Postcranial anatomy of Riograndia guaibensis (Cynodontia: Ictidosauria)

Abstract

Postcranial remains of non-mammaliaform cynodonts provide crucial information documenting the anatomical steps and locomotor advances that led to the mammalian pattern. Here, we present a detailed description of the first postcranial specimens referable to Riograndia guaibensis (Probainognathia, Ictidosauria), an abundant non-mammaliaform cynodont from the Late Triassic of Brazil (Riograndia Assemblage Zone, Candelária Sequence) that is closely related to Mammaliaformes. Functional and ecological implications of these fossils are also investigated, based on scapular and humeral morphology and muscular reconstructions. The pectoral girdle and forelimb show fossorial adaptations, with numerous specializations increasing the mechanical advantages of retractor muscles. R. guaibensis retained a semi-sprawling forelimb, with well-developed adductor muscles to hold the body off the ground. These results confirm that the forelimb posture of non-mammaliaform probainognathians was still similar to that of basal synapsids and differed from the more erect posture and parasagittal function of therians.