Postparietal and prehatching ontogeny of the supraoccipital in Alligator mississippiensis (Archosauria, Crocodylia).

The first record of the postparietal bone of Alligator mississippiensis, documented by transverse histological sections, is presented. It is the first evidence of the presence of this bone within Recent reptiles. The postparietal is present in a specimen with a head length of 32.3 mm. The bone is a small dermal plate lying ventrally and posteriorly to the posterior margin of the parietal and dorsally to the trabecular bone, forming a dorsal surface of the supraoccipital portion of the neural endocranium. The trabecular bone develops perichondrally from the dorsal surface of the tectal cartilaginous bridge spanning between the dorsal portions of the otic capsules and occipital pilae. The bridge probably represents the fused tectum synoticum posterior plus tectum posterius. Later in ontogeny, the bridge ossifies endochondrally. The endochondrally ossifying bridge together with its perichondrally ossifying trabecular bone form the future supraoccipital. The trabecular bone is the integral part of the cranial endoskeleton and ontogenetically distinct from the dermal postparietal bone.