THE TRIASSIC RISE OF REPTILES AND THE EVOLUTION OF THE PHYTOSAURIA (Invited Presentation)

Following the end-Permian extinction (~250 Ma), terrestrial vertebrate diversity recovered to be dominated by reptiles by the Middle Triassic. However, these groups of reptiles, including archosaurs and their closest relatives, are not commonly found until ~30 million years after the extinction in classic Late Triassic deposits (southwestern USA, eastern USA, Germany) despite predictions of an Early Triassic divergence for those clades. One of these, the phytosaurs, is a well known Triassic group with a near-Pangean distribution. This cosmopolitan distribution, combined with morphological disparity spanning the Late Triassic, led to this clade being key to biostratigraphic and biochronologic correlations. Members of Phytosauria superficially resemble living crocodylians, bearing elongated rostra with non-terminal nares, carnivorous dentition, and a sprawling posture, and are hypothesized to have shared a similar semi-aquatic ecology. Though these similarities are present in derived phytosaurs near the end of the lineage in the Late Triassic, there was little information on the timing and sequence of acquisition of those features. Additionally, biogeographic patterns of this widespread clade remained unclear. Recent work has revealed a new Middle Triassic phytosaur from China, indicating that the characteristic elongated rostrum of phytosaurs appeared after the cranial and postcranial modifications associated with enhanced prey capture, predating a similar general trend of morphological change observed in the evolution of crocodylians and their close relatives. New detailed anatomical studies incorporating CT data have brought to light additional features (trigeminal innervation of the jaws for increased sensory capabilities, increased number of sacral vertebrae) that previously were only known in other archosaurs. Reevaluation of specimens from Germany, India, the American west, and the eastern US reveal a single Pangean clade early in the group’s history. This suggests that early ecosystem exploration by the clade through nearshore environments, coupled with modifications to become large, apex predators, lead to the dispersal of phytosaurs across the Tethys and their aquatic dominance for nearly 50 million years until their extinction at the Triassic-Jurassic boundary.