previous authors for decades. Hoffstetter (1962) supposed that hypapophyses can never be reacquired once lost. Subsequently, the same author interpreted all ventral prominences (haemal keel or vestigial pad) as vestiges of ancestral hypapophyses (Hoffstetter 1968); the presence of hypapophyses throughout the trunk portion of the column was therefore considered a primitive condition, whereas their absence posterior to the anterior trunk portion a derived condition. Underwood (1967) stated that the significance of the presence or absence of hypapophyses remained rather difficult to assess. Underwood (1967) argued that it was simpler to believe that they had been lost several times than to believe that they had been acquired several times.

Rage and Albino (1989), based on the examination of the Cretaceous snake Dinilysia Woodward, 1901, in which hypapophyses were present on anterior trunk vertebrae only, came to the conclusion that the lack of hypapophyses in middle and posterior parts of the trunk region was a primitive character in snake evolution. Their standpoint was additionally supported by the fact that virtually all then known basal snakes (including the extinct Cretaceous simoliophiids [i.e., pachyophiids sensu Zaher et al. 2022] and lapparentophiids) displayed the same condition (Rage and Albino 1989). Similarly, Kluge (1991: character 70) considered the presence of “a weakly developed hypapophyseal keel” throughout the trunk portion to be a plesiomorphy, while “a well developed keel” an apomorphy. In his opinion, the fact that the non-Colubroides caenophidian Acrochordus Hornstedt, 1787, and many snakes of the group Colubroides have a hypapophyseal keel (i.e., hypapophysis) suggested the presence of a more general synapomorphy in the advanced snake clade (Kluge 1991).

Subsequently found specimens of Dinilysia gave more clues on the issue, as they possessed interesting features in the third and fourth anterior trunk vertebrae, with these two elements possessing unfused intercentra articulating with a broad, rounded, and concave hypapophysis (the only other squamates showing this same anatomy are certain Cretaceous marine lizards, including mosasaurs, thus deviating from the typical ophidian condition) (Caldwell and Calvo 2008). In addition, more recently described skeletons of Dinilysia and other two Cretaceous snakes, Haasiophis Tchernov et al., 2000, and Najash Apesteguía & Zaher, 2006, revealed the presence of free cervical intercentra attached to distinct hypapophyses of the centra, suggesting that the post atlas-axis intercentra were subsequently lost in other snake lineages (Palci et al. 2013b; Garberoglio et al. 2019). On the other hand, Zaher and Smith (2020) disputed the presence of an articulated intercentrum in the third anterior trunk vertebra of Najash, revealing that it is instead a broken hypapophysis on the specimen. The same authors treated Pachyrhachis Haas, 1979, Haasiophis, and the Miocene–Pleistocene Australian madtsoiid Yurlunggur Scanlon, 1992, as not possessing cervical intercentra – they only confirmed the presence of cervical intercentra in Dinilysia (Zaher and Smith 2020).

The problem seems to be even more complex considering that the presence of muscular subcentral layer is not necessarily automatically linked to the development of hypapophyses (Mosauer 1935; Gasc 1981). Moreover, function of the hypapophyses of posterior trunk vertebrae in some non-caenophidian snakes (e.g., tropidophiids) remains unclear (Malnate 1972).

It is worth adding that the number of anterior trunk vertebrae bearing hypapophyses seems well correlated with the total number of trunk vertebrae. In other words, in snakes with lower total number of trunk vertebrae also the number of those bearing hypapophyses is lower and vice versa. Notable exception, however, are scolecophidians, in which hypapophyses are present in a few anteriormost vertebrae only; this phenomenon is commonly associated with extreme adaptations to burrowing (e.g., Hoffstetter 1968). Recent works have suggested that the presence of continuous hypapophyses throughout the vertebral column in several groups of snakes was indicative that a “cervical identity” (and therefore not a “thoracic identity”) had prevailed the development of ophidian trunk vertebrae (Caldwell 2003; see Garberoglio et al. 2019; but see also Cohn and Tickle 1999 and Head and Polly 2015 for alternative interpretation). It is nevertheless beyond the scope of this paper to assess the neck area and its extent in snakes, an issue that has been a matter of disagreements since the time of Aristotle and with several studies in the past decades having attempted to decipher the issue (Nopcsa 1908, 1923; Hoffstetter and Gasc 1969; Scanlon 2004; Cohn and Tickle 1999; Caldwell 2000, 2019; Tsuihiji et al. 2006, 2012; see also Scanlon 2004 for a thorough discussion on this subject).

Finally, embryological data have suggested that crown snakes have lost all trunk intercentra except in the atlas-axis complex, with only pedicles, downgrowths of the pleurocentra, present (Gauthier et al. 2012). These authors considered pedicles / hypapophyses to be homologous between lizards and snakes, as their scoring for their characters 465 and 466 shows (Gauthier et al. 2012), an opinion subsequently followed by Garberoglio et al. (2019).

Paracotylar foramina

Paracotylar foramina can be situated on the lateral side(s) of the cotyle; in the Cretaceous pachyophiid Simoliophis Sauvage, 1880, though, these foramina are placed more dorsally (see Rage et al. 2016). The presence of paracotylar foramina has been recognized as a derived condition in snakes. But this feature apparently evolved independently several times in different snake taxa. According to Underwood’s (1976) opinion, paracotylar foramina are assumed to have been independently lost within erycids and boids. The presence of such foramina in Boa (but not in most other boids) would place it as primitive compared to other constrictors. Rage (2001) cast doubt on the phylogenetic value of paracotylar foramina as they are, on one hand, indeed absent in basal extant alethinophidians such as scolecophidians and aniliids, but on the other hand, these are present in several primitive Cretaceous forms. Note also that in certain forms that usually lack paracotylar foramina in their trunk vertebrae, these can be instead present in many caudal vertebrae (e.g., erycids; see also discussion in Georgalis 2019). Finally, it should be also highlighted that is some groups, paracotylar foramina can be really tiny and sometimes their observation in dry skeletons may in fact rely on the quality of their skeletonization techniques – some techniques (e.g., dermestid beetles) leave sometimes enough tissue around the cotyle rims, thus masking the paracotylar foramina, while other techniques (e.g., bacterial maceration in water) are more accurate in rendering tiny paracotylar foramina visible in vertebrae (Agustin Scanferla, personal communication).

Parazygosphenal and parazygantral foramina

These are very rare among snakes, both extant and extinct. Parazygosphenal foramina have been documented only in the acrochordid Acrochordus, the dipsadid Synophis Peracca, 1896, the Eocene palaeophiid Palaeophis colossaeus Rage, 1983 (but not any other species of the genus Palaeophis Owen, 1841), the Eocene thaumastophiids Renenutet McCartney & Seiffert, 2016, and Thaumastophis Rage et al., 2008, the Eocene constrictors Itaboraiophis Rage, 2008, and Paulacoutophis Rage 2008, the Cretaceous lapparentophiids Pouitella pervetus Rage, 1988, and Lapparentophis ragei Vullo, 2019 (but not in the type [and sole other] species of the genus Lapparentophis Hoffstetter, 1959 [i.e., Lapparentophis defrennei Hoffstetter, 1959]), and a few Cretaceous madtsoiids (Bogert 1964; Hoffstetter and Gayrard 1964; Rage 1983, 1988, 2008a; Head 2005; LaDuke et al. 2010; McCartney and Seiffert 2016; McCartney et al. 2018; Vullo 2019; Zaher et al. 2021).

