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Abstract

This dissertation focuses on a fundamental feature of the jawed vertebrate (gnathostome) body plan, the vertebral column. The vertebral column has long been used to diagnose the vertebrate clade, but its development and evolutionary history are not well understood. Vertebrae can consist of neural arches and spines, hemal arches and spines, and either a persistent notochord or a series of centra. Vertebral columns preserved in early jawed vertebrate fossils show diversity in morphology and construction, leading to the hypothesis that many components of vertebrae have evolved independently throughout the evolutionary history of jawed vertebrates. Accompanying the hypotheses of independent evolution is evidence that centra in teleost fishes are, at least in part, derived from the notochord, while centra in tetrapods are formed from somitic cells. This dissertation seeks to explore the evolution of vertebral morphology and development across non-tetrapod gnathostomes, to test the hypotheses that vertebral modifications have evolved numerous times and determine how differences in vertebral development have shaped the vertebrae of major gnathostome subgroups. Chapter one provides an introduction to vertebral morphology, gnathostome phylogeny, and the history of the study of vertebral column evolution and development. In chapter two I compile a supertree representing hypotheses of gnathostome relationships and infer ancestral vertebral conditions using parsimony, maximum likelihood, and Bayesian inference. I recover up to ten independent originations of centra, five originations of anterior fusions, and eight originations of polyspondyly throughout gnathostomes. In chapter three I describe vertebral development in the little skate (Leucoraja erinacea), a taxon representative of a neglected group of gnathostomes, the elasmobranchs. I use traditional techniques including histology and wholemount skeletal preparations, as well as modern microCT methods to study embryonic vertebral morphology in a series of little skate embryos. My findings include that the neural arches develop from an unsegmented layer of condensed mesenchyme that extends the length of the body, that centra in the little skate comprise three layers of tissue, and that the mineralized areolar calcification arises from a layer of spindle-shaped cells that seem to be mesenchyme-derived. I test the embryonic origin of the centra and arches in the little skate in chapter four by injecting a fluorescent dye into 1) the ventral portions of the somites and 2) notochord progenitor cells and allowing embryos to develop to near-hatching. I recover dye-labeled cells from the somites in all components of the vertebral skeleton including the neural arches, hemal arches, and all layers of the centra, while notochord-labeled cells remain in the notochord and notochord epithelium. These fate mapping experiments indicate that the vertebral skeleton is primitively somite- derived. Taken together, the data presented in this dissertation demonstrate that actinopterygians, elasmobranchs, and tetrapods all evolved centra independently and have invented distinct patterns and processes in which to build them.

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