Reconstructing Body Size and Center of Mass in Synapsids

Abstract

Body size and center of mass of an organism have an influence on ecology, physiology, and locomotion. Vertebrates are typically preserved in the fossil record only as bones, so reconstructing their full body size requires estimation methods to account for soft tissue. Here, I employ two approaches in particular: volumetric reconstruction and limb scaling. Volume reconstructions were obtained through photogrammetry for an array of non-mammalian synapsids and therapsids in order to generate estimates of body mass and center of mass position. Secondary body mass estimates based on limb scaling were also computed using equations derived from Campione & Evans (2012). Comparison of volumetric and limb scaling approaches across 11 fossil tetrapods showed an overestimation of body mass range when using limb scaling equations, primarily due to the fact that early fossil synapsids have much more robust limb bones. Extant tetrapod limb bone diversity may not span the range of limb bone morphologies present in fossil species and may therefore be inadequate for inferring aspects of Synapsid biology. Further, the results here indicate all specimens likely employed quadrupedal locomotion, and normalized center of mass values are consistent with values observed in modern quadrupedal mammals. These findings contribute to our understanding of body size evolution in the forerunners of mammals and the methods best suited for estimating these characteristics.