On the other hand, parazygantral foramina (i.e., large foramina on either side of the zygantrum) have been considered a diagnostic trait of the extinct Cretaceous–Quaternary madtsoiids but have also been observed in some other Cretaceous genera (Najash, Seismophis Hsiou et al., 2014, Simoliophis, and Norisophis Klein et al., 2017) as well as the Paleocene palaeophiid-like Tuscahomaophis Holman & Case, 1992, the Miocene Colombophis Hoffstetter & Rage, 1977, and extant hydrophiine elapids (Hoffstetter 1961; Smith 1976; Holman and Case 1992; Scanlon 1992, 1993, 2005; Rage 1998; Rieppel et al. 2002; LaDuke et al. 2010; Vasile et al. 2013; Hsiou et al. 2014; Klein et al. 2017; Rio and Mannion 2017; Gómez et al. 2019; Alfonso-Rojas et al. 2023; but see Rage et al. 2016 for a contrary opinion regarding the presence of this feature in Simoliophis). However, the situation with parazygantral foramina appears to be more complex, because depending on the term’s definition, such foramina are observed in several other groups as well. Indeed, Hoffstetter (1961), upon his description of the parazygantral foramina in madtsoiids, pointed out the presence of smaller such foramina in several extant constrictors but considered that they appear in the latter in an inconsistent and irregular pattern and moreover, are tiny and cannot be compared as being homologous with the respective foramina of madtsoiids – he only noted the exception of Sanzinia for which he suggested that it is the sole extant genus whose foramina had a “distant analogy” (“lointaine analogie”) to those of madtsoiids (Hoffstetter 1961:157). Lee and Scanlon (2002) treated these small pit-shaped foramina of certain boids, even those situated at the edges of the postzygapophyses far from the zygantrum (exemplified by them in Eunectes), as parazygantral foramina, though in their phylogenetic analysis, they scored them differently than the “genuine” single and large-sized parazygantral foramina of madtsoiids. Moreover, the presence of small, pit-shaped parazygantral foramina was also highlighted in Corallus (Onary et al. 2018) as well as in pythonids (Scanlon and Mackness 2001). We also documented here similar foramina in Candoia. Besides, structures that “look like” parazygantral foramina have been observed in an array of caenophidians as well, such as many elapids (see Scanlon et al. 2003), psammophiids (Georgalis and Szyndlar 2022), and natricids (see e.g., Szyndlar 1984, 1991b; Zaher et al. 2019). Therefore, it could be the case that such foramina evolved multiple times (i.e., homoplasy), or that they represent a plesiomorphic character lost more than once, as it was suggested by Klein et al. (2017). Whatever the case, parazygantral foramina of madtsoiids seem to be indeed different than those “similar” foramina of other snakes, because in the former group, all trunk and caudal vertebrae have a parazygantral foramen (or several such foramina) in a more or less distinct fossa situated lateral to each of the zygantral facets (Rage 1988, 1998; Scanlon 2005; Vasile et al. 2013; Rio and Mannion 2017; Gómez et al. 2019) – a similar case with a large parazygantral foramina situated in a distinct fossa is otherwise only observed in a few (i.e., not all) vertebrae of Colombophis (Alfonso-Rojas et al. 2023). Such a distinct fossa is not the case for extant snakes e.g., Candoia and Sanzinia. Alfonso-Rojas et al. (2023) investigated the parazygantral foramina of the South American Miocene taxon Colombophis. These authors applied μCT scanning in order to study the intrernal anatomy of parazygantral foramina and demonstrated that the area around the postzygapophyses and the neural arch is very vascularized and the parazygantral foramina and the associated pits are interconnected with the zygantral and lateral foramina (Alfonso-Rojas et al. 2023).

These being said, both parazygantral and (especially) parazygosphenal foramina could have some taxonomic utility for identifications, but as they occur in distantly related groups, they do not allow any phylogenetic considerations by themselves. Moreover, a stricter definition of parazygantral foramina (potentially linked solely to madtsoiids) should ideally be applied – in any case, the utility and significance of such foramina next to the zygantrum of non-madtsoiid snakes should be handled with cautiousness.

Cotyles and condyles

Scolecophidians, Anilius, Cylindrophis, and uropeltids possess dorsoventrally depressed cotyles and condyles; these structures are more or less orbicular in other non-Colubroides snakes. Lee (1997: character 110) considered an oval shape of cotyles and condyles in mid-trunk vertebrae as a primitive feature and an orbicular shape as an advanced feature.

Additionally, Lee (1997: character 98) and Lee and Scanlon (2002: character 194) considered condyles facing posteriorly a primitive condition, whereas condyles facing posterodorsally an advanced feature. Lee (1997) also observed posteriorly facing condyles in uropeltids, aniliids, xenopeltids, and Dinilysia; however, cylindrophiids have posterodorsally facing condyles.

Depressed vs. vaulted neural arch

A depressed neural arch, typical of lizards, has been considered to be plesiomorphic in snakes. It is observed in many non-caenophidian snakes, but also in several advanced taxa (e.g., several caenophidians, for example the fossorial Xenocalamus Günther, 1868). In general, the degree of vaulting of the neural arch is a highly variable feature within snake groups and is apparently correlated with locomotion and life habits. Apparently though it possesses a taxonomic value for genus / species identification. Georgalis et al. (2021a) quantified the vaulting ratio (the height of the roof of the zygantrum to the distance between the outer edges of the postzygapophyses [measured on the midline], to the half-width of the distance between the outer edges of the postzygapophyses), showing that this value is independent of size and does not vary ontogenetically (see Georgalis et al. 2021a: fig. 89).

Neural spine

Lee (1997: character 100) proposed the “high spinous projection” of neural spines to be a primitive state (lizard-like feature) and the “reduced to low ridge” as a derived state. All basal alethinophidians have low neural spines, whereas these are relatively high in many (but not all) advanced snakes. Again, the height of the neural spine is apparently correlated with locomotion and life habits but still it possesses a taxonomic value for genus / species identification. The anteroposterior length of the neural spine is of importance, particularly the ratio of the neural spine length to the centrum length, which has been applied as a taxonomic feature (see Georgalis et al. 2021a; Smith and Scanferla 2022).

Prezygapophyseal accessory processes

Well-developed processes have been considered an apomorphy. They occur in all scolecophidians, but otherwise, this condition is practically absent among non-caenophidian snakes – the only known exceptions are Casarea and especially, Bolyeria and Eryx jayakari.

The presence of well-developed prezygapophyseal accessory processes in Eryx jayakari is very interesting, considering that the structure is distinctly reduced in other members of this well-defined genus.

Although prezygapophyseal accessory processes projecting laterally (or anteriorly) beyond the articular facets in dorsal view in virtually all snakes, this is not the case for Trachyboa gularis, where these structures are expanded posteriorly in dorsal view, a unique trait not observed in any other snake.

Paradiapophyses

Paradiapophyses (facets for ribs), also known as synapophyses, can be in some snakes divided into distinct upper portions (diapophyses) and lower portions (parapophyses) or in others, the diapophyseal and parapophyseal portions cannot be clearly differentiated. In most non-Colubroides, paradiapophyses are relatively simply built. In scolecophidians, Anilius, Cylindrophis, and uropeltids the paradiapophysis is a vertically oval convexity that fits into a corresponding oval concavity on the rib. In other non-Colubroides it can be slightly divided into the upper (diapophyseal) and lower (parapophyseal) portions. This division is, however, not clear and never approaches the truly derived condition characteristic for the Colubroides. It is worth noting that in certain palaeophiid snakes, the bases of the paradiapophyses originate very close to each other – this is observed at an extreme in the species Pterosphenus kutchensis Rage et al., 2003, from the Eocene of India, where, uniquely among all snakes, the long paradiapophyses originate at a common base or their bases are only very narrowly separated (Rage et al. 2003).

