Chañares Formation

The Triassic period documents the origin and diversification of modern amniote lineages and the Late Triassic fossil record of South America has been crucial to shed light on these early evolutionary histories. However, the faunistic changes that led to the establishment of Late Triassic ecosystems are largely ignored because of the global scarcity of fossils from assemblages a few million years older. Here we contribute to fill this gap with the description of a new tetrapod assemblage from the lowermost levels of the Chañares Formation (uppermost Middle–lower Late Triassic epochs) of Argentina, which is older than the other vertebrate assemblages of the same basin. The new assemblage is composed of therapsids, rhynchosaurids and archosaurs, and clearly differs from that of the immediately overlying and well-known historical Chañares vertebrate assemblage. The new tetrapod association is part of a phase of relatively rapidly changing vertebrate assemblage compositions, in a time span shorter than 6 million years, before the diversification of dinosaurs and other common Late Triassic tetrapods in southwestern Pangaea.

Rhynchosaurs first appear in the Early Triassic fossil record and flourished during the late Carnian as the dominant members of several assemblages worldwide. In Argentina, the rhynchosaur record is currently restricted to the Ischigualasto Formation of late Carnian–earliest Norian age. Recent fieldwork in the new locality of Brazo del Puma, in the lowermost levels of the Chañares Formation, yielded three rhynchosaur tooth-bearing bones, which were collected five metres above the contact with the underlying Tarjados Formation. The most complete specimen is the posterior end of the alveolar region of a left dentary. The dentary possesses densely packed tooth rows on the lingual surface and medial half of the occlusal surface, showing longitudinal Zahnreihen. The teeth of the occlusal surface are worn flat and those of the lingual surface are organized in multiple rows, supporting the referral of the specimen to Rhynchosauridae. In addition, the dentary teeth are conical to mesiodistally compressed, resembling the condition observed in hyperodapedontines. The rhynchosaur remains reported here are the oldest collected in Argentina and among the oldest in South America, together with an unnamed form from Brazil. The new rhynchosaur specimens come from levels in which dicynodonts are numerically dominant, whereas cynodonts are considerably less abundant. Accordingly, the specimens reported here bolster faunal differences within the Chañares Formation and add a new faunal component to this already diverse vertebrate assemblage.