Morphological variation in atlas and axis of Neotropical spiny rats (Rodentia, Echimyidae)

The unique morphologies of the first two cervical vertebrae, the atlas and axis, represent a significant innovation in mammalian evolution. These structures support the weight of the head and enable intricate movements of the head and neck. Within Caviomorpha, Echimyidae (spiny rats, coypu, and hutias) exhibit considerable variation in vertebral size and encompass arboreal, terrestrial, semi-fossorial, and semi-aquatic lifestyles, making them a promising model for investigating how vertebral morphology responds to different ecological pressures. We analyzed 99 atlas and 76 axis vertebrae from 23 extant species and the extinct Eumysops chapalmalensis, using two-dimensional geometric morphometrics combined with phylogenetic comparative methods. Linear discriminant analyses were used to infer the locomotor mode of the extinct species. Our results reveal that the morphology of the atlas and axis is shaped by phylogenetic relatedness, vertebral size (allometry), and locomotor modes. Although allometry significantly influenced vertebral shape, its overall effect was limited compared to the stronger role of locomotor specialization. We detected significant associations between vertebral shape and both vertebral size and ecological specialization in most analyzed views. Although each vertebra contributes to identifying the major echimyid clades and their locomotor strategies, the atlas exhibits the strongest overall signal. We also describe morphofunctional adaptations related to head stabilization and movement. Our findings support the hypothesis that E. chapalmalensis was a terrestrial or semi-fossorial rodent. Altogether, the results underscore the value of axial skeletal elements, particularly the atlas and axis, in revealing patterns of adaptive specialization in both extant and extinct species.