Meeting Abstract
Crocodylians represent one of the two crown radiations of Archosauria, the ruling reptiles. Although much has been described regarding the series of transitions from the ancestral archosaur to the ancestral bird, somewhat less has been made of the gradual assembly of the similarly specialized crocodylian body and head. I use new CT data and new specimens from the stem of Archosauria and the stem of Crocodylia to trace the gradual assembly of the crocodile form, focusing on the skull. The divergence between the avian and crocodylian lineages was quickly marked by a distinctive lightening of the skull in the former and reinforcement in the latter, culminating in the heavy, solid crania of Crocodyliformes. The wide crocodylian facial region is a relatively recent innovation, which can be pinpointed with an expanded fossil record of Jurassic and Cretaceous taxa. Some of the molecular developmental underpinnings of the divergence between crocodile and bird faces are also now coming to light. New data implicate very early molecular patterning differences in the morphological divergence of the premaxillary region at the tip of the snout, the maxillary region in the middle of the head, and the brain and skull roof at the back of the cranium. This work, however, represents only a beginning. With the release of several crocodylian genomes, it might now be hoped that the upstream regulatory changes leading to crocodylian- or bird-specific gene expression and phenotype are within reach.
