Hexanchiformes

Hexanchiformes; Temporal range: Early Jurassic–Recent PreꞒ Ꞓ O S D C P T J K Pg N Possible Paleozoic record
	Frilled shark, (Chlamydoselachus anguineus)
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Chondrichthyes
Subclass:	Elasmobranchii
Infraclass:	Euselachii
Superorder:	Selachimorpha
Order:	Hexanchiformes; F. de Buen, 1926
Families
	†Crassonotidae; †Mcmurdodontidae?; †Orthacodontidae; Chlamydoselachidae; Hexanchidae

The Hexanchiformes /hɛkˈsæŋkɪfɔːrmiːz/ are the order consisting of the most primitive types of sharks,^[a] and numbering just seven extant species. Fossil sharks that were apparently very similar to modern sevengill species are known from Jurassic specimens.^[2]

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.

Classification[]

Living species[]

Family Chlamydoselachidae Garman 1884 (frilled sharks)
- Chlamydoselachus Garman, 1884
  - Chlamydoselachus africana Ebert & Compagno, 2009 (Southern African frilled shark)
  - Chlamydoselachus anguineus Garman, 1884 (frilled shark)
Family Hexanchidae J. E. Gray 1851 (cow sharks)
- Heptranchias Rafinesque, 1810
  - Heptranchias perlo (Bonnaterre, 1788) (sharpnose sevengill shark)
- Hexanchus Rafinesque, 1810
  - Hexanchus griseus (Bonnaterre, 1788) (bluntnose sixgill shark)
  - Hexanchus nakamurai Teng, 1962 (bigeyed sixgill shark)
  - Hexanchus vitulus Daly-Engel, 2018 (Atlantic sixgill shark)^[3]^[4]
- Notorynchus Ayres, 1855
  - Notorynchus cepedianus (Péron, 1807) (broadnose sevengill shark)

Extinct species[]

Notidanodon sp. fossil at the Geological Museum, Copenhagen

Family Chlamydoselachidae
- Chlamydoselachus Garman, 1884
  - Chlamydoselachus bracheri Pfeil, 1983
  - Chlamydoselachus fiedleri Pfeil, 1983
  - Chlamydoselachus garmani Welton, 1983
  - Chlamydoselachus goliath Antunes & Cappetta, 2002
  - Chlamydoselachus gracilis Antunes & Cappetta, 2002
  - Chlamydoselachus keyesi Mannering & Hiller, 2008
  - Chlamydoselachus landinii Carrillo-Briceño, Aguilera & Rodriguez, 2014
  - Chlamydoselachus lawleyi Davis, 1887
  - Chlamydoselachus tatere Consoli, 2008
  - Chlamydoselachus thomsoni Richter & Ward, 1990
  - Chlamydoselachus tobleri Leriche, 1929
- Proteothrinax Pfeil, 1983
  - Proteothrinax baumgartneri Pfeil, 1983
Family Crassonotidae
- Crassodontidanus Kriwet & Klug, 2011
  - Crassodontidanus serratus Fraas, 1855
  - Crassodontidanus wiedenrothi Thies, 1983
- Notidanoides Maisey, 1986
  - Notidanoides muensteri Agassiz, 1843
- Notidanus Cuvier, 1816
  - Notidanus amalthei Oppel, 1854
  - Notidanus atrox Ameghino, 1899
  - Notidanus intermedius Wagner, 1862
  - Notidanus nikitini Chabakov & Zonov, 1935
- Pachyhexanchus Cappetta, 1990
  - Pachyhexanchus pockrandti Ward & Thies, 1987
Family Hexanchidae
- Gladioserratus Underwood, Goswami, Prasad, Verma & Flynn, 2011
  - Gladioserratus aptiensis Pictet, 1864
  - Gladioserratus dentatus Guinot, Cappetta & Adnet, 2014
  - Gladioserratus magnus Underwood, Goswami, Prasad, Verma & Flynn, 2011
- Heptranchias Rafinesque, 1810
  - Heptranchias ezoensis Applegate & Uyeno, 1968
  - Heptranchias howellii Reed, 1946
  - Heptranchias karagalensis Kozlov in Zhelezko & Kozlov, 1999
  - Heptranchias tenuidens Leriche, 1938
- Hexanchus Rafinesque, 1810
  - Hexanchus agassizi Cappetta, 1976
  - Hexanchus andersoni Jordan, 1907
  - Hexanchus casieri Kozlov, 1999
  - Hexanchus collinsonae Ward, 1979
  - Hexanchus gracilis Davis, 1887
  - Hexanchus hookeri Ward, 1979
  - Hexanchus microdon Agassiz, 1843
  - Hexanchus tusbairicus Kozlov in Zhelezko & Kozlov, 1999
- Notidanodon Cappetta, 1975
  - Notidanodon brotzeni Siverson, 1995
  - Notidanodon dentatus Woodward, 1886
  - Notidanodon lanceolatus Woodward, 1886
  - Notidanodon loozi Vincent, 1876
  - Notidanodon pectinatus Agassiz, 1843
- Notorynchus Ayres, 1855
  - Notorynchus borealus Jordan & Hannibal, 1923
  - Notorynchus kempi Ward, 1979
  - Notorynchus lawleyi Cigala Fulgosi, 1983
  - Notorynchus primigenius Agassiz, 1843
  - Notorynchus serratissimus Agassiz, 1843
  - Notorynchus subrecurvus Oppenheimer, 1907
- Pachyhexanchus Cappetta, 1990
  - Pachyhexanchus pockrandti Ward & Thies, 1987
- Paraheptranchias Pfeil, 1981
  - Paraheptranchias repens Probst, 1879
- Pseudonotidanus Underwood & Ward, 2004
  - Pseudonotidanus semirugosus Underwood & Ward, 2004
- Welcommia Cappetta, 1990
  - Welcommia bodeuri Cappetta, 1990
  - Welcommia cappettai Klug & Kriwet, 2010
- Weltonia Ward, 1979
  - Weltonia ancistrodon Arambourg, 1952
  - Weltonia burnhamensis Ward, 1979
Family Mcmurdodontidae ?
- Mcmurdodus White, 1968
  - Mcmurdodus featherensis White, 1968
  - Mcmurdodus whitei Turner, & Young, 1987
Family Orthacodontidae
- Occitanodus Guinot, Cappetta & Adnet, 2014
  - Occitanodus sudrei Guinot, Cappetta & Adnet, 2014
- Orthacodus Woodward, 1889
  - Orthacodus longidens Agassiz, 1843
- Sphenodus Agassiz, 1843
  - Sphenodus alpinus Gümbel, 1861
  - Sphenodus longidens Agassiz, 1843
  - Sphenodus lundgreni Davis, 1890
  - Sphenodus macer Quenstedt, 1852
  - Sphenodus nitidus Wagner, 1862
  - Sphenodus planus Agassiz, 1843
  - Sphenodus rectidens Emmons, 1858
  - Sphenodus robustidens Seguenza, 1900
  - Sphenodus tithonius Gemmellaro, 1871
  - Sphenodus virgai Gemmellaro, 1871

