Skeletal Anatomy in the Chondrichthyan Tree of Life
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Crawford, Callie Hendricks
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Abstract
Chondrichthyans (sharks, rays, skates, and chimaeras) are a diverse taxonomic clade inhabiting bodies of water all over the world. As a lineage, chondrichthyans split from the other jawed vertebrates 450 million years ago, the most basal split in the gnathostome vertebrate tree. Although they have been studied for centuries, knowledge about these animals lags behind that of many other vertebrate groups. This work uses Computed Tomography (CT) to explore morphological variation across phylogenetically diverse species of chondrichthyans. CT imaging is a nondestructive method for viewing internal structures of extant and fossilized specimens. After CT scan data acquisition, reconstruction software was used to manually segment the skeletal anatomical into constituent structures, creating 3-Dimensional representations of the structures. In most groups of vertebrate organisms, skeletal structures are made of calcified bone which has high radiopacity, leading to greater contrast between the skeleton and soft tissues. Chondrichthyans, by comparison, have skeletons composed of cartilage which is much less radiopaque than bone, resulting in lower contrast with surrounding tissues. Variations in the skeletal structures are discussed along with notes on calcification within the chondrichthyan orders. This work is presented as a summary of the variation observed in the skeletal anatomy, building upon previous works in chondrichthyan anatomy, expanding the current state of knowledge of the diversity in chondrichthyan fish skeletons. This project is part of a collaborative effort to develop a phylogenetic tree of life for modern chondrichthyans.