Redescription and Phylogenetic Relationships of the Proterochampsid Rhadinosuchus gracilis (Diapsida: Archosauriformes) from the Early Late Triassic of Southern Brazil

Abstract

The proterochampsids are small to medium-sized, quadrupedal and probable semi-aquatic reptiles that were part of the evolutionary radiation of Archosauromorpha during the Triassic. The group is restricted to the Middle and Late Triassic of South America with eight nomi - nal species. Rhadinosuchus gracilis Huene was the first described proterochampsid and comes from the late Carnian-earliest Norian of south - ern Brazil. This genus and species is currently the most enigmatic and poorly known member of the group. For this reason we redescribe here its anatomy and discuss its phylogenetic relationships. We found that this species can be distinguished from other proterochampsids and we dismiss the proposed synonymy with Cerritosaurus binsfeldi Price. Our quantitative phylogenetic analysis found Rhadinosuchus gracilis more closely related to Gualosuchus reigi Romer and Chanaresuchus bonapartei Romer than to other proterochampsids, together forming the subfamily Rhadinosuchinae. Characters supporting this assignment include a maxilla with a distinct longitudinal change in slope between lateral and dorsal surfaces, and dorsal surface of nasal and frontal ornamented by ridges showing a radial pattern. In addition, Rhadinosuchus gracilis was recognized as the sister taxon of Chanaresuchus bonapartei based on a lacrimal with an antorbital fossa that occupies almost half or more of the anteroposterior length of the ventral process. The topology of the phylogenetic analysis shows that the Brazilian species do not form a monophyletic clade and favours multiple dispersal events between the Ischigualasto-Villa Union (northwestern Argentina) and the Parana (southern Brazil) basins .