Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven gill slits on the sides of the head, and pectoral fins that are not fused to the head. modern sharks are classified within the clade selachimorpha (or selachii) and are the sister group to the rays. however, the term "shark" has also been (incorrectly) used to refer to extinct members of the subclass elasmobranchii, which are technically outside the selachimorpha clade. notable examples of improper classification include cladoselache, and various other members of the chondrichthyes class like the holocephalid eugenedontidans. under this broader definition, the earliest known sharks date back t...

Source: Wikipedia