Research Article |
Corresponding author: Georgios L. Georgalis ( dimetrodon82@gmail.com ) Academic editor: Uwe Fritz
© 2023 Zbigniew Szyndlar, Georgios L. Georgalis.
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
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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.
Intracolumnar variation, osteology, Serpentes, Squamata, taxonomy, vertebral counts, vertebral morphology
It is truly wonderful to see the work of hands, feet, fins, performed by a simple modification of the vertebral column.
Richard
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 (
Important descriptions of snake vertebral morphology were subsequently made by
Since the onset of the 20th 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.,
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
Besides, important studies on the vertebral morphology of individual taxa have also been conducted (e.g.,
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.,
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) (
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 (
However, the taxonomic importance of other vertebral structures has been more rarely raised among herpetologists, and this is particularly true for 20th century workers. As summarized by
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.
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 150th 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
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.
Snakes demonstrate a great variability in their total number of vertebrae compared to limbed amniotes (
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 (
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.
Nomenclature of anatomical structures in snake vertebrae (A–M Epicrates cenchria [ISEZ R/437]; N Tropidophis jamaicensis [
In fact, the greatest controversies in naming parts of the ophidian vertebral column concern just the region situated around the cloaca (pericloacal vertebrae sensu
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 (
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.
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
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.
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.
The anatomical terminologies in current use in the (mainly palaeontological) literature, follows those proposed by
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
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).
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
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).
A few preliminary works in the 19th century dealt with vertebrae located at the level of cloaca (e.g.,
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
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.
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.,
Taxonomy of extant snakes follows
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 |
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.,
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.,
The fossil record of scolecophidians is poor, consisting exclusively of vertebrae spanning across a small number of Cenozoic localities (
A considerable amount of skeletal studies in scolecophidians has focused primarily on their cranial anatomy already since the 19th century (e.g.,
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.
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.
In his monographic treatise,
One interesting feature that was addressed by
Finally, one feature that was highlighted by
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
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
Epacrophis boulengeri (Boettger, 1913) (SMF 16700 [holotype]); Epictia albifrons (Wagler, 1824) (
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
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 (
Number of vertebrae. Epacrophis boulengeri (SMF 16700 [holotype]): 185 (160+3+22, including a final fusion); Epictia ater (
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 (
In general, the vertebral counts are highly variable with a single species, even subjected to sexual variation. With 546 vertebrae in total (see
This superfamily represents the sister group to leptotyphlopids and comprises Typhlopidae, Xenotyphlopidae, and Gerrhopilidae (
Typhlopidae is the most speciose family of scolecophidians and is currently distributed over large parts of Africa, Asia, the Americas, Oceania, and southeastern Europe (
Previous figures of vertebrae of extant Typhlopidae have been so far presented by
Acutotyphlops kunuaensis Wallach, 1995 (
Description (Figs
Trunk vertebrae . The morphology of the trunk vertebrae is very similar to leptotyphlopids. See the respective part in Leptotyphlopidae above.
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 (
Data from literature and unpublished data from personal communications: Acutotyphlops infralabialis (Waite, 1918): 289 trunk and cloacal vertebrae plus 13 caudal vertebrae (
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
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 (
The sole so far published figure of gerrhopilid vertebrae has been presented by
Gerrhopilus mirus (Jan, 1860 in Jan & Sordelli 1860–1866) (
Trunk vertebrae. The morphology of the trunk vertebrae is very similar to other scolecophidians. See the respective part in Leptotyphlopidae above.
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 (
Data from literature: Gerrhopilus persephone: 286 vertebrae in total (
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 (
No xenotyphlopid specimen was available for study. Although the cranial anatomy of this species has been recently described in detail (
Number of vertebrae. Data from literature: Xenotyphlops grandidieri: 264 vertebrae in total (
Anomalepididae have been considered to represent the basalmost scolecophidians, mainly due to their peculiar cranial anatomy (
Previous figures of vertebrae of extant Anomalepididae have been so far presented only by
Anomalepis mexicana Jan, 1860 in Jan & Sordelli 1860–1866 (
Trunk vertebrae. The morphology of the trunk vertebrae is strikingly similar to other scolecophidians. See the respective part in Leptotyphlopidae above.
