First volumetric body mass estimate and a new in vivo 3D reconstruction of the oldest Karoo pareiasaur Bradysaurus baini, and body size evolution in Pareiasauria

Pareiasaurs were among the world’s first large terrestrial tetrapods, first appearing during the Guadalupian Epoch, and were later widespread across Pangea during the Lopingian where they formed a significant part of the large herbivore guild. The Bradysauria of the Lower Beaufort Group of the Karoo Basin of South Africa are among the oldest pareiasaurs, yet little is known of their palaeobiology. Here, we present the first volumetric body mass estimate and a new in vivo reconstruction of the earliest Karoo pareiasaur, and the phylogenetically basal-most pareiasaur, Bradysaurus baini, based on two almost complete adult mounted skeletons. Using 3D photogrammetric models of the two skeletons, we calculated a possible body mass range for B. baini by applying different densities for living tissue to reconstructions of differing soft tissue mass. Considering the larger adult Bradysaurus specimen, our volumetric body masses range from a minimum of 851.4 kg to a maximum of 1,276.5 kg, with an overall mean body mass of 1,022 kg, approximately the body weight of a large domestic cow. The study provides a more precise estimate of body mass in early members of Pareiasauridae, and we suggest that they were somewhat smaller than some late Permian taxa such as Scutosaurus karpinskii.