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Shark classification{{TOCleft}} 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 400 to 350 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 familes and genera within the orders are listed in alphabetical order. The Lamniformes include the extinct megalodon, Carcharodon megalodon, which like all extinct sharks is only known from its teeth (the only bone found in these cartilaginous fishes, and therefore the only fossils produced). A reproduction of the jaw was based on some of the largest teeth (up to almost 17 cm (7 in) in length) and suggested a fish that could grow 25 m (80 ft) long to 30.5 m (100 ft). The jaw was realized to be inaccurate, and estimates revised downwards to around 13 m (43 ft) to 15.9 m (52 ft). Taxonomic listing See also | ||||||||
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