Iasmim Michelotti da Costa

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.

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.

Hypercanines, or hypertrophied canines, are observed in a wide range of both extinct and extant synapsids. In non‐mammaliaform cynodonts, the Permo‐Triassic forerunners of mammals, long canines are not uncommon, appearing in several unrelated taxa within the clade. Among them is Trucidocynodon riograndensis, a carnivorous ecteniniid cynodont from the Late Triassic of Brazil, which exhibits a specialized dentition, including spear‐shaped incisors, very long and narrow canines, and sectorial postcanines with distally oriented cusps, all of which have finely serrated margins. Recent synchrotron X‐ray micro‐computed tomography of a large specimen (CAPPA/UFSM 0029; Várzea do Agudo site, Brazil) provides new insights into its lower jaw and dentition, as well as offers the first digital endocast of an ecteniniid. Our study reveals the presence of (i) putatively opened‐root canines in the adult stage and the possible presence of unresorbed remnant of an old canine, which may indicate that the specimen stopped replacing its canines; (ii) lower canines that are longer than the upper canines and, in occlusion, were kept inside deep paracanine fossae that perforated the dorsal surface of the rostrum; (iii) a diastema between the incisors and lower canine, which is absent in the holotype; (iv) advanced brain structures, such as the absence of a pineal body, presence of cerebral hemispheres divided by the interhemispheric sulcus and expanded laterally, and a higher encephalization quotient than non‐mammaliaform prozostrodonts, reflecting the homoplastic evolution of relative brain sizes observed in Triassic cynodont lineages. Finally, the abundance of carnivorous and omnivorous species at the Várzea do Agudo site, where the specimen was found—including the archosauriforms Dynamosuchus collisensis and Stenoscelida aurantiacus—suggests a diverse predator guild that warrants further investigation from a paleoecological perspective.