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 (
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) (
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).
The “true” (“ἀληθινά”) “snakes” (“ὀφίδια”), as their name in Greek readily suggests, Alethinophidia was originally established by
Amerophidia constitute a recently established group based on molecular phylogenies (
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 19th century under the names Ilysiidae (or Ilysioidea) (e.g.,
Although a number of fossil remains and taxa has been referred to aniliids in the past few decades (e.g.,
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
Anilius scytale (Linnaeus, 1758) MNHN-AC-1869.0772; MNHN-AC-1880.1892; MNHN-AC-1887.0901; MNHN-AC-1888.0186.
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.
Trunk/caudal transition. All last trunk, cloacal, and caudal vertebrae retain a relatively flattened haemal keel.
Number of vertebrae
(all for Anilius scytale):
Data from literature and unpublished data from personal communications (all for Anilius scytale): 220 trunk vertebrae plus 4 cloacal vertebrae plus 14 caudal vertebrae (
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
Commonly known as “dwarf boas”, they were for long lumped into an expansive “Boidae” (e.g.,
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 (
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
Previous figures of vertebrae of extant Tropidophiidae have been so far presented by
Trachyboa boulengeri Peracca, 1910 (
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.
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
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 (
In Trachyboa gularis (
Number of vertebrae. Trachyboa boulengeri (
Data from literature: Trachyboa boulengeri: 131–134 trunk and cloacal vertebrae plus 24 caudal vertebrae (
Tropidophis canus (Cope, 1868) (
Trunk vertebrae. The description is primarily based on the complete skeleton of Tropidophis jamaicensis (
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 (
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 7th caudal vertebra (i.e., V 183) of T. jamaicensis (but in T. greenwayi, the slightly grooved hypapophysis already appears from the 3d 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 (
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 (
This is the sister group of amerophidians, comprising the rest of alethinophidians (i.e., uropeltoids, booids, pythonoids, bolyeriids, xenophidiids, and caenophidians).
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 (
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.
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 (
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
We only had available an X-ray of the posterior trunk, cloacal, and caudal regions of a skeleton (
Number of vertebrae. Anomochilus leonardi (
Data from literature and unpublished data from personal communications: Anomochilus leonardi: 265 trunk and cloacal vertebrae plus 17 caudal vertebrae (
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 (
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
Cylindrophis maculatus (Linnaeus, 1758) (
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.
It is worth noting that
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.
Number of vertebrae. Cylindrophis maculatus (
Data from literature and unpublished data from personal communications: Cylindrophis maculatus: 195–209 trunk and cloacal vertebrae plus 8–?9 caudal vertebrae (
It should be noted that recently Cylindrophis ruffus has been recognized as a species complex, with several new cryptic species established (e.g.,
Uropeltids are a moderately diverse family of fossorial snakes, with more than 60 species, currently endemic to India and Sri Lanka (
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
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 (
Previous figures of vertebrae of extant Uropeltidae have been so far presented by
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;
Brachyophidium rhodogaster Wall, 1921 (
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.
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 (
Data from literature: Brachyophidium rhodogaster: 138–143 trunk and cloacal vertebrae plus 9–12 caudal vertebrae (
Melanophidium wynaudense (Beddome, 1863) (
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.
Number of vertebrae. Data from literature and personal communications (all for Melanophidium wynaudense): 219+3+14+fusion (
Platyplectrurus madurensis Beddome, 1877 (
Trunk vertebrae. More elongate than trunk vertebrae of Brachyophidium; otherwise, the morphology is similar to that of other uropeltids.
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.
