Descrição, taxonomia, filogenia e evolução craniana de novos probainognátios não-mamaliaformes (Cynodontia: eucynodontia) sul-brasileiros

Abstract

Cynodontia comprises a large synapsid clade that includes numerous species, both nonMammaliaformes and Mammaliaformes. Non-mammaliaform forms were among the most conspicuous components of the continental paleofauna of terrestrial tetrapods during the Triassic, occurring throughout Pangea. Less inclusive, the clade Probainognathia has a great variety of size and dental morphology, exhibiting an abundant fossil record in the faunal successions of the Middle and Late Triassic, especially in Brazil and Argentina. The present work aims to study the cranial, mandibular, and dental anatomy of new specimens (CAPPA/UFSM 0208, CAPPA/UFSM 0262, and CAPPA/UFSM 0210) of non-mammaliaform probainognathian cynodonts from the Upper Triassic, collected in the region of Quarta Colônia, central region of Rio Grande do Sul. The specimens were CT scanned, and their anatomy formally described. The materials present diagnostic morphological structures of both the Probainognathia clade and the less inclusive Prozostrodontia clade. CAPPA/UFSM 0262 and CAPPA/UFSM 0208 correspond to a new genus and species of non-mammaliaform prozostrodontian cynodont, Agudotherium gassenae Stefanello et al., 2020a. The new taxon consists of a left dentary (holotype, CAPPA/UFSM 0262) and a right dentary (paratype, CAPPA/UFSM 0208), with preserved incisors, canines, and postcanines. The new species clearly differs from other known non-mammaliaform Carnian and Norian cynodonts in having a unique combination of morphological features. Agudotherium gassenae provides new evidence on the diversity of non-mammaliaform probainognathian cynodonts and the knowledge of the adaptive irradiation of Prozostrodontia, which occurred during the late Triassic period. CAPPA/UFSM 0210 comprises a new and more complete specimen of Prozostrodon brasiliensis (BARBERENA; BONAPARTE; TEIXEIRA, 1987), represented by a complete skull with disarticulated dentaries. Given the completeness of the specimen, the new material from P. brasiliensis provides additional anatomical information for the posterior region of the skull, helping to fill in gaps in the evolutionary path of the transformations undergone within the nonMammaliaformes Cynodontia clade, which led to the currently known Mammalian pattern. The new anatomical data, through cladistic investigation, reveal a new endemic clade of South American cynodonts – Prozostrodontidae. This clade illustrates the first radiation of Carnian prozostrodontians to western Gondwana. Finally, the set of specimens studied contributes to the knowledge and understanding of the cranial evolution and diversification of non-Mammaliaform Prozostrodontia during the late Triassic period.