Anatomy and development of skull-neck boundary structures in the skeleton of the extant crocodylian Alligator mississippiensis

A system-by-system approach dominates morphological and evolutionary study; however, some structures that are better understood within the context of an interface between two systems or traditional units remain less well understood. As part of a larger goal to clarify aspects of skull-neck boundary evolution, we herein describe the morphology and development of the occiput and atlas-axis complex in the crocodylian Alligator mississippiensis. We apply micro-computed tomography scanning, clearing and double staining, and histological analyses to skull-neck boundary structures at three stages of development (embryonic stage 22, 23, and hatchling). Regions of ossification that could possibly pertain to a postparietal were found adjacent to the parietal bone and supraoccipital; however, these were not deemed convincing and are considered part of the supraoccipital. Within the atlas-axis complex, the proatlas appears as two discrete cartilaginous elements in Stage 22 that ossify together at Stage 23. Posterior to the proatlas, the atlas-axis complex is composed of two centra, each with cervical ribs ventrally and neural arches dorsally that begin ossifying at Stage 23. Histology and clearing and staining of Stages 22 and 23 embryos reveal a discrete atlas intercentrum applied to the ventral part of the occipital condyle of the skull. Posterior to this is a cartilage that appears to be a co-chondrified atlas pleurocentrum, axis intercentrum, and axis pleurocentrum. Ossification of this cartilaginous structure produces discrete atlas inter- and pleurocentra, as well as a singular axis centrum. Together these data are discussed with reference to clarifying historical discrepancies concerning elements at the crocodylian skull-neck boundary.