Hexanchiformes

A frilled shark, a member of the Hexanchiformes

**Hexanchiformes** are the order consisting of the most primitive types of sharks, and numbering just six extant species. Fossil sharks that were apparently very similar to modern sevengill species are known from specimens dating back to the Jurassic.

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 from 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 hexanchids would be considered the oldest extant order of elasmobranchs) .

The frilled shark (*Chlamydoselachus anguineus*), is very different from the cow sharks, and it has been proposed that it should be moved to its own order, Chlamydoselachiformes.

## Taxonomy

Below is a list of living and extinct members of the Hexanchiformes:

* **Family Chlamydoselachidae** (frilled sharks)
	+ Genus *Chlamydoselachus*
		- *Chlamydoselachus africana* (Southern African frilled shark)
		- *Chlamydoselachus anguineus* (Frilled shark)
		- *Chlamydoselachus bracheri*†
		- *Chlamydoselachus fiedleri*†
		- *Chlamydoselachus garmani*†
		- *Chlamydoselachus goliath*†
		- *Chlamydoselachus gracilis*†
		- *Chlamydoselachus keyesi*†
		- *Chlamydoselachus landinii*†
		- *Chlamydoselachus lawleyi*†
		- *Chlamydoselachus tatere*†
		- *Chlamydoselachus thomsoni*†
		- *Chlamydoselachus tobleri*†
	+ Genus *Proteothrinax*†
		- *Proteothrinax baumgartneri*†
* **Family Hexanchidae** (Cow sharks)
	+ Genus *Heptranchias*
		- *Heptranchias perlo* (sharpnose sevengill shark)
	+ Genus *Hexanchus*
		- *Hexanchus griseus* (bluntnose sixgill shark)
		- *Hexanchus nakamurai* (bigeyed sixgill shark)
		- *Hexanchus vitulus* (Atlantic sixgill shark)
		- *Hexanchus agassizi*†
		- *Hexanchus andersoni*†
		- *Hexanchus casieri*†
		- *Hexanchus collinsonae*†
		- *Hexanchus gracilis*†
		- *Hexanchus hookeri*†
		- *Hexanchus microdon*†
		- *Hexanchus tusbairicus*†
	+ Genus *Notorynchus*
		- *Notorynchus cepedianus* (broadnose sevengill shark)
		- *Notorynchus borealus*†
		- *Notorynchus kempi*†
		- *Notorynchus lawleyi*†
		- *Notorynchus primigenius*†
		- *Notorynchus serratissimus*†
		- *Notorynchus subrecurvus*†
	+ Genus *Gladioserratus*†
		- *Gladioserratus aptiensis*†
		- *Gladioserratus dentatus*†
		- *Gladioserratus magnus*†
	+ Genus *Heptranchias*†
		- *Heptranchias ezoensis*†
		- *Heptranchias howellii*†
		- *Heptranchias karagalensis*†
		- *Heptranchias tenuidens*†
	+ Genus *Notidanodon*†
		- *Notidanodon brotzeni*†
		- *Notidanodon dentatus*†
		- *Notidanodon lanceolatus*†
		- *Notidanodon loozi*†
		- *Notidanodon pectinatus*†
	+ Genus *Pachyhexanchus*†
		- *Pachyhexanchus pockrandti*†
	+ Genus *Paraheptranchias*†
		- *Paraheptranchias repens*†
	+ Genus *Pseudonotidanus*†
		- *Pseudonotidanus semirugosus*†
	+ Genus *Welcommia*†
		- *Welcommia bodeuri*†
		- *Welcommia cappettai*†
	+ Genus *Weltonia*†
		- *Weltonia ancistrodon*†
		- *Weltonia burnhamensis*†
* Family Crassonotidae†
	+ Genus *Crassodontidanus*†
		- *Crassodontidanus serratus*†
		- *Crassodontidanus wiedenrothi*†
	+ Genus *Notidanoides*†
		- *Notidanoides muensteri*†
	+ Genus *Notidanus*†
		- *Notidanus amalthei*†
		- *Notidanus atrox*†
		- *Notidanus intermedius*†
		- *Notidanus nikitini*†
	+ Genus *Pachyhexanchus*†
		- *Pachyhexanchus pockrandti*†
* Family Mcmurdodontidae†?
	+ Genus *Mcmurdodus*†
		- *Mcmurdodus featherensis*†
		- *Mcmurdodus whitei*†
* Family Orthacodontidae†
	+ Genus *Occitanodus*†
		- *Occitanodus sudrei*†
	+ Genus *Orthacodus*†
		- *Orthacodus longidens*†
	+ Genus *Sphenodus*†
		- *Sphenodus alpinus*†
		- *Sphenodus longidens*†
		- *Sphenodus lundgreni*†
		- *Sphenodus macer*†
		- *Sphenodus nitidus*†
		- *Sphenodus planus*†
		- *Sphenodus rectidens*†
		- *Sphenodus robustidens*†
		- *Sphenodus tithonius*†
		- *Sphenodus virgai*†