Research Article |
Corresponding author: John R. Wible ( jwible99@gmail.com ) Academic editor: Ingmar Werneburg
© 2022 John R. Wible.
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Citation:
Wible JR (2022) The history and homology of the os paradoxum or dumb-bell-shaped bone of the platypus Ornithorhynchus anatinus (Mammalia, Monotremata). Vertebrate Zoology 72: 143-158. https://doi.org/10.3897/vz.72.e80508
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Abstract
The os paradoxum or dumb-bell-shaped bone is a paired bone occurring in the middle of the specialized bill of the platypus Ornithorhynchus anatinus. It has been variously considered as a neomorph of the platypus, as the homologue of the paired vomer of sauropsids, or as a part of the paired premaxillae. A review of the near 200-year history of this element strongly supports the os paradoxum as a remnant of the medial palatine processes of the premaxillae given its ontogenetic continuity with the premaxillae and association with the vomeronasal organ and cartilage, incisive foramen, and cartilaginous nasal septum. In conjunction with this hypothesis, homologies of the unpaired vomer of extant mammals and the paired vomer of extant sauropsids are also supported. These views are reinforced with observations from CT scans of O. anatinus, the Miocene ornithorhynchid Obdurodon dicksoni, and the extant didelphid marsupial Didelphis marsupialis. At the choanae, Obdurodon has what appears to be a separate parasphenoid bone unknown in extant monotremes.
Didelphis marsupialis, Obdurodon dicksoni, parasphenoid, premaxilla, vomer, vomeronasal organ
First illustrated by
To aid the reader in following the history of study regarding the os paradoxum, an overview of the anatomical domain in a placental and a lizard is presented.
Figure
Cynocephalus volans (= Galeopithecus philippinensis, G. volans), cranium in ventral view. A, fetus, 14 cm from snout to root of tail, with palatal parts of maxillae and palatines removed to expose the nasal cartilages; modified from
Each of the elements named above exhibits considerable diversity across mammals. Regarding the adult premaxilla, Chiroptera, for example, shows a broad range of morphologies (
Figure
Nearly 200 years ago, Johann Friedrich
Ornithorhynchus anatinus, cranium, atlas, and partial axis in ventral view, modified from
The first alternative view on the homologies of the os paradoxum came from
After
Crania in ventral view. A, D, Sphenodon punctatus (modified from
Ornithorhynchus anatinus, anterior rostrum of ‘mammary foetuses’ in ventral view, modified from
Five years later,
In 1929, De Beer, a proponent of Wilson’s and Broom’s view on the os paradoxum, proposed the following three stages in the evolution of the prevomer and premaxilla: (1) forming from separate ossifications, as in sauropsids (Fig.
The ontogeny of the platypus skull was notably documented by
Ornithorhynchus anatinus, 122 snout-tail length posthatchling, modified from
In 1940, two paleontologists, Parrington and Westoll, revisited the evidence from embryology and the fossil record. Their synthesis convincingly debunked the homological hypotheses that the prevomer of lizards = the os paradoxum of the platypus = the medial palatine process of the premaxilla of therians (
In his monograph on the development of the skull of the echidna Tachyglossus aculeatus,
In 1978, Presley and Steel reported on an ontogenetic series of ten stages of Ornithorhynchus, including some specimens studied previously by
In his 1981 book “The Mammalian Skull,” Moore included a synopsis of the arguments for and against the hypotheses of Wilson and Broom, and he clearly settled on the ‘against’ side. He questioned the value of ossification centers as a guide to a bone’s phylogenetic history, and the case of the two centers in the human vomer I noted above is one of many appropriate examples of this issue. He noted that the embryological evidence used by Wilson and Broom and supported by
The three most recent views on the os paradoxum that I have found in the literature cover the possible hypotheses about this bone except for the one settled on by most researchers in the wake of
CT scans of the following specimens were studied, with structures of interest segmented in Avizo 2020.3 (FEI SAS a part of Thermo Fisher Scientific).
