Hexanchiformes

The Hexanchiformes /hɛkˈsæŋkɪfɔːrmiːz/ are the order consisting of the most primitive types of sharks,[lower-alpha 1] and numbering just seven extant species. Fossil sharks that were apparently very similar to modern sevengill species are known from Jurassic specimens.[2]

Hexanchiformes
Temporal range: Early Jurassic–Recent [1] 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

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

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
  • 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
    • 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
    • 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 Image 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

See also

Footnotes
  1. 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
  1. Froese, Rainer, and Daniel Pauly, eds. (2009). "Hexanchiformes" in FishBase. January 2009 version.
  2. Allen, Thomas B. (1999). The Shark Almanac. New York: The Lyons Press. p. 45. ISBN 1-55821-582-4.
  3. "New shark species confirmed: Genetic testing finds a different sixgill shark". ScienceDaily. Retrieved 2020-08-21.
  4. "New species of shark discovered through genetic testing". phys.org. Retrieved 2020-08-21.
  5. 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.
  • 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)
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