Chondrichthyan phylogeny: a look at the evidence

Chondrichthyans (sharks, rays and holocephalans) are subjected to cladistic analysis in order to identify possible monophyletic groups. Chondrichthyan monophyly is established on the basis of several apomorphic characters, of which the most convincing is the presence of a mineralized layer of prismatic perichondral tissue of a unique type. Modern elasmobranchs are united with the Jurassic Palaeospinax by several synapomorphies. Palaeospinax and some other fossil sharks are sequenced as successive sister-groups to modern elasmobranchs. It is concluded that elasmobranchs and chimaeroids are monophyletic sister-groups, but that sharks are not monophyletic unless holocephalans are included.