Shark classification
Sharks belong to the superorder Selachimorpha in the subclass Elasmobranchii in the class Chondrichthyes. The Elasmobranchii also include rays and skates; the Chondrichthyes also include Chimaeras. It is currently thought that the sharks form a polyphyletic group: in particular, some sharks are more closely related to rays than they are to some other sharks. The first sharks appeared in the oceans 350 to 400 million years ago. Most of the species we know today are as old as the Jurassic period.
Listed below are extant species of shark in taxonomic order. Sharks are spread across eight orders which are listed in roughly evolutionary order (from primitive to modern), and the families and genera within the orders are listed in alphabetical order.
Hexanchiformes: Examples from this group include the cow sharks, frilled shark and even a shark that looks upon first inspection to be a marine snake.
Squaliformes: Examples from this group include the bramble sharks, dogfish sharks and rough sharks, and prickly shark.
Pristiophoriformes: These are the sawsharks, with an elongated, toothed snout that they use for slashing the fishes that they subsequently eat.
Squatiniformes: Angel sharks.
Heterodontiformes: They are commonly referred to as the bullhead or horn sharks.
Orectolobiformes: They are commonly referred to as the carpet sharks, including zebra sharks, nurse sharks, wobbegongs and the whale shark.
Carcharhiniformes: They are commonly referred to as the groundsharks, and some of the species include the blue, tiger, bull, reef and oceanic whitetip sharks (collectively called the requiem sharks) along with the houndsharks, catsharks and hammerhead sharks. They are distinguished by an elongated snout and a nictitating membrane which protects the eyes during an attack.
Lamniformes: They are commonly referred to as the mackerel sharks. They include the goblin shark, basking shark, megamouth shark, the thresher sharks, shortfin and longfin mako sharks, and great white shark. They are distinguished by their large jaws and ovoviviparous reproduction. The Lamniformes include the extinct megalodon, Carcharodon megalodon, which like many extinct sharks is only known from its teeth and a few partially preserved vertebrae