Number of vertebrae. Platyplectrurus madurensis (
Data from literature: Platyplectrurus madurensis: 166–167 trunk and cloacal vertebrae plus ?12–14+ caudal vertebrae (
Plectrurus perroteti Duméril & Bibron in Duméril & Duméril, 1851 (
Trunk vertebrae. More elongate than trunk vertebrae of Brachyophidium; otherwise, the morphology is similar to that of other uropeltids.
Trunk/caudal transition. The morphology is similar to that of other uropeltids.
Number of vertebrae. Plectrurus perroteti (
Data from literature: Plectrurus perroteti: 154 trunk and cloacal vertebrae plus 12+ caudal vertebrae (
Rhinophis blythii Kelaart, 1853 (
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
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
Data from literature and unpublished data from personal communications: Rhinophis blythii: 152–157 trunk and cloacal vertebrae plus 7–11 caudal vertebrae (
Teretrurus sanguineus (Beddome, 1867) (
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 (
Uropeltis arcticeps (Günther, 1875) (
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.
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 (
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
The lowest vertebral counts in the entire family Uropeltidae were reported by
Constrictores represents a recently defined group encompassing Booidea and Pythonoidea, plus tentatively also Bolyeriidae and Xenophidiidae (
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 (
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 (
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
Bolyeria multocarinata (Boié H in Boié F, 1827) (
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.
Trunk/caudal transition. Unknown.
Number of vertebrae. Data from literature: Bolyeria multocarinata: 198 trunk vertebrae plus 87 cloacal and caudal vertebrae (
Casarea dussumieri (Schlegel, 1837) (MNHN-AC-1993.3382;
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.
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
Data from literature (all for Casarea dussumieri): 225 trunk and cloacal vertebrae (a few anterior vertebrae missing) plus 126 caudal vertebrae (
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 (
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
Number of vertebrae. Xenophidion acanthognathus (
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 (
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 (
This group comprises only a single species, Calabaria reinhardtii, distributed in Central and Western Africa (
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
Calabaria reinhardtii (Schlegel, 1848) (
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 40th vertebra; haemal keel flattened, weakly developed; subcentral grooves deep; paracotylar foramina absent; asymmetrical subcentral foramina occasionally present in some 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 5th or 6th 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):
Data from literature: Calabaria reinhardtii: 236 trunk and cloacal vertebrae plus 26+ caudal vertebrae (
Sanziniidae represents a small lineage of booids, pertaining to two genera (Acrantophis and Sanzinia) and four species, that are endemic to Madagascar (
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:
Previous figures of vertebrae of extant Sanziniidae were so far only presented by
Acrantophis dumerili Jan, 1860 in Jan & Sordelli 1860–1866 (
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 80th vertebra approximately; haemal keel in following vertebrae well-developed, ridge-like; paracotylar foramina absent.
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
Data from literature: Acrantophis dumerili: less than 300 (in this approximately 230 trunk) vertebrae (based on
Sanzinia madagascariensis (Duméril & Bibron, 1844) (MNHN-ZA-AC-1900.0122A;
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 60th vertebra; haemal keel in more posterior vertebrae well-developed, ridge-like; paracotylar foramina absent in most vertebrae – in Sanzinia examined by
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
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]);
Data from literature (all for Sanzinia madagascariensis): ~300 vertebrae in total, of which ~225 trunk (based on
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.,
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.,
Vertebrae of Boidae have regularly and extensively appeared in the literature, including some of the first studies of snake vertebral morphology (e.g.,
Boa constrictor Linnaeus, 1758 (
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 60th and 70th vertebrae; haemal keel in more posterior vertebrae well-developed, ridge-like; paracotylar foramina present.
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 (
It should be noted that Boa constrictor has recently been recognized as a species complex, with other cryptic species recognized (
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).
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 5th caudal vertebra. Normally paired haemapophyses first appear on the 6th 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 (
Corallus caninus (Linnaeus, 1758) (PIMUZ A/III 1024; SMF PH 47; SMF PH 48; SMF PH 54; SMF PH 182); Corallus cropanii (Hoge, 1954) (
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.
