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| Avicranium renestoi Pritchard & Nesbitt, 2017
Illustration: Matt Celeskey @clepsydrops
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Abstract
The Triassic Period saw the first appearance of numerous amniote lineages (e.g. Lepidosauria, Archosauria, Mammalia) that defined Mesozoic ecosystems following the end Permian Mass Extinction, as well as the first major morphological diversification of crown-group reptiles. Unfortunately, much of our understanding of this event comes from the record of large-bodied reptiles (total body length > 1 m). Here we present a new species of drepanosaurid (small-bodied, chameleon-like diapsids) from the Upper Triassic Chinle Formation of New Mexico. Using reconstructions of micro-computed tomography data, we reveal the three-dimensional skull osteology of this clade for the first time. The skull presents many archaic anatomical traits unknown in Triassic crown-group reptiles (e.g. absence of bony support for the external ear), whereas other traits (e.g. toothless rostrum, anteriorly directed orbits, inflated endocranium) resemble derived avian theropods. A phylogenetic analysis of Permo-Triassic diapsids supports the hypothesis that drepanosaurs are an archaic lineage that originated in the Permian, far removed from crown-group Reptilia. The phylogenetic position of drepanosaurids indicates the presence of archaic Permian clades among Triassic small reptile assemblages and that morphological convergence produced a remarkably bird-like skull nearly 100 Myr before one is known to have emerged in Theropoda.
KEYWORDS: reptilia, phylogeny, convergence, Permo-Triassic extinction, evolutionary radiation, Triassic
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| Avicranium renestoi Pritchard & Nesbitt, 2017
Illustration: Matt Celeskey @clepsydrops
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| Figure 2. Line drawing of the restored skull of Avicranium renestoi based on the three-dimensional surface renderings of skull elements in AMNH FARB 30834. |
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| Figure 3. Reconstructed skull of Avicranium renestoi based on rearticulated three-dimensional surface rendering of the skull bones of AMNH FARB 30834. Callouts include (a) reconstructed endocast in dorsal view, (b) skull roof in dorsal view, (c) postorbital complex (consisting of postfrontal and postorbital) in anterior view, (d) braincase and stapes in posterior view, (e) palatal complex in ventral view, (f) left quadrate in posterior view and (g) braincase and stapes in left lateral view. All bones have been rearticulated based on the facets of the reconstructed elements.
Abbreviations: fb, forebrain; mb, midbrain; pa, parietal; pf, postfrontal; pl, palatine; po, postorbital; pt, pterygoid; qu, quadrate; st, stapes; su, supratemporal.
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Systematic palaeontology
Diapsida
Drepanosauromorpha
Drepanosauridae
Avicranium renestoi, n. gen., n. sp.
Etymology: Avicranium, from aves (Latin for bird) and cranium (Latin for cranium), in reference to the suite of bird-like morphologies present in the holotype skull; renestoi, for Silvio Renesto, who described much of the drepanosauromorph fossil record from Triassic Italy.
Holotype: AMNH FARB 30834, partial skull and articulated cervical series. Additional drepanosaurid caudal vertebrae and limb fragments are preserved in the block, but are not clearly associated with the individual to which the skull and cervical vertebrae belong.
Locality: Coelophysis Quarry (‘siltstone member’, Chinle Formation). Recovered during preparation of the holotype block of the shuvosaurid pseudosuchian Effigia okeeffeae by S.J.N. [12].
Diagnosis: Specimens for anatomical comparisons are listed in appendix C. A drepanosaurid diapsid differing from Hypuronector limnaios, Megalancosaurus preonensis and Vallesaurus cenensis (the only drepanosauromorphs with skull material) in the complete absence of teeth, a dorsoventrally taller retroarticular process with a triangular shape in lateral view, and cervical neural spines with subequal anteroposterior lengths and transverse widths.
Adam C. Pritchard and Sterling J. Nesbitt. 2017. A Bird-like Skull in A Triassic Diapsid Reptile Increases Heterogeneity of the Morphological and Phylogenetic Radiation of Diapsida. ROYAL SOCIETY OPEN SCIENCE. DOI: 10.1098/rsos.170499
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