Re-assessing the age of the type locality of Nythosaurus larvatus (Therapsida, Cynodontia) and implications on the evolutionary dynamics of cynodonts

Nythosaurus larvatus was one of the first cynodonts named and, for a long time, the only therapsid for which the endocranial cast was described. The holotype and only skull of this species is thus a landmark in the history of therapsid palaeontology. It is believed to have come from Commissie Drift 303 (Free State Province, South Africa), a locality that has been traditionally dated to the Lystrosaurus declivis Assemblage Zone. However, this age was mostly based on the synonymy between Nythosaurus larvatus and Thrinaxodon liorhinus, the former being the only fossil indisputably reported from this locality and the latter being an index taxon for the Lystrosaurus declivis Assemblage Zone. A recent revision of Nythosaurus larvatus also questioned the age of Commissie Drift. Here, we report the result of our fieldwork to re-assess the age of this locality. We find that the stratigraphy conforms to the lower Burgersdorp Formation and that the fossil tetrapod fauna does not match the Lystrosaurus declivis Assemblage Zone. The overall scarcity of fossil bones, absence of plants and dicynodonts, and relative dominance of amphibian remains all point to an assignation to the Langbergia–Garjainia Subzone of the Cynognathus Assemblage Zone, which is consistent with the stratigraphy. The discovery of new material attributable to cf. Nythosaurus bolsters the notion that this taxon is from the late Olenekian. Nythosaurus larvatus can safely be considered a valid taxon, and may in fact occur elsewhere in this subzone.