Pterapophyses

This is a very rare structure in snake vertebrae, observed solely in Erycidae, Charinaidae, the tropidophiid Trachyboa, and especially, the extinct Palaeophiidae, but it is anyway doubtful whether these structures across these mentioned clades are indeed homologous. In some taxa they are prominent (e.g., trunk vertebrae of palaeophiids, caudal vertebrae of erycids and charinaids), while in (the trunk, cloacal, and caudal vertebrae of) Trachyboa they are small, having the shape of distinct tubercles on the neural arch. It is further worth noting that Nopcsa (1923) also reported that the anterior trunk vertebrae of scolecophidians bear similar bumps above their postzygapophyses, which he regarded as corresponding to the pterapophyses of Palaeophiidae.

Other features of trunk vertebrae

Several other useful characters of trunk vertebrae, in part based on fossil snakes, have been documented in the literature and/or used in phylogenetic analyses (e.g., Rage 1984, 1987, 1988; Lee and Scanlon 2002; Scanferla and Canale 2007; Gauthier et al. 2012; Smith 2013; Palci et al. 2020; Zaher and Smith 2020). Such characters include: the depression of the vertebrae; the presence or absence of parapophyseal processes; the depth of the posterior median notch of the neural arch seen in dorsal view; the arqual ridges (sensu Scanferla and Canale 2007) of the neural arch; the direction of the major axis of the prezygapophyseal articular facets; the subcentral foramina that can be uniform (small and paired throughout the vertebrae) or asymmetrical / irregular (either small and paired, absent, or single and large, in different vertebrae; e.g., typhlopids); the presence or absence of a precondylar constriction.

Additional apophyses on caudal vertebrae

The presence of additional apophyses on caudal vertebrae was repeatedly raised as a synapomorphy shared by members of erycids and charinaids (Hoffstetter and Rage 1972; Underwood 1976; Kluge 1993b). Hoffstetter (1955) considered these caudal peculiarities of vertebrae of erycids and charinaids as indicative of their fossorial habits, making a parallelism with their overall “resemblance” with the dorsal vertebrae of the African fossorial soricid mammal Scutisorex Thomas, 1913. The latter genus is the only mammal in which most of its dorsal vertebrae interlock with a bone to bone articulation, representing the most extreme vertebral modification among all mammals (Allen 1917; Stanley et al. 2013; Smith and Angielczyk 2020). This complex vertebral morphology in Scutisorex is characterized by numerous accessory intervertebral articulations and massively expanded transverse processes of most of its dorsal vertebrae, providing them with an incredible strength that allows this tiny (ca. 50–70 g) mammal to lift very heavy objects (Stanley et al. 2013; Smith and Angielczyk 2020), with cases of even 70 kg humans standing on its back for several minutes (Allen 1917). Of course, the vertebral accessory processes of this mammal are much different than those of erycids and charinaids but in any case, it denotes that complex vertebral morphologies can evolve across different vertebrate lineages.

It is further worth noting that a complex morphology is also present in the posteriormost caudal vertebra of the elapid Toxicocalamus ernstmayri O’Shea, Parker & Kaiser, 2015, from Papua New Guinea, but is absent from all other congeneric species of the Papuan genus Toxicocalamus Boulenger, 1896 (O’Shea et al. 2015, 2018; Roberts et al. 2022). Judging from the single published X-ray image (see O’Shea et al. 2018: fig. 8H), the complex nature of posteriormost caudal vertebrae of Toxicocalamus ernstmayri is evident, yet it cannot be ascertained whether and/or how these structures resemble those of erycids and/or charinaids. A detailed description and figuring of these caudal vertebrae of Toxicocalamus is highly anticipated but in any case, we consider that any resemblance should only be superficial. In addition, a highly peculiar tail morphology is observed in the Iranian viperid Pseudocerastes urarachnoides Bostanchi et al., 2006, that forms a knob-like structure (Bostanchi et al. 2006); such structure facilitates its life habits, with the species using its tail to lure its prey (Martínez del Marmol et al. 2016).The single published X-ray image of the holotype of Pseudocerastes urarachnoides depicts the caudal vertebrae (Bostanchi et al. 2006: figs 11–12), however, the figure is not adequate to deduce any conclusion about the vertebral morphology. It is worth mentioning that, similarly to the case of Toxicocalamus above, such tail morphology is absent (or at least not to that extent) in other species of the genus Pseudocerastes Boulenger, 1896. These being said, it is evident that peculiar caudal vertebral morphologies can evolve in distantly related groups and moreover, these can differ substantially even within a single genus. In any case, such structures are of utmost taxonomic utility, being quintessential in genus and species level identifications of fossil remains.

Subcentral structures around the cloaca

It seems that subcentral structures in the last trunk and anterior cloacal vertebrae develop independently from those in the preceding and following parts of the column. This view is supported by the fact that small hypapophyses (or at least some tubercle-like structures) appear on the last trunk vertebra (sometimes two, rarely three last trunk vertebrae) even in the case when no subcentral structures are present on preceding trunk vertebrae. Even in snakes possessing hypapophyses throughout the trunk portion of the column, the hypapophyses on the last trunk and cloacal vertebrae display somewhat different morphology (e.g., Viperidae). Indeed, there is a substantial difference between the hypapophyses present on posterior trunk vertebrae (“posterior hypapophyses” in Underwood’s writings) and those on last trunk and cloacal vertebrae (“cloacal hypapophyses” of Hoffstetter) (see Hoffstetter 1961, 1968). In succeeding vertebrae of the cloacal region, i.e., usually posterior cloacal vertebrae, the subcentral structures disappear or diminish in size. Then, they are followed by morphologically different structures of caudal vertebrae. Smith (2013) suggested that the presence of hypapophyses around the cloacal area (his pericloacal vertebrae) represents a derived condition.

Ophidian haemapophyses

The haemapophyses of snakes are structures that project ventrally from the centra of the caudal vertebrae (and sometimes, more rarely, also of the cloacal vertebrae), which almost never contact each other distally. Some exceptions to this “never contact rule” of haemapophyses have nevertheless been described, particularly among males of some extant colubrids (in which the distal tips of the haemapophyses may be more elongate and bent inwards contacting at the midline; Keiser 1970) or in some extant hydrophiine elapids (in which the haemapophyses can be very elongated and sometimes fused distally to form a complete arch; McDowell 1969). Other deviations in the shape of the haemapophyses can also occur. In one caenophidian, additional appendages of haemapophyses are present: Periergophis Georgalis et al., 2019, from the Neogene of Greece possesses distinct tubercles in its haemapophyses, termed as “haemapophyseal tubercles” (Georgalis et al. 2019). Interestingly, the same taxon possessed distinct tubercles also in the haemal keel of its posterior trunk vertebrae (termed “haemal keel tubercles”; see Georgalis et al. 2019). On the other hand, there are certain snake groups in which only a single process is observed in the postcloacal vertebrae (e.g., Uropeltidae) or in which subcentral structures are totally or nearly absent in the caudal series.

In lizards, ventral pedicles (also spelled in the literature as peduncles or pedicels) for articulation with the chevrons are prominently developed in several extant and extinct large anguimorphs (Hoffstetter and Gasc 1969; Gauthier et al. 2012; Georgalis et al. 2021d) and their presence has been considered a synapomorphy (Rieppel 1980). Among snakes, such pedicles for articulation with chevrons are present only in madtsoiids and some pachyophiids (e.g., Rage and Escuillié 2000; Scanlon and Lee 2000; LaDuke et al. 2010; Vasile et al. 2013).

