Morphology of the Neurocentral Junction during Postnatal Growth of Alligator (Reptillia, Crocodylia).

Abstract

The two main parts of a vertebra, the centrum and neural arch, form independently during early developmental stages in nearly all vertebrates, and they typically fuse together in later growth stages. Fusion between centrum and neural arch is the result of ossification of a thin cartilage layer (neurocentral synchondrosis) between them. The timing of neurocentral fusion varies considerably within the vertebral column and among species, especially in archosaurian reptiles, and may be related to changes in body size and/or locomotion. Despite the importance of neurocentral fusion to our understanding of archosaur evolution, basic information about this process and how it changed through time remains poorly understood.

In this dissertation, morphology of neurocentral sutures and vertebrae in crocodilians (Reptilia, Archosauria) is explored. In Chapter 2, the detailed cell- and tissue-level morphology of neurocentral sutures in the vertebrae of Alligator mississippiensis is documented. In chapter 3, complexity of neurocentral sutures are quantified, and changes related to differences in vertebral position, ontogenetic age, and phylogeny are examined. In Chapter 4, allometric changes in vertebrae of Alligator are quantified and investigated in relation to key ontogenetic events. As seen in some craniofacial bones in various vertebrates, neurocentral fusion may affect changes in relative size and shape of certain vertebral structures (e.g., centrum, neural spine, transverse processes, neural canal) during growth. In chapter 5, data examined in crocodilians (chapters 2–4) are applied to various fossil archosaurs from the Early Mesozoic to investigate the origin and evolutionary significance of two unique features of neurocentral sutures, delayed neurocentral fusion and complex neurocentral sutures.