New specimen bolsters the presence of stahleckeriid dicynodonts in the Ladinian- Earliest Carnian levels of the Chañares Formation, La Rioja Province, Argentina

Abstract

Proterosuchidae represents the oldest substantial diversification of Archosauromorpha andProterosuchus is stratigraphically the lowermost new taxon after the Permo-Triassic mass extinctionin the Karoo of South Africa. Therefore, proterosuchids play a key role in understanding the bioticrecovery after this mass extinction. Proterosuchidae has been treated as a wastebasket taxon towhich at least 17 species have been referred, but a recent revision reduced its taxonomic content tofive species within the genera Archosaurus , Proterosuchus and ? Chasmatosaurus ?, from the latestPermian of Russia and the Induan of South Africa and China. Additionally, a few isolatedproterosuchid bones were also reported more than 50 years ago from the Induan Panchet Formationof India. We conducted fieldwork in this unit to expand our knowledge of the Panchet proterosuchidassemblage. We collected multiple vertebrate specimens, including fishes, temnospondyls,dicynodonts, and proterosuchids; the latter including a few cranial and limb bones, and multiplecervical, dorsal, sacral and caudal vertebrae. The presacral vertebrae of the Panchet proterosuchidclosely resembles those of the Early Triassic Russian species Chasmatosuchus rossicus andChasmatosuchus magnus , but they also possess a unique combination of features indicating referral toa new species. Our phylogenetic analysis recovered the Panchet taxon and Chasmatosuchus spp.within Proterosuchidae, closer to each other than to a Proterosuchus -? Chasmatosaurus ? clade;whereas the Permian Archosaurus behaves as a wildcard taxon within Proterosuchidae. Thus, thetaxonomic diversity of Proterosuchidae is substantially expanded here, indicating a broaderdiversification of the clade in the immediate aftermath of the Permo-Triassic mass extinction.