Reassessment of the postcranial anatomy of Prozostrodon brasiliensis and implications for postural evolution of non-mammaliaform cynodonts

ABSTRACT

Postcranial anatomical studies on non-mammaliaform cynodonts are essential for understanding postural and locomotor advances in mammalian evolution and provide information about the transition from the sprawling posture of basal synapsids to a more erect posture. Nevertheless, the number of functional analyses of this group remains small because little postcranial material has been discovered and the articular surfaces of bones are rarely preserved. Here, we present a detailed description of the postcranial skeleton of Prozostrodon brasiliensis (Probainognathia, Prozostrodontia), a non-mammaliaform cynodont from the Upper Triassic of Brazil (Hyperodapedon Assemblage Zone, Candelária Sequence), closely related to Mammaliaformes. The study adds new information on postcranial morphology, and functional implications are investigated. Prozostrodon brasiliensis had a semisprawling forelimb and a more therian-like erect hind limb. These results confirm that the evolution of an erect posture and a new muscular orientation of the hind limb occurred within Cynodontia before the appearance of the clade Mammaliaformes.

ACKNOWLEDGMENTS

We thank CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico) for the financial support. We thank also the crew of the PaleoVertebrate Laboratory, UFRGS, for continuous support. Photographs were skillfully taken by L. F. Lopes (UFRGS). We especially thank S. D. Chapman and P. Gill (NHMUK), M. Lowe (UMZC), G. Veron (MNHN), D. Sanches Venturini (MVP), A. M. Ribeiro (Fundação Zoobotânica-RS), and C. L. Schultz (UFRGS) for permission to access the respective collections. Finally, we thank reviewers J. Fröbisch and C. Kammerer and editor T. Martin for comments and valuable contributions, which greatly improved the manuscript.