1 - Elasmobranchs and Their Extinct Relatives: Diversity, Relationships, and Adaptations Through Time

Abstract

1. Introduction 2. Systematic and Phylogenetic Framework of Chondrichthyan Diversity 2.1. Names, Taxa, and Characters 2.2. Chondrichthyan Diversity and Interrelationships 3. Environments and Adaptations 4. Conclusion Current views about chondrichthyan phylogeny and systematics are briefly reviewed, with particular reference to the living and fossil taxa that are, or have been, once referred to as “elasmobranchs.” Recent reviews of early fossil chondrichthyans suggest that the last common ancestor of the living elasmobranchs and holocephalans probably lived by the end of the Devonian period, about 370–380 Myr ago, but a number of Paleozoic, shark-like chondrichthyans are currently regarded as stem chondrichthyans that diverged before the last common ancestor of all living taxa. Stem holocephalans display an amazing morphological diversity that reflects adaptations to very diverse benthic habitats. By contrast, both stem elasmobranchs and stem chondrichthyans are generally shark-like and were probably adapted to a pelagic mode of life. The earliest evidence for tessellated prismatic calcified cartilage, the “signature” of euchondrichthyans (i.e., all chondrichthyans which possess tessellated calcified prismatic cartilage), is about 400 Myr old, but scales and teeth tentatively assigned to chondrichthyans have been recorded from earlier periods. The “acanthodians,” a paraphyletic ensemble of Paleozoic fishes known since about 445 Myr are currently regarded as possible stem chondrichthyans that diverged before the rise of euchondrichthyans.