1) The head, neck, and thorax of male Ornithorhynchus anatinus,
2) The cranium of the Miocene ornithorhynchid Obduodon dicksoni,
3) The cranium of Didelphis marsupialis, du baa 0164. Duke University provided access to these data, the collection of which was funded by NSF BCS 1552848 (to D.M. Boyer) and NSF DBI 1458192 (to G.F. Gunnell). The CT image series included 1,910 tiff images [doi: 10.17602/M2/M58076]. X, Y, and Z spacing is 0.0690778 mm.
As with any anatomical contribution, it is important to acknowledge the source of terminology. The substance of this paper concerns a near 200-year debate concerning the homologies of bony elements of the mammalian rostrum. The terminology employed is wholly dependent on the side of the debate taken. Here, I follow the terms used by the main proponents of the debate. I usually try to conform to the Nomina Anatomica Veterinaria (NAV 2017), using anglicized versions of the Latin terms. However, NAV (2017) has a limited number of terms for the anterior rostrum.
Institutional Abbreviations: du baa, Duke University, Biological Anthropology and Anatomy, Durham, North Carolina, USA;
This male platypus specimen (Fig.
Ornithorhynchus anatinus,
The os paradoxum, here accepted as the fused medial palatine processes of the premaxillae, lies anterior to the palatal processes of the maxillae, separated from them by a narrow gap (Fig.
The vomer is recessed posteriorly from the anterior margin of the palatal processes of the maxillae and sits on short septal processes of the maxillae (Fig.
For comparison between the os paradoxum and the corresponding bones in therians, Figure
Didelphis marsupialis, du baa 0164, bone isosurfaces derived from CT scans. Cranium in A, dorsal and D, ventral views. Premaxillae in B, dorsal, C, anterior, and E, ventral views. F, premaxillae and vomer (anterior part) in anterior view. Scale for A and D is 25 mm; scale for B–C and E-F is 10 mm. Abbreviations: ap, alveolar process; b, body, fp, facial process; I1, upper first incisor; ii, incisive incisure; lp, lateral palatine process; mp, medial palatine process; mxf, facet for maxilla; pcf, paracanine fossa; pss, paraseptal shelf; sp, septal process; vo, vomer; vp, vomerine process; vs, vomer shelf.
For descriptive purposes, the medial palatine process can be divided into anterior and posterior parts, which are roughly subequal in length on the right premaxilla. In the anterior part, the left and right medial palatine processes contact on the midline and their convex ventral surfaces are exposed on the palate (Fig.
Obdurodon dicksoni,
To date, a parasphenoid has not been reported for extant monotremes, despite the number of ontogenetic stages that have been studied (e.g.,
The mammalian skull has ossifications widely considered as evolutionary novelties. Well-known examples include entotympanics, independent elements in the auditory bulla (e.g.,
Although the mammalian skull includes neomorphic structures, the usual first course of action for neontologists and paleontologists is to identify and name structures in the context of elements already known in other taxa, with the implicit assumption that creating novelties is less parsimonious. The platypus os paradoxum is an instructive example in that early invocations of novelty by
Ontogeny has identified the os paradoxum as forming as part of the medial palatine processes of the premaxillae. Anatomical comparison of the elements in adult Ornithorhynchus (Fig.
I thank the organizers of this special number, Ingmar Werneburg and Irina Ruf, for the invitation to submit an article in celebration of Wolfgang Maier. I first met Wolfgang in Frankfurt am Main in 1980 when he sponsored me for an extended stay at the Anatomisches Institut where I conducted much of my Ph.D. research in the embryological collections housed there. This was a life-changing opportunity for me that greatly influenced the trajectory of my research career and I am forever grateful to Wolfgang for the role he played. Additionally, it is hard to find a topic on the mammalian head that Wolfgang’s work has not impacted, the current paper being no exception. My contribution allowed me to revisit literature produced by some of my anatomical and paleontological heroes, including the late Robert Presley, another colleague who impacted my career. For access to the CT scans studied here, I thank Michael Archer, Ted Macrini, and Doug Boyer. Funding for this project is from National Science Foundation Grant DEB 1654949 and the R.K. Mellon North American Mammal Research Institute.