Hexanchiformes
Family Common name Genera Species Description
Chlamydoselachidae Frilled sharks 1 extant 1 extinct 2 extant 12 extinct Frilled sharks contain only two extant species of deepsea creatures which are typically weakened in areas closer to the surface . The most widely known species still surviving is the frilled shark , known as a living fossil , along with the Southern African frilled shark , found along coastal areas of South Africa . Several extinct species are known
†Crassonotidae † Crassonotidae 4 8 Extinct
Hexanchidae Cow sharks 3 extant 5 extinct 5 extant 31 extinct Cow sharks are considered the most primitive of all the sharks , because their skeletons resemble those of ancient extinct forms , with few modern adaptations . Their excretory and digestive systems are also unspecialised , suggesting that they may resemble those of primitive shark ancestors . Their most distinctive feature , however , is the presence of a sixth , and , in two genera , a seventh , gill slit , in addition to the five found in all other sharks . They range from 1.4 metres ( 4.6 ft ) to over 5.5 metres ( 18 ft ) in adult body length
†Mcmurdodontidae † Mcmurdodontidae 1 2 Extinct
†Orthacodontidae † Orthacodontidae 3 12 Extinct
Species
The Hexanchiformes are the order consisting of the most primitive types of sharks, and numbering just seven extant species. Fossil sharks that were apparently very similar to modern sevengill species are known from Jurassic specimens. Hexanchiform sharks have only one dorsal fin, either six or seven gill slits, and no nictitating membrane in the eyes. Shark teeth similar to those modern hexanchids are known from Devonian deposits in Antarctica and Australia, as well as Permian deposits in Japan. If these are in fact hexanchids, this may be the only extant order of elasmobranchs to have survived after the Permian extinction (and by extension, the oldest extant order of elasmobranchs). The frilled sharks of the genus Chlamydoselachus are very different from the cow sharks, and have been proposed to be moved to a distinct order, Chlamydoselachiformes.