Long bone scaling of caseid synapsids: a combined morphometric and cladistic approach

Abstract

In this study, the evolution of the long bones in a group of basal synapsids was analysed, belonging to the caseids (Synapsida, Caseasauria, Caseidae), a crucial Palaeozoic group of terrestrial vertebrates. During their evolutionary history, caseids transitioned from small faunivorous tetrapods such as Eocasea martini (reaching ~20 cm total length), to gigantic herbivores such as Cotylorhynchus hancocki (reaching almost 7 m in length). In this analysis, morphometric techniques (i.e. Principal Component Analysis, Reduced Major Axis (regression) slopes) and phylogenetic methods (construction of metric trees) were used in a comparative and integrated way in order to study the major changes in the long bones in the course of caseid evolution, and identify in which nodes and terminal branches more extreme repatterning of structures is concentrated. The analysis showed a decoupling between the long bones of the fore- and hindlimbs, with the hind leg remaining more conservative and congruent to a basal pattern. By contrast, the forelimbs (especially the ulna and radius) show greater restructuring in the course of evolution, with an allometric strengthening not limited to species of very large size. An overbuilt appendicular skeleton was detected as early as taxa small to medium in size, indicating that the strengthening is not related in a simple and linear way to a structural response to gigantic body size. Because this robust structure is already present in small- to medium-sized taxa, it may have been subsequently exapted, allowing to reach the colossal body size of more derived members of Caseidae, which rank among the largest reached throughout the Palaeozoic.