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.
Highlights
The Triassic has long been recognized as a critical interval in the history of vertebrate life, especially in terms of the diversification of important Mesozoic taxa
In the light of the extensive new data on the cranial anatomy of Drepanosauromorpha provided by AMNH FARB 30834, we integrated the taxon into a phylogenetic analysis focused on terrestrial Permo-Triassic Diapsida and early Sauria
These results indicate that drepanosauromorphs represent a deep divergence within Diapsida, earlier than that of crown-group reptiles, but one that persisted through the Permo-Triassic Extinction (PTE) and radiated deep within the Triassic [11,27]
Summary
The Triassic has long been recognized as a critical interval in the history of vertebrate life, especially in terms of the diversification of important Mesozoic taxa It saw the global recovery from the biodiversity crash of the Permo-Triassic Extinction (PTE), and the first appearances of major diapsid reptile clades that would typify Mesozoic ecosystems (e.g. Dinosauromorpha, Lepidosauria, Pseudosuchia, Pterosauria, Ichthyosauria Sauropterygia) [1,2,3,4,5,6,7]. Were major taxonomic categories established during the Triassic Period, but suites of morphological features—suites which would typify many Mesozoic and Cenozoic diapsid reptile clades—emerged in a variety of unrelated Triassic species As yet, this pattern of convergent morphologies is well established in archosaurs and their close relatives, a possible consequence of overall larger body size and higher preservational potential [19,20].
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