Postcranial anatomy and osteohistology of an early prozostrodont cynodont from the Upper Triassic of Brazil

Prozostrodon brasiliensis, a probainognathian cynodont of the clade Prozostrodontia, is recognized as a key taxon for understanding the origin of Mammaliaformes and mammals. Despite detailed investigations of its cranial anatomy, the postcranial skeleton has received comparatively less attention. In this contribution, we present a multi-proxy approach to study new postcranial elements assigned to an individual of P. brasiliensis from the Marchezan outcrop (Hyperodapedon Assemblage Zone of Santa Maria Supersequence, Upper Triassic, Rio Grande do Sul, Brazil), integrating anatomical, osteohistological, and phylogenetic data. The analyzed elements include parts of the pectoral girdle (left scapulocoracoid and left clavicle), forelimb (right and left humeri), hindlimb (left femur, tarsal, and metatarsal elements), and caudal vertebrae. The postcranial skeleton exhibits a mosaic of plesiomorphic and derived features, indicating an intermediate morphological (and possibly functional) condition within Prozostrodontia. The anatomy of the scapulocoracoid and humerus suggests a transitional forelimb posture, whereas the femur and hindfoot indicate a generalized, predominantly terrestrial locomotor pattern. Elongated amphicoelous caudal vertebrae further suggest a proportionally long and functional tail, likely contributing to balance and body stabilization. Osteohistological analyses of the humerus and femur show predominance of a woven-parallel complex, consistent with relatively high growth rates. The presence of two growth lines indicates cyclical deposition, whereas the absence of an external fundamental system (EFS) and the lack of a marked decrease in vascularization suggest that the individual had not yet reached skeletal maturity at the time of death. A phylogenetic analysis was performed incorporating newly defined humeral characters and a revised set of postcranial characters modified from previous datasets. Phylogenetic mapping of postcranial characters reveals recurrent patterns of homoplasy within Probainognathia, highlighting the anatomical diversity of non-mammaliaform cynodonts. Together, these results provide new insights into the postcranial record of Prozostrodon brasiliensis and reinforce its significance for understanding the postcranial evolution of the mammal forerunners.