Osteology of the pectoral girdle and forelimb of Siriusgnathus niemeyerorum, a Late Triassic traversodontid cynodont from Brazil.

Abstract

Traversodontids are non-mammaliaform cynodonts, herbivorous to omnivorous, that lived during the Triassic. In the Late Triassic, some members of this group increased in body size relative to other cynodont lineages and evolved postcranial adaptations associated with locomotion. The Late Triassic Siriusgnathus niemeyerorum is one such traversodontid. Originally described in 2018 based on cranial material, the species has since been the focus of considerable advances in our understanding of its skull anatomy; however, its postcranial skeleton remains undescribed. The present study examined the postcranial osteology of this traversodontid, focusing on the pectoral girdle and forelimb of the specimen CAPPA/UFSM 0109, paratype of the species. These remains show that Si. niemeyerorum presents a scapulocoracoid with concave anterior and posterior margins and a lateroventrally oriented glenoid fossa formed by the scapula and coracoid; a humerus with a short diaphysis and a broadly open deltopectoral crest; an ulna with a developed olecranon process; and a manus with a phalangeal formula of 2-3-3-3-3. One of the distal carpals (dc1) is greatly elongated and morphologically similar to a metacarpal, a feature observed in some gomphodonts. The combination of traits present in the scapulocoracoid (glenoid) and the robustness of the humerus suggests a lateralized ("sprawling") forelimb posture in Si. niemeyerorum, similar to that inferred for Exaeretodon argentinus and Exaeretodon riograndensis. In summary, this study expands current knowledge of traversodontid postcranial anatomy and provides new insights into the locomotor adaptations among non-mammaliaform cynodonts.