Osteology of the skull of Tropidosuchus romeri (Archosauriformes: Proterochampsidae)

Abstract

Tropidosuchus romeri is a member of the clade Proterochampsidae (non-Archosauria Archosauriformes) collected from Upper Triassic (lower Carnian) outcrops of the Chañares Formation of the Ischigualasto-Villa Unión Basin (La Rioja Province, Argentina). This species is a non-rhadinosuchine proterochampsid known from several specimens, including almost complete skeletons. Here, we describe and compare in detail the skull of T. romeri based on direct observation of specimens and the three-dimensional segmentation of one of them based on computed microtomography scans. Contrasting with previous studies, we were able to determine sutures between bones and individualize each element digitally. Tropidosuchus romeri shares with rhadinosuchines the presence of a crest on the lateral surface of the jugal and a main body of the postorbital with a projection onto the orbit. Features such as a short preorbital region (less than half of skull length), well-developed anterolateral processes of the frontals, premaxilla without anteroventrally oriented alveolar margin, a maxilla restricted to the lateral wall of the rostrum, and a skull roof ornamentation composed of crests without a radial arrangement are probably autapomorphies. Future studies will focus on the postcranial anatomy and on an integration of the new information to test quantitatively the phylogenetic relationships of the species.