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 (
Data from literature and unpublished data from personal communications: Corallus annulatus: 286 trunk vertebrae plus 4 cloacal vertebrae plus 103 caudal vertebrae (
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 cenchria (Linnaeus, 1758) (ISEZ R/437; ISEZ R/458 [juvenile]; ISEZ R/459; SMF PH 25; SMF PH 26).
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 50th and 60th vertebrae; haemal keel well developed, ridge-like; subcentral grooves deep; paracotylar foramina absent.
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 (
Eunectes murinus (Linnaeus, 1758) (ISEZ R/103; ISEZ R/265; ISEZ R/266;
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.
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 (
Data from literature: Eunectes deschauenseei Dunn & Conant, 1936: 238 trunk vertebrae plus 3 cloacal vertebrae plus 45 caudal vertebrae (
This group of island boas comprises a single genus, Candoia, with five known species, distributed across several islands of the Pacific Ocean (
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
Candoia aspera (Günther, 1877) (
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 (
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 5th caudal vertebra.
Number of vertebrae. Candoia carinata (
Data from literature: Candoia aspera: 144 trunk and cloacal vertebrae plus 21+ caudal vertebrae (
The current concept of Erycidae is confined to the genus Eryx, which comprises 13 species distributed in southeastern Europe, southwestern Asia, and Africa (
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 (
Previous figures of vertebrae of extant Erycidae were so far presented by
Eryx colubrinus (Linnaeus, 1758) (ISEZ R/370; MGPT-MDHC 172; SMF PH 24; SMF PH 203 [juvenile]; SMF PH 204 [juvenile];
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
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 1st to 5th 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 (
Data from literature and unpublished data from personal communications: Eryx colubrinus: 187–190 trunk and cloacal vertebrae plus 28–29 caudal vertebrae (
An interesting feature of Eryx is a very low variation in the total number of vertebrae within the genus.
Charinaidae currently comprise two genera, Charina and Lichanura, with four species, distributed in North and northern Central America (
The fossil record attests a higher diversity of charinaids in the past, including also extinct species of the extant genera (see
Note that the original name of the family, Charinidae, recently had its spelling emended to Charinaidae by a formal decision of
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).
Previous figures of vertebrae of extant Charinaidae have been so far presented by
Charina bottae (Blainville, 1835) (MNHN-RA-0730 [part of the skeleton of the holotype];
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. 50th vertebra; haemal keel broad, weakly developed; paracotylar foramina absent.
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 3rd 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 (
Data from literature (all for Charina bottae): 215 trunk and cloacal vertebrae plus 44 caudal vertebrae (
Lichanura orcutti Stejneger, 1889 (
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. 50th vertebra; haemal keel weakly developed to absent; paracotylar foramina present in a minority of trunk vertebrae.
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 (
Data from literature (all for Lichanura trivirgata): 235±2 trunk and cloacal vertebrae plus 52+ caudal vertebrae (Lichanura roseofusca of
Long considered as a subfamily of Tropidophiidae (Ungaliopheinae of
Ungaliophiidae currently comprise two genera, Exiliboa and Ungaliophis, with only three species in total, inhabiting continental Central and northern South America (
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,
Ungaliophiid vertebrae have only been rarely figured. In fact, previous figures of vertebrae of extant Ungaliophiidae have been so far only presented by
Exiliboa placata Bogert, 1968 (MVZ Herps 137126 [Morphosource.org: Media 000076130, ark:/87602/m4/M76130];
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.
The mid-trunk vertebra of the same species, illustrated by
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.
Number of vertebrae
(all for Exiliboa placata).
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) (
Ungaliophis continentalis Müller, 1880 (
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.
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
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 (
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 comprises the Old World Pythonidae and Xenopeltidae plus the American Loxocemidae (
Xenopeltidae represents a monotypic family, with a single genus, Xenopeltis, with only three species distributed in southeastern Asia (
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
Xenopeltis unicolor Reinwardt in Boié, 1827 (
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
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).