Species[]

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.^[5] 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

Footnotes[]

^ The Chimaeras (Holocephali) are arguably more primitive than the Hexanchiformes, but arguably may not be sharks, depending on whether "sharks" are taken to be superorder Selachimorpha or instead class Chondrichthyes (all cartilaginous fish).

References[]

^ Froese, Rainer, and Daniel Pauly, eds. (2009). "Hexanchiformes" in FishBase. January 2009 version.
^ Allen, Thomas B. (1999). The Shark Almanac. New York: The Lyons Press. p. 45. ISBN 1-55821-582-4.
^ "New shark species confirmed: Genetic testing finds a different sixgill shark". ScienceDaily. Retrieved 2020-08-21.
^ "New species of shark discovered through genetic testing". phys.org. Retrieved 2020-08-21.
^ Matt's, J. & Last P.R. (1998). Paxton, J.R. & Eschmeyer, W.N. (eds.). Encyclopedia of Fishes. San Diego: Academic Press. p. 61. ISBN 0-12-547665-5.

External links[]

Froese, Rainer, and Daniel Pauly, eds. (2011). "Chlamydoselachidae" in FishBase. February 2011 version. (Fish Base family reference)
Froese, Rainer, and Daniel Pauly, eds. (2011). "Hexanchidae" in FishBase. February 2011 version. (Fish Base family reference)

[2] The Chimaeras (Holocephali) are arguably more primitive than the Hexanchiformes, but arguably may not be sharks, depending on whether "sharks" are taken to be superorder Selachimorpha or instead class Chondrichthyes (all cartilaginous fish).

[FB-1] Froese, Rainer, and Daniel Pauly, eds. (2009). "Hexanchiformes" in FishBase. January 2009 version.

[Allen_1999-3] Allen, Thomas B. (1999). The Shark Almanac. New York: The Lyons Press. p. 45. ISBN 1-55821-582-4.

[4] "New shark species confirmed: Genetic testing finds a different sixgill shark". ScienceDaily. Retrieved 2020-08-21.

[5] "New species of shark discovered through genetic testing". phys.org. Retrieved 2020-08-21.

[EoF-6] Matt's, J. & Last P.R. (1998). Paxton, J.R. & Eschmeyer, W.N. (eds.). Encyclopedia of Fishes. San Diego: Academic Press. p. 61. ISBN 0-12-547665-5.

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