The first braincase of the basal aetosaur Aetosauroides scagliai (Archosauria: Pseudosuchia) from the Upper Triassic of Brazil

Abstract

The phylogenetic relationships of Pseudosuchia, the crocodile-line of Archosauria, are still poorly resolved, in part, due to the lack of crucial braincase information for several key taxa. Recently, erpetosuchids and ornithosuchids have been recovered as close relatives to Aetosauria, sharing several braincase features. Here we provide the description of the first braincase of the basal aetosaur Aetosauroides based on specimens from the Upper Triassic Candelária Sequence of Brazil. Our study revealed the presence of an exoccipital lateral ridge and a medial ridge on the supraoccipital (both shared with all aetosaurs and erpetosuchids, but absent in ornithosuchids) and an anterolateral exit for the internal carotids (shared with all aetosaurs and ornithosuchids, but not with erpetosuchids). Aetosauroides lacks a medial contact between the exoccipitals (shared with the aetosaurs Desmatosuchus smalli and Tecovasuchus) and possesses a single hypoglossal foramen (contrasting with Stagonolepis robertsoni and Desmatosuchus spurensis). It also differs from the putative Argentinian Aetosauroides (PVSJ 326) by the presence of a ridge connecting the basal tubera medially (contrasting also with all stagonolepidoideans) and by a bulbous and ventrolaterally recurved basipterygoid process (contrasting with Paratypothorax). These features show that the braincase of aetosaurs is suitable for providing further phylogenetic information and may contribute to resolving controversies within Pseudosuchia relationships.