Revision of the holotype of Rhadinosuchus gracilis (Archosauriformes: Proterochampsidae) from the early Late Triassic of southern Brazil.

DESOJO, J.B; RAUHUT, O.W.; EZCURRA, M.

Munich

Congreso; Annual meeting of the German Palaeontological Association Paläontologische Gesellschaft; 2010

Paläontologische Gesellschaft

The Proterochampsidae includes a clade of non­-archosaur archosauriforms recorded in Middle to Late Triassic outcrops of the South American Paraná Basin (Brazil) and Ischigualasto Villa-Unión Basin (Argentina). This lineage consisted of semi-aquatic animals bearing sub-triangular skulls in dorsal view with highly ornamented roof bones. The Late Triassic proterochampsids are represented by Proterochampsa barrionuevoi and Chanaresuchus sp. in Argentina and Proterochampsa nodosa, Cerritosaurus binsfeldi, and Rhadinosuchus gracilis in Brazil. Of these late proterochampsids, the most enigmatic is Rhadinosuchus, originally described by von Huene from the Zahn Sanga in beds belonging to the Hyperodapedon Assemblage Zone (Ischigualastian). The lack of further studies since its original description currently obscured its phylogenetic affinities and taxonomic validity. A reappraisal of the holotype of Rhadinosuchus has allowed the recognition of characters considered synapomorphic for Proterochampsidae, namely the presence of external nares located dorsally and posterior to the tip of snout and dermal sculpturing on the frontals and nasals consisting largely of sagittal ridges. Rhadinosuchus also exhibits a strongly elongated antorbital region representing more than 50% of the skull length, a condition recognized in “proterosuchids”, phytosaurs, and proterochampsids among basal archosauriforms. A unique combination of apomorphies allows the distinction of Rhadinosuchus from other basal archosauriforms, including autapomorphic maxillae with a laterally inflatened anterior portion and sub-thecodont maxillary dentition, in which the alveoli are medially opened. Accordingly, we present here evidence supporting the taxonomic validity of Rhadinosuchus gracilis and its assignmet to Proterochampsidae. Thus, the alfa-taxonomic diversity of proterochampsids in the Late Triassic of Brazil reaches that recorded during the Middle Triassic of Argentina, which was traditionally considered as showing the peak of proterochampsid diversity. The current understanding of the group shows that the diversity of proterochampsids did not meagre towards the end of the Triassic, but the morphological diversity of the lineage increased in Ischigualastian times.