Abstract As the sole living rhynchocephalian, the tuatara (Sphenodon punctatus) provides important comparative information for fossil rhynchocephalians and lepidosaurs in general regarding expected intraspecific variability, ontogeny, and sexual dimorphism. The axial skeleton of Sphenodon punctatus is described here in detail, using Computed Tomography (CT)-scans and a comprehensive sample (N = 33) of different ontogenetic stages and sexes. The new description adds to already existing literature, and confirms some consistent morphological characters, such as number of vertebrae, position of accessory processes, and ossification patterns. We conducted 3D geometric morphometrics analyses to better understand regionalization of the axial skeleton, ontogenetic changes, and sexual dimorphism in Sphenodon. The morphology of presacral vertebrae is influenced by size and shows negative allometry, with increasing relative height of the neural spine, but decreasing relative vertebral width and length in larger individuals. We compare the morphology and ontogenetic implications of Sphenodon with Mesozoic fossil relatives. Some taxa, especially those from the Jurassic of Europe, show substantial differences in number, proportions, and morphology of vertebrae, including shifts in ossification timing, loss of vertebral intercentra, and changes in rib morphology. We highlight osteological features, such as ossification timing and morphology, that may correlate with taxonomy, systematics, and ecomorphology of Mesozoic rhynchocephalians.
Beccari et al. (Fri,) studied this question.