New specimen of Dinodontosaurus (Therapsida, Anomodontia) from west-central Argentina (Chañares Formation) and a reassessment of the Triassic Dinodontosaurus Assemblage Zone of southern South America

Abstract

The Chañares Formation is known worldwide for its diverse tetrapod assemblage, including representatives of proterochampsid archosauromorphs, early offshoots of the crocodilian lineage, precursors of the dinosaurian lineage, and dicynodont and cynodont therapsids. The dicynodont Dinodontosaurus, originally described nearly 80 years ago, is the most common dicynodont therapsid in the assemblage. Here we present a new specimen of this taxon, focusing on the basicranium structure using 3D modeling and digital segmentation of the individual bones to provide a novel description of this region of the skull. Among the newly-recognized anatomical structures are a rod-like pila antotica in the prootic, the presence of a dorsal conical projection of the parasphenoid, and the anterior extension of the supraoccipital between the prootic and the parietal. The Chañares tetrapod assemblage was historically correlated with the Dinodontosaurus Assemblage Zone of Southern Brazil, originally due to the presence of this large dicynodont, as well as the cynodont Massetognathus, and more recently by absolute dates of the fossil-bearing strata. Both assemblages are similar in diversity and taxonomy, particularly in their non-mammalian therapsids; however their body-size distributions are very different. These differences in body-size are likely not reflecting a taphonomic bias but local variations in climate that promoted more open landscapes in southern Brazil in contrast to western Argentina, where more humid conditions prevailed during the early Late Triassic.