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Cow shark

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Cow sharks
Temporal range: Upper Jurassic–Recent[1]
Possible Permian occurrence
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Chondrichthyes
Subclass: Elasmobranchii
Order: Hexanchiformes
Family: Hexanchidae
J. E. Gray, 1851
Genera

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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]

Description

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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 unspecialized, 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.[3]

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.[4] The first pair are not connected across the throat.[5] They range from 1.4 to 5.5 m (4.6 to 18.0 ft) in adult body length.

These cylindrical sharks have a ventral mouth with compressed, comb-like teeth in the lower jaw and smaller, pointed teeth in the upper jaw. They have a short, angular and spinless dorsal fin. The pelvic fins are smaller than the angular pectoral fins. The caudal fin has a notch towards the end.[5]

Biology

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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]

Fossil Record

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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

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View of the six gill openings of Hexanchus nakamurai

The 37 species of cow shark (five of which are extant), in 10 genera, are:[5][1]

References

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  1. ^ a b Froese, Rainer; Pauly, Daniel (eds.). "Family Hexanchidae". FishBase. February 2011 version.
  2. ^ Allen, 45
  3. ^ Burrow, Carole J.; Hovestadt, D. C.; Turner, Sue. "New information on the Devonian shark Mcmurdodus, based on material from western Queensland, Australia". www.academia.edu. Retrieved 2016-06-03.
  4. ^ 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.
  5. ^ a b c Compagno, Leonard J. V.; Dando, Marc; Fowler, Sarah L.; Compagno, Leonard; Fowler, Sarah (2005). A field guide to the Sharks of the world. Collins field guide. London: Collins. ISBN 978-0-00-713610-0.
  6. ^ Allen, Thomas B. The Shark Almanac. New York: The Lyons Press, 1999. ISBN 1-55821-582-4
  7. ^ Klug, Stefanie and Kruiwet, Jurgen, “A new Jurassic cow shark (Chondrichthyes, Hexanchiformes) with comments on Jurassic hexanchiform systematics” Swiss Journal of Geosciences, 12 November 2011. https://www.researchgate.net/publication/234060567
  8. ^ "New shark species confirmed: Genetic testing finds a different sixgill shark".
  9. ^ "New species of shark discovered through genetic testing".
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