Shark is the naming term of all members of Selachimorpha suborder in the subclass Elasmobranchii, in the class Chondrichthyes. The Elasmobranchii also include rays and skates; the Chondrichthyes also include Chimaeras. The first shark-like chondrichthyans appeared in the oceans 400 million years ago, developing into the crown group of sharks by the Early Jurassic. Listed below are extant species of shark. Sharks are spread across 557 described and 23 undescribed species in eight orders. The families and genera within the orders are listed in alphabetical order. Also included is a field guide to place sharks into the correct order.
Cow and frilled sharks (2 Families) ORDER HEXANCHIFORMES Family Chlamydoselachidae (frilled sharks) Genus Chlamydoselachus Garman, 1884 Chlamydoselachus africana Ebert & L. J. V. Compagno, 2009 (Southern African frilled shark) Chlamydoselachus anguineus Garman, 1884 (frilled shark) Family Hexanchidae (cow sharks) Genus Heptranchias Rafinesque, 1810 Heptranchias perlo (Bonnaterre, 1788) (sharp-nose seven-gill shark) Genus Hexanchus Rafinesque, 1810 Hexanchus griseus (Bonnaterre, 1788) (blunt-nose six-gill shark) Hexanchus nakamurai Teng, 1962 (big-eyed six-gill shark) Hexanchus vitulus Springer and Waller, 1969 (Atlantic six-gill shark) Genus Notorynchus Ayres, 1855 Notorynchus cepedianus (Péron, 1807) (broad-nose seven-gill shark)
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