Number of vertebrae
(all for Xenopeltis unicolor). MGPT-MDHC 117: 222 (188+3+31);
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 (
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” (
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
Loxocemus bicolor Cope, 1861 (MVZ Herps 143487 [Morphosource.org: Media 000076134, ark:/87602/m4/M76134];
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.
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
Number of vertebrae
(all for Loxocemus bicolor). MVZ Herps 143487: 309 (262+4+43, including a final fusion);
Data from literature: Loxocemus bicolor: 267 trunk vertebrae plus unknown number of cloacal and caudal vertebrae (
Pythonids comprise some the largest and most impressive snakes of all time (
Molecular data and fossil evidence support the origination of pythonids already by the Paleogene (
Pythons comprise more than 40 extant species, distributed over large parts of Africa, Asia, and Australia (
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.,
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 (
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
Python bivittatus Kuhl, 1820 (ISEZ R/327; ISEZ R/461; NHMW 35675); Python curtus Schlegel, 1872 (MGPT-MDHC 106; MGPT-MDHC 107;
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.
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 (
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 (
The total value of 435 vertebrae for Python molurus by
Malayopython reticulatus (Schneider, 1801) (ISEZ R/436 [juvenile]); Malayopython timoriensis (Peters, 1876) (SMF PH 27 [juvenile]).
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).
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 (
Aspidites melanocephalus (Krefft, 1864) (
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.
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 (
Data from literature (all for Aspidites melanocephalus): 337 trunk vertebrae plus 5 cloacal vertebrae plus 52 caudal vertebrae (
Morelia spilota (Lacépède, 1804) (MNHW Reptilia-0328; SMF PH 4; SMF PH 6; SMF PH 8; SMF PH 67); Morelia viridis (Schlegel, 1872) (
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
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 [
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 (
Data from literature and unpublished data from personal communications: Morelia spilota: 371 vertebrae in total (
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 amethistina (Schneider, 1801) (
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.
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.
Number of vertebrae. Simalia amethistina (
Data from literature: Simalia amethistina: 323 trunk vertebrae plus 106 cloacal and caudal vertebrae (
Antaresia childreni (Gray, 1842) (
Trunk vertebrae. The morphology is relatively similar to that of Simalia above. Neural arch moderately vaulted.
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 (
Apodora papuana (Peters & Doria, 1878) (
Trunk vertebrae. The morphology is relatively similar to that of Simalia above.
Trunk/caudal transition. The morphology is relatively similar to that of Simalia above.
Number of vertebrae. Apodora papuana (
Bothrochilus boa (Schlegel, 1837) (
Trunk vertebrae. The morphology is relatively similar to that of Simalia above. Neural arch moderately vaulted.
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 (
Leiopython albertisii (Peters & Doria, 1878) (SMF PH 50).
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.
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
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) (
Liasis mackloti Duméril & Bibron, 1844 (
Trunk vertebrae. The morphology is relatively similar to that of Simalia above.
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 (
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 (
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.,
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.,
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.
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
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.
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) (
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 (
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.,
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 (
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
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
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
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.
Scolecophidians, Anilius, Cylindrophis, and uropeltids possess dorsoventrally depressed cotyles and condyles; these structures are more or less orbicular in other non-Colubroides snakes.
Additionally,
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.
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 (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
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
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.,
The presence of additional apophyses on caudal vertebrae was repeatedly raised as a synapomorphy shared by members of erycids and charinaids (
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 (
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
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;
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 (
Haemapophyses have been considered to represent the fusion of the caudal intercentra to the vertebral pleurocentra in snakes (
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
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 (
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 (
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) (
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.
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
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
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
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;
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
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.
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
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 (
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 (
For sharing valuable unpublished information on vertebral counts of different taxa, we are grateful to Sarin Tiatragul (Australian National University, Canberra), Claudia Koch (
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
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)
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
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)
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