Evolution of the reptile spine reveals independent trajectories to axial skeletal complexity in amniotes

The evolution of complex, highly regionalized and heterogeneous axial skeletons within amniotes has traditionally been considered a unique characteristic of Pan-Mammalia, with limited complexity evolving independently in some reptiles. The ability to resolve axial skeletal evolution across Amniota remains restricted due to the lack of studies comprehensively exploring axial skeletal complexity through deep time outside of Pan-Mammalia. Here, we combine 3D geometric morphometrics of vertebral morphology with maximum likelihood model testing in a phylogenetic context to quantify regionalization and morphological heterogeneity in the presacral vertebral column of reptiles and representative tetrapod outgroups. We recover evidence for the evolution of four regions at least four times independently within amniotes, highly heterogeneous axial skeletal anatomies in archosaurs, and no evidence for uniquely complex vertebral anatomies in mammals. Heterogeneity is positively associated with body size in most reptile clades except for theropod dinosaurs, which also reduce regionalization toward the avian crown. The evolution of volancy is correlated with high heterogeneity, potentially associated with functional modularity of the cervical and dorsal regions. Our results indicate that complex axial skeletons arose independently and repeatedly in reptiles in addition to mammals, variably associated with the remarkable diversity in size, body form, function, and ecology across amniotes.