A Triassic tetrapod footprint assemblage from southern South America: palaeobiogeographical and evolutionary implications

A new Triassic tetrapod track assemblage recorded from levels of the Cuyana Basin (west-central Argentina) is described. The tracks came from multiple track-bearing horizons of the Upper Triassic (Carnian) Portezuelo Formation that indicate recurrent playa to mudflat conditions in a marginal lacustrine succession. This footprint assemblage is the most important one described up to now for the lower Mesozoic of South America, and forms a significant estimate of the fauna represented during that time in the region. Therefore, in order to understand the faunal significance of the assemblage, the track-makers were identified within a phylogenetic context. Although at a coarse taxonomic level, this analysis indicates the presence of non-mammalian therapsids (small cynodonts and large dicynodonts) and a quite diverse archosaur fauna that includes members of both crurotarsal archosaurs and dinosaurs (sauropodomorphs and theropods). Comparisons between the studied assemblage and other Early Mesozoic tetrapod track records from Gondwana suggest close relationships with those described from the Upper Triassic of the Karoo Basin in South Africa. In addition, the presence of putative basal sauropodomorph dinosaur tracks in the Portezuelo levels suggests that the stratigraphic range of the group extends back into the Carnian, farther than that documented by the known body-fossil record of the group in western Gondwana.