Cow shark

Cow sharks Temporal range: Upper Jurassic–Recent[1] PreꞒ Ꞓ O S D C P T J K Pg N Possible Permian occurrence
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Chondrichthyes
Order:	Hexanchiformes
Family:	Hexanchidae J. E. Gray, 1851
Genera
See text

Cow sharks are a shark family, the Hexanchidae, characterized by an additional pair or pairs of gill slits. Its 37 species are placed within the 10 genera: Gladioserratus, Heptranchias, Hexanchus, Notidanodon, Notorynchus, Pachyhexanchus, Paraheptranchias, Pseudonotidanus, Welcommia, and Weltonia.^[2][3]

Cow sharks are considered the most primitive of all the sharks, as their skeletons resemble those of ancient extinct forms, with few modern adaptations. Their excretory and digestive systems are also unspecialised, suggesting they may resemble those of primitive shark ancestors. A possible hexanchid tooth is known from the Permian of Japan, making the family a possible extant survivor of the Permian-Triassic extinction.^[4]

Their most distinctive feature, however, is the presence of a sixth, and, in two genera, a seventh, gill slit, in contrast to the five found in all other sharks.^[5] They range from 1.4 to 5.5 m (4.6 to 18.0 ft) in adult body length.

Cow sharks are ovoviviparous, with the mother retaining the egg cases in her body until they hatch. They feed on relatively large fish of all kinds, including other sharks, as well as on crustaceans and carrion.^[6]

The only fossil records of the cow shark consist of mainly only isolated teeth. Although skeletal remains for this species have been found from the Jurassic time period, these have been very rare and have only been found in the "Late Jurassic lithographic limestones of South Germany, Nusplingen, Solnhofen, and late Cretaceous calcareous sediments of Lebanon." Due to these sparse records some scientists conclude that the cow shark is now a more "diverse and numerous species".^[7]

Species[]

The 37 species of cow shark, in 10 genera, are:^[1]

• †Gladioserratus Underwood, Goswami, Prasad, Verma & Flynn, 2011
  • † Pictet, 1864
  • † Guinot, Cappetta & Adnet, 2014
  • † Underwood, Goswami, Prasad, Verma & Flynn, 2011
• Heptranchias Rafinesque, 1810
  • Heptranchias perlo (Bonnaterre, 1788) (sharpnose sevengill shark)
  • † Applegate & Uyeno, 1968
  • †Heptranchias howelli Reed, 1946
  • † Kozlov in Zhelezko & Kozlov, 1999
  • † Leriche, 1938
• Hexanchus Rafinesque, 1810
  • Hexanchus griseus (Bonnaterre, 1788) (bluntnose sixgill shark)
  • Hexanchus nakamurai Teng, 1962 (bigeyed sixgill shark)
  • Hexanchus vitulus Daly-Engel, 2018^[8][9] (atlantic sixgill shark)
  • † Cappetta, 1976
  • † Jordan, 1907
  • † Kozlov, 1999
  • †Hexanchus collinsonae Ward, 1979
  • † Davis, 1887
  • † Ward, 1979
  • † Agassiz, 1843
  • † Kozlov in Zhelezko & Kozlov, 1999
• †Notidanodon Cappetta, 1975
  • † Siverson, 1995
  • † Woodward, 1886
  • † Woodward, 1886
  • † Vincent, 1876
  • † Agassiz, 1843
• Notorynchus Ayres, 1855
  • Notorynchus cepedianus (Péron, 1807) (broadnose sevengill shark)
  • † Jordan & Hannibal, 1923
  • † Ward, 1979
  • † Cigala Fulgosi, 1983
  • † Agassiz, 1843
  • † Agassiz, 1843
  • † Oppenheimer, 1907
• † Cappetta, 1990
  • † Ward & Thies, 1987
• † Pfeil, 1981
  • † Probst, 1879
• † Underwood & Ward, 2004
  • † Underwood & Ward, 2004
• †Welcommia Klug & Kriwet, 2010
  • † Cappetta, 1990
  • †Welcommia cappettai Klug & Kriwet, 2010
• † Ward, 1979
  • † Arambourg, 1952
  • † Ward, 1979

View of the six gill openings of Hexanchus nakamurai

References[]

External links[]

• ReefQuest Centre: Order Hexanchiformes: Cow Sharks