Haemapophyses have been considered to represent the fusion of the caudal intercentra to the vertebral pleurocentra in snakes (Hoffstetter and Gasc 1969; Rieppel and Head 2004), while other workers regarded that extant taxa have lost the intercentra throughout the vertebral column (with the exception of the atlas-axis complex) and that the haemapophyses are actually downgrowths of the vertebral centra and, as such, homologous to the haemapophyses of lizards (e.g., Gauthier et al. 2012; Palci et al. 2013b). It has also been suggested that haemapophyses are homologous with the hypapophyses present in preceding vertebrae and that they represent remnants of the intercentra (Keiser 1970). Smith (2013) considered that, although paired haemapophyses can be absent in some extant basal snakes, the fact that these are present (partially) in Tropidophiidae, in Najash, in most Constrictores, as well as in several extant anguimorph lizards, was strongly indicative that the absence of paired haemapophyses in all postcloacal vertebrae represented a derived character.

Consequently, it could be regarded that the presence of paired haemapophyses potentially represents the most primitive condition in snakes. In that case, there are two independent derivatives of this primitive state: first, the absence (reduction) of any subcentral structures in caudal vertebrae (of scolecophidians and other lower snakes); second, the transformation of paired haemapophyses into a single hypapophysis (as in uropeltids). The latter supposition is based on the observation of the tropidophiids Trachyboa and Tropidophis, which possess on their caudal vertebrae partly “true” hypapophyses and partly grooved distally hypapophyses. A possible opposite transformation, i.e., the split of a hypapophysis into paired haemapophyses, would seem less probable. An intermediate condition, namely the incomplete reduction of haemapophyses along with their partial transformation into a single structure, can be observed in the sanziniids Acrantophis and Sanzinia (as well as in the extinct Oligocene snake Rottophis Szyndlar & Böhme, 1996; see Szyndlar and Böhme 1996), though this reduction in Sanzinia could be ontogenetically (or generally intraspecifically) variable (Smith 2013).

Fossil skeletons of Cretaceous snakes have shed some valuable light on the evolution of these structures. Indeed, recently described skeletal material of Dinilysia, Eupodophis Rage & Escuillié, 2002, Haasiophis, and Najash, as well as the Pleistocene madtsoiid Wonambi naracoortensis Smith, 1976, revealed a caudal pattern that deviates from that of other known snakes (Scanlon and Lee 2000; Rieppel et al. 2002; Rieppel and Head 2004; Palci et al. 2013b; Garberoglio et al. 2019): in these taxa, free unfused postcloacal intercentra (true chevrons) were articulated with distinct ventral projections of the centrum (haemapophyses); a similar case was observed also in the anterior trunk vertebrae, with free cervical intercentra attached to distinct hypapophyses of the centra (Palci et al. 2013b; Garberoglio et al. 2019). Garberoglio et al. (2019) accordingly suggested that in basal fossil snakes, free unfused intercentra were present in the anterior trunk (“cervical”) and caudal vertebrae, and they were subsequently lost in later evolving snakes. On the basis of the Dinilysia example, Garberoglio et al. (2019) further suggested that the presence of these unfused postcloacal intercentra (true chevrons) articulating with ventral projections of the centrum, was indicative of the scenario that haemapophyses of extant snakes were homologous to the caudal subcentral structures of lizards, with chevron elements being totally lost. The condition in some of the Cretaceous snakes seems to be analogous to the condition in Varanidae, where hypapophyses (“downgrowths” or “pedicles”) and intercentra are present in the cervical region, and pedicles and chevrons are present in the tail, while this seems to be the case potentially also in glyptosaurids (Sullivan 1979).

Another important point is the presence of paired haemapophyses in cloacal vertebrae. We observed this condition only very rarely in non-Colubroides (see above Epicrates cenchria, Morelia spp., and Xenopeltis unicolor [and even in one specimen of Python molurus]). Moreover, in most non-Colubroides snakes, haemapophyses are absent also on the first or more caudal vertebrae (such vertebrae are also called pygals); this condition is reminiscent of that occurring in most lizards (Hoffstetter and Gasc 1969; Garberoglio et al. 2019).

The presence of haemapophyses in cloacal vertebrae is typical for Colubroidea (Colubridae s.l. of older literature), however, their distribution throughout the cloacal portion of the column differs across various genera. In natricids, for instance, the haemapophyses are present on posterior cloacal vertebrae, whereas more anterior cloacal vertebrae are provided with hypapophyses. But in certain colubrid genera (e.g., Coluber Linnaeus, 1758), well-developed haemapophyses are present on all cloacal vertebrae (or even on the last trunk vertebra). Without extensive studies of axial skeletons of colubroid snakes it is impossible to estimate which condition is prevailing in this huge and diverse assemblage.

The Elapidae (most African and Asiatic genera examined) are similar in the above aspect to natricids and possess haemapophyses on the last cloacal vertebra only. The elapid genus Walterinnesia Lataste, 1887 (five specimens examined) approaches the condition of non-Colubroides: the haemapophyses are absent in the cloacal portion of the column. In other elapoids, distinct patterns can be observed. For example, in the pseudoxyrhophiid Duberria lutrix (Linnaeus, 1758) there are no haemapophyses in caudal vertebrae but instead there is a thick hypapophysis (or prominent haemal keel, depending on the definition) (McCartney et al. 2014). Other pseudoxyrhophiids nevertheless, have typical haemapophyses in their caudal vertebrae (McCartney et al. 2014). In atractaspidids paired haemapophyses are present in the caudal series (Thireau 1967) and the same seems to be also the case with cyclocorids (Weinell et al. 2020), though in all species of the latter lineage at least some or even many caudal vertebrae also possess single hypapophyses or at least slightly grooved hypapophyses (Morphosource.org: KUBI H 330056 [Media 000047780; ark:/87602/m4/M47780], KUBI H 203012 [Media 000049617; ark:/87602/m4/M49617], KUBI H 329413 [Media 000045319; ark:/87602/m4/M45319], KUBI H 337269 [Media 000045320; ark:/87602/m4/M45320], and KUBI H 323386 [Media 000047812; ark:/87602/m4/M47812]). In certain psammophiids, such as Malpolon Fitzinger, 1826, haemapophyses can appear even from the last trunk vertebra (see Georgalis and Szyndlar 2022: fig. 7).

Virtually all members of the Viperidae (almost all genera examined) display a very distinct pattern, where haemapophyses in the anterior cloacal (as well as last trunk) vertebrae resemble (or are replaced by!) distally forked hypapophyses; “normally” built haemapophyses appear on the posterior (last) cloacal vertebrae. This peculiar condition, observed in all studied viperid species, seems unique among snakes.

Comparisons and taxonomic implications

The following vertebral features can be observed in scolecophidians, cylindrophiids, and uropeltids: elongate centrum, depressed cotyle and condyle, depressed neural arch, absent or very shallow median notch of the neural arch, absent or poorly developed haemal keels in mid- and posterior trunk vertebrae, vestigial (or absent) neural spine shifted posteriorly, and very low number of caudal vertebrae. It is necessary to add that (contrary to opinions of some authors) the neural spines (or rather, vestiges of the spine) of the above snakes seem to be similar morphologically to one another; the only significant difference appears to be that the spine of scolecophidians is present in anterior trunk vertebrae only, whereas it is (hardly) visible in more posterior vertebrae of the other two groups. Also, the absence (or shallowness) of the posterior median notch of the neural arch is evident in the three aforementioned ophidian groups, although not in all species. The haemal keel of trunk vertebrae is poorly developed in aniliids and cylindrophiids but in none of these groups is it totally absent, marking a smooth centrum as in scolecophidians. Notably, the smooth centrum of scolecophidian vertebrae is reminiscent of the situation in some fossorial lizards (e.g., amphisbaenians, see Georgalis et al. 2018; dibamids, see Xing et al. 2018; certain anguids, see Čerňanský et al. 2019). No subcentral structures are visible in the caudal portion of the column of either scolecophidians or Cylindrophis (except for a moderately developed ridge-like keel in the last cloacal and two succeeding caudal vertebrae of Cylindrophis ruffus), whereas uropeltids possess prominent hypapophyses throughout the cloacal and caudal portions. We consider these similarities as merely due to convergence. As for anomochilids, their vertebral morphology remains unknown.

