Osteology and ontogeny of the Permian dicynodont Rastodon procurvidens (Synapsida, Therapsida) based on micro-CT scanning and its phylogenetic and biogeographical implications

Dicynodonts were a globally distributed group of herbivorous synapsids that first appeared during the middle Permian (Guadalupian) and survived the Permian-Triassic Mass Extinction, until their extinction in the Late Triassic. Their fossil record in South America is almost entirely concentrated in the Triassic, with only two described Permian species. One of these, Rastodon procurvidens, was found in the Rio do Rasto Formation in Rio Grande do Sul, Brazil, and represents one of the best-preserved Permian therapsids from South America. The phylogenetic position of Rastodon within dicynodonts remains debated, with competing hypotheses leading to radically different conclusions regarding the early biogeography of dicynodonts. In this study, we present a detailed description of both the cranial and postcranial skeleton of the R. procurvidens holotype using microcomputed tomography. Phylogenetic analyses under both maximum parsimony and Bayesian criteria support the placement of Rastodon within the emydopoid clade Kingoriidae. These findings provide the first evidence of Emydopoidea and Kingoriidae in South America and underscore the limited knowledge of dicynodonts during the Guadalupian, particularly outside southern Africa. Furthermore, they suggest an earlier origin of Kingoriidae and the possibility that the group originated in west rather than east Gondwana.