Review of the fossil record of early dinosaurs from South America, and its phylogenetic implications

Triassic beds from Argentina and Brazil provide the most relevant fossil record of early dinosauriforms in terms of numerical abundance and taxonomic diversity. This record currently represents the best source to understand the origin and early evolutionary radiation of dinosaurs. In the present paper we offer an updated review focused on the available evidence of Carnian dinosaurs from this continent, but we also discuss the record of Triassic dinosaur precursors and the evolution of Triassic dinosaurs in other continents. It is clear that, aside the agreed taxonomic composition of some particular dinosaurian subclades (e.g., Herrerasauridae, Neotheropoda), there is no consensus about early dinosaur phylogeny, and our paper is not the exception. Recent years witnessed the discovery of several new early dinosaurian taxa, as well as reviews of the taxonomic allocation of several renowned forms such as Lagerpeton, Lewisuchus, Pisanosaurus, and Eoraptor. New analyses demonstrate that evidence supporting the taxonomic referrals of pre-Norian dinosaurs to Theropoda, Sauropodomorpha and Ornithischia are tenuous, at best. Here we present new anatomical observations and comparisons for each of these South American early dinosauriforms with the aim to test previous phylogenetic interpretations. Evidence from South America allows reviewing the phylogenetic relationships of taxa from other continents, including Tawa, Chindesaurus, and Daemonosaurus, which are here suggested to nest within Herrerasauria. Evidence at hand indicates that herrerasaurs were a successful clade of archaic predatory saurischians that inhabited both South and North America, and probably also India and Europe.