Head‐turning morphologies: Evolution of shape diversity in the mammalian atlas–axis complex

Abstract

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. Although 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, whereas 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.

The atlas of StW 573 and the late emergence of human-like head mobility and brain metabolism

“…In particular, the atlas, or first cervical vertebra, acts as the main interface between the head and the axial skeleton and, therefore, plays a critical role in directing and stabilizing head movements (rev. in 4,5 ). As such, previous studies have emphasized the functional link between the morphology of the atlas, head mobility, and postural and locomotor repertoires in catarrhines (e.g.…”

“…As such, previous studies have emphasized the functional link between the morphology of the atlas, head mobility, and postural and locomotor repertoires in catarrhines (e.g. [5][6][7] ). More specifically, while the shape and curvature of the articular facets at the atlanto-occipital joint determines the range of head motions in terms of flexion and extension, the atlanto-axial joint is responsible for lateral rotation of the atlas and the head (rev.…”

“…More specifically, while the shape and curvature of the articular facets at the atlanto-occipital joint determines the range of head motions in terms of flexion and extension, the atlanto-axial joint is responsible for lateral rotation of the atlas and the head (rev. in [4][5][6][7][8]. Combined analysis of the superior and inferior articular facets has the potential to provide essential information about the range of head motions in Australopithecus, which has only been superficially assessed due to the scarcity and fragmentary nature of cervical vertebrae in the hominin fossil record 9,10 .…”

Evolutionary and morphological patterns underlying carnivoran body shape diversity

“…Instead, evolution toward more elongate bodies must occur through reduction of body depth, elongation of the head, and/or elongation of the individual vertebrae. Previous work has shown that changes in vertebral shape can generate diverse body shapes from longnecked giraffes with elongate cervical vertebrae to fusiform dolphins with shortened and fused cervical vertebrae (Buchholtz 2001;Buchholtz and Schur 2004;Arnold et al 2017;Vander Linden et al 2019). Furthermore, more recent work has investigated the evolutionary processes that led to increase in the complexity and regionalization of the mammalian vertebral column (Jones et al 2018a, b;Jones et al 2019).…”

“…2017; Vander Linden et al. 2019). Furthermore, more recent work has investigated the evolutionary processes that led to increase in the complexity and regionalization of the mammalian vertebral column (Jones et al.…”