Meeting Abstract
Sharks and their relatives belong to the major vertebrate group, Chondrichthyes, with an evolutionary history that extends back over 450 million years. Recent research has focused on an improved understanding of the phylogenetic relationships of chondrichthyans, in particular stem-group relatives of the crown groups Elasmobranchii (sharks and rays) and Holocephali (chimaeroids). Knowledge of these relationships is crucial to understanding how chondrichthyan teeth, within a patterned dentition, have evolved in the elasmobranchs, including in new model taxa such as the catshark Scyliorhinus and the Little Skate Leucoraja. Development of the dentition in the elasmobranchs, and particularly in sharks like Scyliorhinus, is becoming increasingly well understood, including genes involved in tooth regeneration and rotatory successive replacement. Rays like the Little Skate show very similar processes with respect to tooth addition, as new teeth are iteratively added to sets across the jaw. In an evolutionary sense, teeth arranged into files on the jaw first appear in stem chondrichthyans known as acanthodians (420-250 million years ago), with this character, rotatory succession, retained in stem relatives of the elasmobranchs and holocephalans, representing the primitive condition for sharks and rays. However, the holocephalans are particularly notable, with extant representatives (crown group holocephalans) characterized by dentitions lacking teeth, lost during the evolution of the group; neither tooth germs, nor successive teeth have been observed in embryos or adults. Exactly when, and how, these teeth were lost, compared to the elasmobranchs is an area of research ripe for exploration.
