Atlas morphology, scaling and locomotor behaviour in primates, rodents and relatives (Mammalia: Euarchontoglires)

Abstract

The mammalian atlas, or first cervical vertebra, supports the weight of the head and enables complex movements of the head and neck via articulations with the cranium and axis (second cervical vertebra). Despite these ubiquitous functional roles, the morphology of the atlas varies widely among mammals, suggesting that its shape may be associated with allometry, ecology, behaviour or a combination of these factors. Here, we test the hypotheses that atlas shape is correlated with body size, head size and locomotor behaviour in Euarchontoglires, a diverse group of mammals including primates, rodents, lagomorphs, tree shrews and colugos. We use three-dimensional geometric morphometrics to quantify the shape of the atlas vertebra in 64 species, and use a phylogenetic comparative framework to examine trends in atlas shape across a broad swath of the mammalian tree. Our results indicate that body size is associated with atlas morphology across euarchontogliran mammals, whereas head size and locomotion are not. Strong phylogenetic signal in atlas shape among all groups reiterates the importance of considering evolutionary non-independence of traits, and we further discuss integration of phylogenetic and shape data.

functional morphology, cervical vertebrae, mammals, evolution, phylogenetic comparative methods