Some sources extend the term 'shark' as an informal category including extinct members of Chondrichthyes (cartilaginous fish) with a shark-like morphology, such as hybodonts. Modern sharks are classified within the clade Selachimorpha (or Selachii) and are the sister group to the Batoidea ( rays and kin). 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.
Clockwise from top left: spiny dogfish, Australian angelshark, whale shark, great white shark, horn shark, frilled shark, scalloped hammerhead and Japanese sawshark representing the orders Squaliformes, Squatiniformes, Orectolobiformes, Lamniformes, Heterodontiformes, Hexanchiformes, Carcharhiniformes and Pristiophoriformes respectively.
