Meeting Abstract
Although the vertebral column is a defining feature of vertebrates, little is known about the evolution and development of this fundamental structure. The vertebrate axial column consists of several components, including a notochord, centrum, neural arches, hemal arches, various processes, and ribs. The presence and condition of many of these structures in different vertebrate groups is not well documented. To analyze variation and morphological diversity of axial columns across major groups of gnathostomes, we examined extant and extinct species of chondrichthyans, osteichthyans, and stem gnathostomes. We performed parsimony-based ancestral state reconstructions on a supertree of gnathostomes that was assembled from recent phylogenies. Our results indicate that centra have evolved independently as many as nine times. Other instances of convergence include polyspondyly and fused anterior vertebrae (synarcuals and the Weberian apparatus). Centrum development is known to be diverse: teleost centra can form the notochord, somites, or both, while tetrapod centra form exclusively from the somite. To investigate what appears to be multiple origins of centra, and to add a necessary comparison with data from other gnathostome models, we treated a range little skate embryos (Leucoraja erinacea) with Lugol’s iodine and obtained microCT scans of the axial column. The scans reveal a complex construction: an inner calcification constricts the notochord and an outer calcification surrounds both the notochord and the inner structure. Such homoplasy and diversity among vertebrate centra indicate novel developmental patterns that have yet to be explored throughout major vertebrate lineages.
