On the evolutionary morphology of the mammalian neck : disparity and constraints

The constancy of seven cervical vertebrae in the neck of almost all mammals is one of the most outstanding examples of morphological stasis in vertebrae evolution. It is yet unclear how morphological disparity is manifested in the neck of mammals in accordance to variation in body size, ecology, ad head use. This thesis aims to gain deeper insights into which cervical traits (despite the fixed number) are further constrained and how disparity emerges in the mammalian neck. Finally, the role of the regionalization or modularization of the neck is examined and discussed. Different approaches are applied to analyze different aspects of neck morphology and regionalization. The in-depth analysis of vertebral allometry across a large sample of mammalian species is used to reveal which role body size and neck length play in neck disparity. Subsequently, neck musculoskeletal organization and modularity are characterized by seven network parameters and interpreted in their morphological context. The evolution of neck organization before and after the CretaceousTertiary boundary is evaluated using a disparity-through-time analysis. Finally, the cervical Hox gene pattern in extant tree sloths is inferred from the shape differences among their cervical vertebrae. Different modes of regionalization of the mammalian neck were suggested in earlier studies and this concept is extended in this thesis. Altogether, they represent a regionalization or modularity on different levels of integration (i.e., structural, developmental, and functional). The morphological-developmental modules provide the cue to the stiffness and softness in evolutionary change of the mammalian neck.