Trunk vertebrae of scolecophidians and uropeltids are strikingly similar to each other by their general appearance; in both groups (unlike in other living snakes) the direction of the major axis of the prezygapophyseal articular facets approximates the direction of the major axis of the vertebra. However, scolecophidians differ from both uropeltids and almost all remaining non-Colubroides by having relatively long prezygapophyseal accessory processes. It is worth noting that Dowling (1975) and Dowling and Duellman (1978) envisaged an expanded concept of Typhlopoidea, which included both scolecophidians and uropeltids. Of course, all these similarities are strictly linked to their similar fossorial ecology and do not reflect any close relationships. At the same time, the monophyly of the Uropeltidae is well grounded, also supported by the presence of hypapophyses in caudal vertebrae.

Within scolecophidians, it is almost impossible to reliably distinguish vertebrae of leptotyphlopids, typhlopoids, and anomalepidids from each other. Some characters could indeed be useful for family-level identifications (e.g., the presence of a single subcentral foramen in many vertebrae of typhlopids), but the variability of these features should first be adequately assessed in multiple vertebrae of multiple taxa.

Vertebrae of Anilius are in many aspects similar to those of the above-mentioned snakes: elongate centrum, depressed cotyle and condyle, depressed neural arch, shallow (but not absent) median notch of the neural arch, very low number of caudal vertebrae, and lack of haemapophyses or hypapophyses in caudal vertebrae. However, vertebrae of Anilius are heavily built in comparison with those of the aforementioned snakes and provided (in the anterior trunk portion) with a prominent (very thick and plate-like in shape) hypapophysis, replaced (in more posterior vertebrae) by a distinct haemal keel. The neural spine, although strongly reduced, occupies most of the neural arch length and by no means can be considered vestigial.

The old traditional concept of Tropidophiidae has been shown to be paraphyletic, with true Tropidophiidae lying next to aniliids, while ungaliophiids ranked within booids. Vertebral morphology fully corroborates the distinction, as it considerably differs in Tropidophis and Trachyboa (Tropidophiidae) on one hand, and Ungaliophis and Exiliboa (Ungaliophiidae) on the other.

A number of peculiarities shared by Tropidophis and Trachyboa, especially the presence of a hypapophysis (haemal keel of other authors) in posterior trunk vertebrae, followed by a hypapophyses and haemapophyses in cloacal and caudal vertebrae, demonstrate well the distinctiveness of Tropidophiidae. However, the supposed close affinities of tropidophiids with aniliids inferred by most molecular phylogenetic analyses are not corroborated at all by vertebral morphology, which remains rather distinct among the two groups.

Most members of booids and pythonoids, collectively grouped into Constrictores, display virtually a similar generalized pattern in their vertebral morphology: massively built vertebrae, with a generally low ratio of centrum length / neural arch width (<1.1), a high neural spine, a relatively thick zygosphene, and a distinct haemal keel in the middle and posterior trunk portion, followed by caudal vertebrae provided with haemapophyses. However, several ingroups deviate significantly from the generalized Constrictores vertebral pattern (e.g., ungaliophiids, xenopeltids, and candoiids).

The family Bolyeriidae, comprising Bolyeria and Casarea, is the one of the only groups (along with Tropidophiidae and Candoiidae) within the non-caenophidian snakes, possessing hypapophyses throughout the trunk portion of the column, a condition characteristic for several lineages of the Caenophidia Hoffstetter, 1939. Notably, the hypapophyses of bolyeriids are more prominent than in tropidophiids or candoiids. Additionally, the change of hypapophyses into haemapophyses in the cloacal/caudal transition that is observed in Bolyeriidae is somehow reminiscent of the condition typical for snakes of the group Colubroides, in particular natricids and elapids. Bolyeriids further resemble Colubroides in the light construction of their trunk vertebrae. For the sister group of bolyeriids, the enigmatic Xenophidiidae, the vertebral morphology remains unknown. Judging, however, from the single published description of Wallach and Günther (1998), xenophidiids also possess hypapophyses in their posterior trunk vertebrae, which are also characterized by expanded blade-like haemapophyses (larger than the neural spines) in their caudal vertebrae. Bolyeriids and xenophidiids are apparently closely related indeed but their exact affinities within Constrictores (or even their inclusion in that group) remain uncertain.

Within Booidea, the typical constrictor vertebral morphology is retained in Boidae, but besides there are several distinct derivatives from the above standard. Of particular notice are the two Malagasy genera Sanzinia and Acrantophis which make up the family Sanziniidae and both possess haemapophyses reduced to keels (partly grooved in Sanzinia – though this reduction in that latter genus could be ontogenetically variable; Smith 2013). In addition, a few species of the Pacific Islands genus Candoia (Candoiidae) have hypapophyses throughout the trunk portion of the column. The above morphological peculiarities, along with geographical isolation, support placement of these two lineages in their own distinct families, Sanziniidae and Candoiidae respectively.

Vertebrae of Calabaria do not differ from the generalized booid pattern except for the caudal vertebrae devoid of haemapophyses. Such pattern on the caudal vertebrae denies the placement of Calabaria in erycids as proposed by Kluge (1993b). We consider that Calabaria is aptly placed in its own booid family.

The small burrowing snakes Eryx (Erycidae) as well as Charina and Lichanura (Charinaidae) share the presence of additional apophyses in the caudal vertebrae, a feature unique among snakes. According to molecular phylogenetic analyses, Charinaidae represents the sister group of Ungaliophiidae instead of Erycidae, however, such relationship is not reflected by vertebral morphology.

The most striking condition of trunk vertebrae of Ungaliophiidae (genera Ungaliophis and Exiliboa) is their elongation and light construction. Another important distinct feature observed in Ungaliophis is the presence of a keel instead of haemapophyses in caudal vertebrae. Such keel is present throughout the caudal series and only disappears near the tip of the tail.

Vertebrae of Xenopeltis possess a number of peculiarities in their morphology. Its vertebral morphology strongly differs from other snake groups. Its heavily built and elongated (centrum longer than wide) vertebrae differ distinctly from other snakes by a number of distinct features, especially the strange hypapophyses on anterior (but not ateriormost) trunk vertebrae as well as the shape of the neural spine. The shift of haemapophyses from caudal to cloacal vertebrae observed in this genus is somehow reminiscent of the similar condition in Colubroides.

Loxocemus possesses an axial skeleton that it is consistent with the morphology of several other constrictors, however, distinctive features in certain vertebral structures (in particular the shape of the subcentral ridges) do exist.

Finally, the vertebral morphology of the Pythonidae (except mainly for Python curtus and some Australian taxa) is relatively homogenous. In many aspects and general morphology, vertebrae of pythonids are rather reminiscent of those of large boids.

Acknowledgements

This project was conceived as an idea almost four decades ago, when one of us (ZS) was intensively studying snake skeletons in multiple institutes across the word, meticulously focusing on intracolumnar and intraspecific variability, and making these detailed drawings that appear in this work. ZS made the first such drawing in 1989 in Paris (Université Paris VI [= Université Pierre & Marie Curie]): this was the specimen MNHN-AC-1909.0007 of Xenopeltis unicolor (Figs 133–135 in the present work). Subsequently, the largest amount of these drawings was made by ZS during his stay in Bonn (ZFMK; 1991–1992), followed by several other drawings of specimens conducted in other parts of Europe, North America, Australia, and of course, his host institute, the Institute of Systematics and Evolution of Animals, in the Polish Academy of Sciences in Krakow (ISEZ).

