Data from: Head-turning morphologies: evolution of shape diversity in the mammalian atlas-axis complex

Mammals flex, extend, and rotate their spines as they perform behaviors
critical for survival, such as foraging, consuming prey, locomoting, and
interacting with conspecifics or predators. The atlas-axis complex is a
mammalian innovation that allows precise head movements during these
behaviors. While morphological variation in other vertebral regions has
been linked to ecological differences in mammals, less is known about
morphological specialization in the cervical vertebrae, which are
developmentally constrained in number but highly variable in size and
shape. Here, we present the first phylogenetic comparative study of the
atlas-axis complex across mammals. We used spherical harmonics to quantify
3D shape variation of the atlas and axis across a diverse sample of
species, and performed phylogenetic analyses to investigate if vertebral
shape is associated with body size, locomotion, and diet. We found that
differences in atlas and axis shape are partly explained by phylogeny, and
that mammalian subclades differ in morphological disparity. Atlas and axis
shape diversity is associated with differences in body size and
locomotion; large terrestrial mammals have craniocaudally elongated
vertebrae, while smaller mammals and aquatic mammals have more compressed
vertebrae. These results provide a foundation for investigating functional
hypotheses underlying the evolution of neck morphologies across mammals.