Chondrichthyan Evolution, Diversity, and Senses

Abstract

Chondrichthyans are one of two major clades of living jawed vertebrates, with a rich fossil record potentially extending back to the Late Ordovician (455 million years ago, mya). The main groups of chondrichthyans include the chimaeroids, sharks, and skates and rays. This chapter outlines the major events in chondrichthyan evolution, focusing on features of the cranium, jaw and jaw musculature, and gill arch skeleton. The “spiny sharks” (acanthodians) and other stem chondrichthyans have recently been shown to exhibit a mosaic of chondrichthyan and osteichthyan characters. Taxa such as iniopterygians and chondrenchelyiforms, resolved as stem group chimaeroids, appear in the Carboniferous and display dramatic body forms and unusual fin morphology. Chondrichthyans also show a considerable range of dentitions, both in terms of morphology and development, particularly modified in the chimaeroids. In addition to their differing tooth morphologies, chondrichthyans have several types of jaw suspensions to support a range of feeding and breathing modes. Sharks have well-developed brains that vary according to the environment rather than phylogeny. Their senses are also well-developed and finely tuned to best perform in their particular ecological niche. The long evolutionary history of chondrichthyans and their great diversity as well as the retention of some primitive characters make them good models for evolutionary and developmental studies.