For this project we studied a vast array of specimens from multiple Institutions throughout the globe, and therefore we are sincerely grateful to a large number of colleagues and curators which made this study feasible. More particularly, for the loan and/or access of specimens under their care, we would like to thank Wolfgang Böhme (ZFMK), Claudia Corti (MSNS), Patrick Couper (QM), Massimo Delfino (MGPT-MDHC), Darrel Frost (AMNH), Heinz Grillitsch, Silke Schweiger, and Georg Gassner (NHMW), Arnold Kluge, Gregory Schneider, and Fred Kraus (UMMZ), Colin McCarthy (NHMUK), Jean-Claude Rage, Ivan Ineich, Nour-Eddine Jalil, and Salvador Bailon (MNHN), Anatoly Tokar (SIZNASU), Jens Vindum (CAS), Harold Voris (FMNH), Van Wallach and Stevie Kennedy-Gold (MCZ), Emily Braker (UCM), David Boyd (LSUMZ), Georg Zug (USNM), Marta Calvo Revuelta (MNCN), Judit Vörös (HNHM), Bartosz Borczyk and Tomasz Skawinski (MNHW), Gunther Köhler and Linda Mogk (SMF), and Krister Smith (SMF PH).

New μCT images of the holotype of Epacrophis boulengeri (SMF 16700) were kindly provided by Krister Smith. 3D models of Liotyphlops beui (SAMA R40142) were kindly provided by Alessandro Palci (Flinders University, Adelaide). 3D models of Typhlophis squamosus (MNHN-RA-1999.8306) were kindly provided by Anthony Herrel (MNHN) and Aurélien Lowie (Ghent University). 3D models of Gerrhopilus mirus (FMNH 178534 [Morphosource.org: Media 000413003, ark:/87602/m4/413003 and Media 000413000, ark:/87602/m4/413000]) were kindly provided by Jaimi Gray (UF Herp). 3D models of Anomalepis mexicana (FMNH 22853 [Morphosource.org: Media 000383763, ark:/87602/m4/383763] and MCZ Herp R-29220 [Morphosource.org: Media 000415858, ark:/87602/m4/415858]) were kindly provided by Juan Daza (Sam Houston State University, Huntsville) and Camilo Andres Linares-Vargas (Universidad del Valle, Cali). 3D models of Amerotyphlops brongersmianus (FMNH 195928 [Morphosource.org: Media 000065157, ark:/87602/m4/M65157]) were kindly provided by Agustin Scanferla (CONICET). Claudia Koch (ZFMK) kindly provided an X-ray image of the holotype of Epictia guayaquilensis (ZMB 4508). Camilo Andres Linares-Vargas and Juan Daza kindly provided X-rays of Liotyphlops bondensis. John Scanlon kindly provided photographs of Apodora papuana (AMS R16488). Petros Lymberakis (NHMC) kindly skeletonized a specimen (NHMC80.3.114.20) of Eryx jaculus. Dave Cundall (Lehigh University) kindly provided the X-ray of Anomochilus leonardi (NHMUK 1946.1.17.4). For facilitating access to specimens from Morphosource, we acknowledge Stephanie Baumgart, David Boyd, David Blackburn, Emily Braker, Rafe Brown, Carla Cicero, Matt Gage, Rich Glor, Sharon Grant, Jaimi Gray, Anthony Herrel, Janeen Jones, Stevie Kennedy-Gold, Michelle Koo, Coen Long, Joshua Mata, Ana Motta, Ramon Nagesan, April Neander, Shion Otsuka, Gregory Pandelis, Zachary Randall, Laura Rincón, Nelson Rios, Gregory Schneider, Mackenzie Shepard, Carol Spencer, Edward Stanley, Greg Watkins-Colwell, Kate Webbink, Tina Wu, and Alice Zhang.

For sharing valuable unpublished information on vertebral counts of different taxa, we are grateful to Sarin Tiatragul (Australian National University, Canberra), Claudia Koch (ZFMK), Jason Head (University of Cambridge), Jingsong Shi (IVPP), John Scanlon, Agustin Scanferla (CONICET), Petros Lymberakis (NHMC), and Krister Smith (SMF). For help with 3D segmentation of μCT scanned data, we thank Kacper Węgrzyn (Warsaw University of Life Sciences). For her valuable help with the literature, we specially thank Beata Babicz (ISEZ).

Special thanks go also to Michael Lee, John Scanlon, and the late Jean-Claude Rage and Garth Underwood who critically read an older, preliminary version of this paper. The quality of the manuscript was enhanced by the thorough and valuable comments and suggestions made by the editor Uwe Fritz and the three reviewers, Agustin Scanferla, Martin Ivanov, and Krister Smith.

ZS acknowledges support from the grant 6 P04C 020 08 of the Committee of Scientific Research of Poland (1995–1997) and a research fellowship awarded by the Alexander von Humboldt Foundation of Germany (1991–1992); on this opportunity, ZS expresses his cordial thanks to Wolfgang Böhme and other colleagues from (then) the Herpetologische Abteilung of the Alexander Koenig Museum (now ZFMK) for two beautiful years he spent in Bonn. GLG acknowledges funding from the Ulam Program of the Polish National Agency for Academic Exchange (PPN/ULM/2020/1/00022/U/00001), as well as from the Forschungskredit of the University of Zurich, grant no. [FK-20-110], travel grants SYNTHESYS ES-TAF-5910 (MNCN), SYNTHESYS AT-TAF-5911 (NHMW), SYNTHESYS HU-TAF-6145 (HNHM), SYNTHESYS GB-TAF-6591 (NHMUK), and SYNTHESYS FR-TAF_Call4_035 (MNHN), and a travel grant from Marcelo Sánchez-Villagra (PIMUZ) and the Georges and Antoine Claraz-Donation for enabling him to travel and study multiple snake skeletons. We finally thank the oVert Project (NSF DBI: 1701714: D. Blackburn, G. Erbach, V. Behari, S.P. Vijayakumar and E. Stanley) and the Morphosource repository (Duke University; https://www.Morphosource.org) for facilitating access to the online μCT scan data of snake skeletons.

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Appendix 1

List of genera of material examined (alphabetically).

Acrantophis (Sanziniidae)

Acutotyphlops (Typhlopidae)

Afrotyphlops (Typhlopidae)

Amerotyphlops (Typhlopidae)

Anilios (Typhlopidae)

Anilius (Aniliidae)

Anomalepis (Anomalepididae)

Anomochilus (Anomochilidae)

Antaresia (Pythonidae)

Antillotyphlops (Typhlopidae)

Apodora (Pythonidae)

Argyrophis (Typhlopidae)

Aspidites (Pythonidae)

Boa (Boidae)

Bolyeria (Bolyeriidae)

Bothrochilus (Pythonidae)

Brachyophidium (Uropeltidae)

Calabaria (Calabariidae)

Candoia (Candoiidae)

Casarea (Bolyeriidae)

Charina (Charinaidae)

Chilabothrus (Boidae)

Corallus (Boidae)

Cubatyphlops (Typhlopidae)

Cylindrophis (Cylindrophiidae)

Epacrophis (Leptotyphlopidae)

Epicrates (Boidae)

Epictia (Leptotyphlopidae)

Eryx (Erycidae)

Eunectes (Boidae)

Exiliboa (Ungaliophiidae)

Gerrhopilus (Gerrhopilidae)

