Anatomical Details of Aetosauria (Archosauria: Pseudosuchia) as Revealed by an Articulated Posterior Skeleton from the Upper Triassic Ischigualasto Formation, San Juan Province, Argentina

Abstract. Aetosaurs are an early-diverging clade of “crocodile-line” archosaurs whose oldest records come from Argentina and Brazil. Articulated skeletons of aetosaurs are rare, but offer insight into their paleobiology. We describe here an incomplete, articulated posterior skeleton of an aetosaur from the Upper Triassic Ischigualasto Formation of San Juan Province, Argentina. It resembles Aetosauroides, the only aetosaur genus currently recognized from the Ischigualasto Formation, but lacks apomorphies that would allow us to assign it to that taxon, appears to be more robust than the holotype of A. scagliai, and preserves a different tail morphology. We identify the specimen as a basal aetosaur in part because the few exposed dorsal and lateral osteoderms have a typical ornamentation of radially distributed pits, grooves, and ridges emanating from a “center of ossification”. Although the specimen is incomplete and exposed primarily in ventral view from the sacrum posteriorly, it preserves many anatomical features not often preserved in aetosaurs, including extensive appendicular armor and a well-preserved caudal ventral carapace. The latter apparently consists of only two columns of ventral osteoderms, and preserves a large cloacal vent proximally. Posteriorly, the ventral paramedian osteoderms fuse to form a single element, something that has not previously been demonstrated in aetosaurs. The arrangement of osteoderms around the vent is distinct from that seen in A. scagliai. The ventral caudal osteoderms differ from many other aetosaurs in that they do not transition from wider than long to longer than wide, indicating that the specimen had a relatively abbreviated tail.