A robust sauropodomorph specimen from the Upper Triassic of Argentina and insights on the diversity of the Los Colorados Formation

The Late Triassic rocks document the first steps of the early dinosaur evolutionary radiation. Although the oldest dinosaurs were not abundant in their assemblages, sauropodomorphs achieved a wide taxonomic diversity and high abundance towards the Triassic-Jurassic boundary. In South America, this pattern is documented in the Ischigualasto-Villa Unión Basin of northwestern Argentina, in which dinosaurs achieved a numerical dominance over other tetrapods during the deposition of the upper levels of the late Norian-Rhaetian Los Colorados Formation. In this contribution we enrich the faunal list of this assemblage with the description of a new medium-sized basal sauropod specimen with a very robust tibial morphology. This new specimen differs from the other known sauropodomorphs described for the Los Colorados Formation and increases the alpha-diversity recorded for this group. A phylogenetic analysis recovered the new specimen at the base of Sauropoda and closely related to Lessemsaurus, Antetonitrus, and other basal sauropods. These results match with the high degree of robustness observed in the tibia of the specimen reported here, which closely approaches the morphology documented for other basal sauropods and departs from the morphospace occupied by non-sauropod sauropodomorphs. A two step pattern of tibia robustness increase is observed in the sauropodomorph phylogeny, a pattern that coincides and could be related with the acquisition of more habitual quadrupedal gait achieved by basal sauropods.