Grypotyphlops (Typhlopidae)

Helminthophis (Anomalepididae)

Indotyphlops (Typhlopidae)

Leiopython (Pythonidae)

Leptotyphlops (Leptotyphlopidae)

Liasis (Pythonidae)

Lichanura (Charinaidae)

Liotyphlops (Anomalepididae)

Loxocemus (Loxocemidae)

Madatyphlops (Typhlopidae)

Malayopython (Pythonidae)

Melanophidium (Uropeltidae)

Mitophis (Leptotyphlopidae)

Morelia (Pythonidae)

Myriopholis (Leptotyphlopidae)

Platyplectrurus (Uropeltidae)

Plectrurus (Uropeltidae)

Python (Pythonidae)

Rena (Leptotyphlopidae)

Rhinophis (Uropeltidae)

Rhinotyphlops (Typhlopidae)

Sanzinia (Sanziniidae)

Siagonodon (Leptotyphlopidae)

Simalia (Pythonidae)

Teretrurus (Uropeltidae)

Tetracheilostoma (Leptotyphlopidae)

Trachyboa (Tropidophiidae)

Tricheilostoma (Leptotyphlopidae)

Tropidophis (Tropidophiidae)

Typhlophis (Anomalepididae)

Typhlops (Typhlopidae)

Ungaliophis (Ungaliophiidae)

Uropeltis (Uropeltidae)

Xenopeltis (Xenopeltidae)

Xerotyphlops (Typhlopidae)

Appendix 2

List of species of material examined (alphabetically).

Acrantophis dumerili

Acutotyphlops kunuaensis

Afrotyphlops punctatus

Afrotyphlops steinhausi

Amerotyphlops brongersmianus

Amerotyphlops microstomus

Anilios erycinus

Anilios torresianus

Anilius scytale

Anomalepis mexicana

Anomochilus leonardi

Antaresia childreni

Antillotyphlops hypomethes

Apodora papuana

Argyrophis diardii

Argyrophis muelleri

Aspidites melanocephalus

Boa constrictor

Bolyeria multocarinata

Bothrochilus boa

Brachyophidium rhodogaster

Calabaria reinhardtii

Candoia aspera

Candoia bibroni

Candoia carinata

Casarea dussumieri

Charina bottae

Chilabothrus angulifer

Chilabothrus subflavus

Corallus caninus

Corallus cropanii

Corallus hortulana

Cubatyphlops biminiensis

Cylindrophis maculatus

Cylindrophis ruffus

Epacrophis boulengeri

Epicrates cenchria

Epictia albifrons

Epictia ater

Epictia borapeliotes

Epictia columbi

Epictia guayaquilensis

Eryx colubrinus

Eryx conicus

Eryx elegans

Eryx jaculus

Eryx jayakari

Eryx johnii

Eryx miliaris

Eryx muelleri

Eryx tataricus

Eunectes murinus

Eunectes notaeus

Exiliboa placata

Gerrhopilus mirus

Grypotyphlops acutus

Helminthophis frontalis

Indotyphlops braminus

Leiopython albertisii

Leptotyphlops nigricans

Leptotyphlops scutifrons

Liasis mackloti

Lichanura orcutti

Lichanura trivirgata

Liotyphlops albirostris

Liotyphlops beui

Liotyphlops bondensis

Loxocemus bicolor

Madatyphlops arenarius

Malayopython reticulatus

Malayopython timoriensis

Melanophidium wynaudense

Mitophis pyrites

Morelia spilota

Morelia viridis

Myriopholis longicauda

Platyplectrurus madurensis

Platyplectrurus trilineatus

Plectrurus perroteti

Python bivittatus

Python curtus

Python molurus

Python regius

Python sebae

Rena humilis

Rena myopica

Rena segrega

Rhinophis blythii

Rhinophis sanguineus

Rhinotyphlops lalandei

Sanzinia madagascariensis

Siagonodon borrichianus

Siagonodon cupinensis

Siagonodon septemstriatus

Simalia amethistina

Simalia boeleni

Tetracheilostoma bilineatum

Teretrurus sanguineus

Trachyboa boulengeri

Trachyboa gularis

Tricheilostoma bicolor

Tropidophis canus

Tropidophis feicki

Tropidophis greenwayi

Tropidophis haetianus

Tropidophis jamaicensis

Tropidophis melanurus

Tropidophis semicinctus

Tropidophis taczanowskyi

Typhlophis squamosus

Typhlops gonavensis

Typhlops lumbricalis

Ungaliophis continentalis

Uropeltis arcticeps

Uropeltis ceylanica

Uropeltis melanogaster

Uropeltis woodmasoni

Xenopeltis unicolor

Xerotyphlops syriacus

Xerotyphlops vermicularis

Appendix 3

List of genera for which numbers of vertebrae are provided (alphabetically).

Acrantophis (Sanziniidae)

Acutotyphlops (Typhlopidae)

Afrotyphlops (Typhlopidae)

Amerotyphlops (Typhlopidae)

Anilios (Typhlopidae)

Anilius (Aniliidae)

Anomalepis (Anomalepididae)

Anomochilus (Anomochilidae)

Antaresia (Pythonidae)

Antillotyphlops (Typhlopidae)

Apodora (Pythonidae)

Argyrophis (Typhlopidae)

Aspidites (Pythonidae)

Boa (Boidae)

Bolyeria (Bolyeriidae)

Bothrochilus (Pythonidae)

Brachyophidium (Uropeltidae)

Calabaria (Calabariidae)

Candoia (Candoiidae)

Casarea (Bolyeriidae)

Charina (Charinaidae)

Chilabothrus (Boidae)

Corallus (Boidae)

Cubatyphlops (Typhlopidae)

Cyclotyphlops (Typhlopidae)

Cylindrophis (Cylindrophiidae)

Epacrophis (Leptotyphlopidae)

Epicrates (Boidae)

Epictia (Leptotyphlopidae)

Eryx (Erycidae)

Eunectes (Boidae)

Exiliboa (Ungaliophiidae)

Gerrhopilus (Gerrhopilidae)

Grypotyphlops (Typhlopidae)

Habrophallos (Leptotyphlopidae)

Helminthophis (Anomalepididae)

Indotyphlops (Typhlopidae)

Leiopython (Pythonidae)

Leptotyphlops (Leptotyphlopidae)

Letheobia (Typhlopidae)

Liasis (Pythonidae)

Lichanura (Charinaidae)

Liotyphlops (Anomalepididae)

Loxocemus (Loxocemidae)

Madatyphlops (Typhlopidae)

Malayopython (Pythonidae)

Malayotyphlops (Typhlopidae)

Melanophidium (Uropeltidae)

Mitophis (Leptotyphlopidae)

Morelia (Pythonidae)

Myriopholis (Leptotyphlopidae)

Namibiana (Leptotyphlopidae)

Platyplectrurus (Uropeltidae)

Plectrurus (Uropeltidae)

Python (Pythonidae)

Ramphotyphlops (Typhlopidae)

Rena (Leptotyphlopidae)

Rhinoleptus (Leptotyphlopidae)

Rhinophis (Uropeltidae)

Rhinotyphlops (Typhlopidae)

Sanzinia (Sanziniidae)

Siagonodon (Leptotyphlopidae)

Simalia (Pythonidae)

Teretrurus (Uropeltidae)

Sundatyphlops (Typhlopidae)

Tetracheilostoma (Leptotyphlopidae)

Trachyboa (Tropidophiidae)

Tricheilostoma (Leptotyphlopidae)

Trilepida (Leptotyphlopidae)

Tropidophis (Tropidophiidae)

Typhlophis (Anomalepididae)

Typhlops (Typhlopidae)

Ungaliophis (Ungaliophiidae)

Uropeltis (Uropeltidae)

Xenopeltis (Xenopeltidae)

Xenophidion (Xenophidiidae)

Xenotyphlops (Xenotyphlopidae)

Xerotyphlops (Typhlopidae)

Appendix 4

List of species for which numbers of vertebrae are provided (alphabetically).

