Dicynodonts (Therapsida: Anomodontia) of South America

Abstract

The dicynodont fauna of South America is comprehensively reviewed. Permian South American dicynodont body fossils are known only from the Rio do Rasto Formation of southern Brazil, and include a specifically uncertain representative of the trans-Gondwanan genus Endothiodon and the endemic Rastodon procurvidens. The majority of known South American dicynodonts are Triassic kannemeyeriiforms, with numerous records from Argentina and Brazil. Traditionally, the stratigraphically lowest Triassic dicynodont-bearing beds in South America were thought to be those of the Argentine Puesto Viejo Group. Historically considered Middle Triassic on biostratigraphic grounds, radioisotopic dating has instead suggested that they are at least partially Late Triassic. Re-examination of the dicynodont fossils from the Puesto Viejo Group provides evidence for taxonomic differentiation between its two component formations. The Quebrada de los Fósiles Formation contains two kannemeyeriiform morphotypes: a shansiodontid identified as the first South American record of Tetragonias and a kannemeyeriid (Kannemeyeria aganosteus sp. nov.). The overlying Río Seco de la Quebrada Formation contains the historically problematic “Kannemeyeria” argentinensis, here recovered as a kannemeyeriid but placed in a distinct genus, Acratophorus gen. nov. The South American endemic shansiodontid Vinceria andina is recorded only from the Argentine Cerro de las Cabras Formation, of uncertain age but possibly coeval with the Puesto Viejo Group. Definitive Late Triassic dicynodonts are well known from the Argentine Agua de la Peña Group and Brazilian Santa Maria Supersequence. The phylogenetic position and alpha taxonomy of the most abundant South American dicynodont, Dinodontosaurus, are problematic. Minimally, two species of this genus should be recognized (D. tener in Brazil and D. brevirostris in Argentina), but extensive variation in cranial morphology is present among the Brazilian specimens. Better-preserved material and more extensive study are required to determine whether this variation is taxonomically significant. In addition to the eponymous genus, the Brazilian Dinodontosaurus Assemblage Zone contains at least two stahleckeriids: Stahleckeria potens and Sangusaurus sp. No dicynodonts are known from the overlying Hyperodapedon Assemblage Zone in Brazil, but the correlative Ischigualasto Formation of Argentina yields the abundant stahleckeriid Ischigualastia jenseni. A single dicynodont genus (the stahleckeriid Jachaleria) is present in the South American Norian, and both nominal species (J. colorata from Argentina and J. candelariensis from Brazil) are here recognized as valid. Our results indicate that individual dicynodont species were more geographically and stratigraphically restricted than previously thought, but also that there is extensive overlap between South American and African assemblages at higher taxonomic (i.e., genus) levels.