Hexanchiformes

Hexanchiformes
Temporal range: Middle Jurassic–Recent

[1]

Possible Paleozoic record

Frilled shark, Chlamydoselachus anguineus
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Chondrichthyes
Subclass: Elasmobranchii
Superorder: Selachimorpha
Order: Hexanchiformes
F. de Buen, 1926
Families

†Crassonotidae
†Mcmurdodontidae?
†Orthacodontidae
Chlamydoselachidae
Hexanchidae

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

Classification

Living species

Extinct species

Notidanodon sp. fossil at the Geological Museum, Copenhagen

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		 4
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.[3] 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

References

  1. ↑ Froese, Rainer, and Daniel Pauly, eds. (2009). "Hexanchiformes" in FishBase. January 2009 version.
  2. ↑ Allen, 45
  3. ↑ 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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