Acrantophis dumerili

Acrantophis madagascariensis

Acutotyphlops infralabialis

Acutotyphlops kunuaensis

Acutotyphlops solomonis

Afrotyphlops angeli

Afrotyphlops angolensis

Afrotyphlops anomalus

Afrotyphlops bibronii

Afrotyphlops blanfordii

Afrotyphlops brevis

Afrotyphlops calabresii

Afrotyphlops congestus

Afrotyphlops cuneirostris

Afrotyphlops decorosus

Afrotyphlops elegans

Afrotyphlops fornasinii

Afrotyphlops gierrai

Afrotyphlops liberiensis

Afrotyphlops lineolatus

Afrotyphlops manni

Afrotyphlops mucruso

Afrotyphlops nigrocandidus

Afrotyphlops obtusus

Afrotyphlops punctatus

Afrotyphlops rondoensis

Afrotyphlops schlegelii

Afrotyphlops schmidti

Afrotyphlops steinhausi

Afrotyphlops tanganicanus

Afrotyphlops usambaricus

Amerotyphlops brongersmianus

Amerotyphlops microstomus

Amerotyphlops reticulatus

Anilios ammodytes

Anilios australis

Anilios bicolor

Anilios bituberculatus

Anilios diversus

Anilios endoterus

Anilios erycinus

Anilios fossor

Anilios ganei

Anilios guentheri

Anilios hamatus

Anilios kimberleyensis

Anilios leptosomus

Anilios ligatus

Anilios nigrescens

Anilios pilbarensis

Anilios pinguis

Anilios proximus

Anilios torresianus

Anilios waitii

Anilius scytale

Anomalepis aspinosus

Anomalepis mexicana

Anomochilus leonardi

Anomochilus monticola

Anomochilus weberi

Antaresia childreni

Antillotyphlops hypomethes

Apodora papuana

Argyrophis diardii

Argyrophis muelleri

Aspidites melanocephalus

Boa constrictor

Boa imperator

Bolyeria multocarinata

Bothrochilus boa

Brachyophidium rhodogaster

Calabaria reinhardtii

Candoia aspera

Candoia carinata

Candoia paulsoni

Casarea dussumieri

Charina bottae

Chilabothrus angulifer

Chilabothrus striatus

Chilabothrus subflavus

Corallus annulatus

Corallus batesii

Corallus caninus

Corallus cookii

Corallus cropanii

Corallus hortulana

Corallus ruschenbergerii

Cyclotyphlops deharvengi

Cylindrophis maculatus

Cylindrophis ruffus

Epacrophis boulengeri

Epicrates alvarezi

Epicrates assissi

Epicrates cenchria

Epicrates crassus

Epicrates maurus

Epictia albifrons

Epictia albipuncta

Epictia ater

Epictia borapeliotes

Epictia columbi

Epictia guayaquilensis

Epictia magnamaculata

Epictia munoai

Epictia phenops

Epictia rioignis

Epictia tenella

Epictia tricolor

Eryx colubrinus

Eryx conicus

Eryx elegans

Eryx jaculus

Eryx jayakari

Eryx johnii

Eryx miliaris

Eryx muelleri

Eryx somalicus

Eryx tataricus

Eunectes deschauenseei

Eunectes murinus

Eunectes notaeus

Exiliboa placata

Gerrhopilus mirus

Gerrhopilus persephone

Grypotyphlops acutus

Habrophallos collaris

Helminthophis frontalis

Indotyphlops braminus

Leiopython albertisii

Leptotyphlops conjunctus

Leptotyphlops emini

Leptotyphlops nigricans

Leptotyphlops scutifrons

Letheobia caeca

Letheobia coecata

Letheobia crossii

Letheobia debilis

Letheobia decorosa

Letheobia feae

Letheobia gracilis

Letheobia graueri

Letheobia kibarae

Letheobia leucosticta

Letheobia lumbriciformis

Letheobia newtoni

Letheobia obtusa

Letheobia pallida

Letheobia praeocularis

Letheobia rufescens

Letheobia somalica

Letheobia stejnegeri

Letheobia sudanensis

Letheobia swahilica

Letheobia toritensis

Letheobia uluguruensis

Letheobia wittei

Letheobia zenkeri

Liasis fuscus

Liasis mackloti

Lichanura orcutti

Lichanura trivirgata

Liotyphlops albirostris

Liotyphlops bondensis

Liotyphlops ternetzii

Loxocemus bicolor

Madatyphlops andasibensis

Madatyphlops arenarius

Madatyphlops boettgeri

Madatyphlops domerguei

Madatyphlops platyrhynchus

Malayopython reticulatus

Malayopython timoriensis

Malayotyphlops luzonensis

Melanophidium wynaudense

Mitophis asbolepis

Mitophis calypso

Mitophis leptepileptus

Mitophis pyrites

Morelia spilota

Morelia viridis

Myriopholis longicauda

Myriopholis phillipsi

Namibiana occidentalis

Platyplectrurus madurensis

Platyplectrurus trilineatus

Plectrurus perroteti

Python bivittatus

Python brongersmai

Python curtus

Python molurus

Python regius

Python sebae

Ramphotyphlops depressus

Ramphotyphlops flaviventer

Ramphotyphlops lineatus

Rena dissecta

Rena dulcis

Rena humilis

Rena maxima

Rena myopica

Rena segrega

Rhinoleptus koniagui

Rhinophis blythii

Rhinophis philippinus

Rhinophis sanguineus

Rhinotyphlops ataeniatus

Rhinotyphlops boylei

Rhinotyphlops lalandei

Rhinotyphlops leucocephalus

Rhinotyphlops schinzi

Rhinotyphlops scorteccii

Rhinotyphlops unitaeniatus

Sanzinia madagascariensis

Siagonodon borrichianus

Siagonodon cupinensis

Siagonodon exiguum

Siagonodon septemstriatus

Simalia amethistina

Simalia boeleni

Sundatyphlops polygrammicus

Teretrurus sanguineus

Tetracheilostoma bilineatum

Tetracheilostoma breuili

Tetracheilostoma carlae

Trachyboa boulengeri

Trachyboa gularis

Tricheilostoma bicolor

Trilepida affinis

Trilepida brasiliensis

Trilepida dimidiata

Trilepida fuliginosa

Trilepida jani

Trilepida joshuai

Trilepida macrolepis

Trilepida nicefori

Trilepida pastusa

Trilepida salgueiroi

Tropidophis cacuangoae

Tropidophis canus

Tropidophis greenwayi

Tropidophis haetianus

Tropidophis jamaicensis

Tropidophis melanurus

Tropidophis semicinctus

Typhlophis squamosus

Typhlops gonavensis

Typhlops jamaicensis

Typhlops lumbricalis

Typhlops platycephalus

Typhlops pusillus

Typhlops richardii

Typhlops rostellatus

Ungaliophis continentalis

Ungaliophis panamensis

Uropeltis arcticeps

Uropeltis dindigalensis

Uropeltis melanogaster

Uropeltis ocellata

Uropeltis pulneyensis

Uropeltis rubrolineata

Uropeltis woodmasoni

Xenopeltis hainanensis

Xenopeltis intermedius

Xenopeltis unicolor

Xenophidion acanthognathus

Xenophidion schaefferi

Xenotyphlops grandidieri

Xerotyphlops socotranus

Xerotyphlops vermicularis