Meeting Abstract
Crown archosaurs and their relatives (=archosauromorphs) possess disparate cranial morphologies and body plans, which are unmatched by other contemporary vertebrates in the Mesozoic. General body plans of Triassic archosauromorphs (e.g. azendohsaurids, shuvosaurids, aetosaurs) were repeated by other archosaurs (e.g. early sauropodomorphs, ornithomimids, ankylosaurs) later in the Mesozoic. Here we present a case of convergence between a new archosauriform from the Late Triassic of Texas and Cretaceous pachycephalosaurid dinosaurs. Like pachycephalosaurids, this specimen preserves a thickened skull roof with obliterated cranial sutures, an expanded posterior skull margin, and large orbits. CT data reveal large, laterally expanded olfactory bulbs and a complete left osseous labyrinth. The anterior semicircular canal has the largest diameter of the three, indicating increased sensitivity to changes in pitch. Our cladistic analysis of Archosauromorpha recovers this taxon as an archosauriform based on the presence of an antorbital fenestra and associated fossa in the lacrimal and an ossified laterosphenoid. The similarity between this taxon and pachycephalosaurids over 100 million years later illustrates the early exploration of cranial morphospace. Distinctive ecomorphological modifications of Jurassic and Cretaceous dinosaurs increasingly are represented in Triassic archosauromorphs, implying that faunal and ecological shifts interpreted to occur among dinosaurs later in the Mesozoic possibly occurred much earlier during the Triassic archosauromorph radiation.
