The axial skeleton is a key diagnostic character of vertebrate animals. Comprising the vertebral column and ribs, the axial skeleton is a support structure from which all appendages of the vertebrate body are suspended. Reptilia is a hugely diverse clade of vertebrates, with regards to body form, body size, function, and behaviour. However, despite this established diversity, we know little about the diversity of the axial skeleton across Reptilia, or how axial skeleton evolution is related to development, body form, or function. In this thesis I investigate the diversity and evolution of the axial skeleton across Reptilia, divided into the following chapters: Chapter 1: Introduction; Chapter 2: Axial skeleton morphology; Chapter 3: Combining geometric morphometrics and maximum likelihood modelling to quantify regionalization: Methodology and sensitivity analyses; Chapter 4: Multiple independent trajectories to highly regionalized axial skeleton morphologies across Reptilia: Implications for development and function; Chapter 5: Axial skeletal segmentation across Reptilia: Phylogenetic, ecological, and developmental correlates; and Chapter 6, conclusion: Mammals are not a uniquely regionalized clade: A complex evolutionary history of axial skeleton morphology, function, and development across Reptilia. My results show multiple independent evolutionary trajectories to high degrees of regionalization across Reptilia, demonstrating a greater complexity in axial skeleton evolution, and a greater diversity in axial skeleton morphology across Reptilia, than previously expected. Further to this, I model the evolution of segment counts across Reptilia, comparing segment evolution with body mass and ecological traits. I establish that segmentation played an integral role in the evolution of body form and ecology across Reptilia. High segment counts are associated with aquatic and fossorial ecologies regardless of phylogeny, whereas high relative cervical counts are strongly associated with the acquisition of powered flight in birds, but not in pterosaurs. Overall, I uncover a complex history of axial skeleton evolution across Reptilia. Each extant clade has its own evolutionary trajectory toward highly regionalized forms, and variable trajectories to high and low segment counts. Previous interpretations of axial skeleton diversity across Vertebrata have focused on the origin of mammals, inferring uniquely complex axial skeleton morphologies across Mammalia. However, morphometric analysis here reveals multiple independent gains and losses of regionalization across Reptilia, associated with adaptations to specialised body forms. Amongst sampled taxa, extant crocodilians, most extant squamates, and many extant birds are just as regionalized as sampled mammals. This implies greater degrees of axial skeleton diversity across vertebrates than previously thought and presents the potential for uncovering similar complexity in axial skeleton macroevolution across other vertebrate clades, and Vertebrata as a whole.