Skull anatomy and endocranial casts of Siriusgnathus niemeyerorum (Cynodontia: Cynognathia) from the Late Triassic of Brazil: implications for the evolution of traversodontids

Traversodontidae was a widely distributed clade of Triassic cynodonts with several representatives in South America. The increasing number of specimens unearthed from this continent in recent years provides an opportunity to re-evaluate several aspects of known species. This study aims to provide an in-depth description of the skull and endocranial anatomy of the traversodontid Siriusgnathus niemeyerorum (Candelária Sequence, Brazil, Upper Triassic) using high-resolution CT scans. Several skull regions poorly known for the species, such as the lateral wall of the braincase, basicranium, orbitosphenoid bone, and postdentary bones, are described here. The cranial endocast is also revisited, and the cranial nerves are reconstructed for the first time for the species. We also present the first description of the inner ear of S. niemeyerorum and discuss it in the context of early-diverging epicynodonts and cynognathians, including traversodontids. Finally, we found that the maxillary canal of gomphodontosuchines is less ramified than in other studied traversodontids and cynognathians. This resembles transformations observed in probainognathians and could indicate increased facial mobility in gomphodontosuchines compared to other traversodontids. Overall, this study provides a comprehensive understanding of the skull of S. niemeyerorum, deepening our knowledge of traversodontids—the most abundant clade of Triassic cynodonts.