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Hexanchus

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Hexanchus is a genus of six-gilled sharks. Compagno (1984) considered two species to be valid, Hexanchus vitulus SPRINGER & WALLER 1969 a circumglobal (spotty and not in the eastern Pacific) warm temperate to tropical species, and the larger (to 4.8 m) Hexanchus griseus (BONNATERRE, 1788). The later species is wide ranging and inhabits similar waters, circumglobally. Compagno points out that H. griseus has six files of the "comb-like" lateral teeth per side, while H. vitulus has only five.

Species Edit

  • Hexanchus agassizi CAPPETTA,1976 Lower Eocene of England, Eocene of New Jersey & Australia and the Oligocene of Australia & Russia,
  • H. gigas (SISMONDA, 1857) (possibly = H. griseus) Miocene and/or Pliocene of Europe, Japan North and South America, and
  • H. microdon (AGASSIZ, 1843) from the Upper Cretaceous of Africa, Europe and Japan.

Teeth Edit

Excluding the small posteriors, the upper teeth are characterized by a large, slender, distally inclined cusp and the lowers by their saw-like, multi-cusped design. These lower laterals have a rectangular shape. The root is laterally elongated and labio-lingually compressed. The root faces are flat and the lingual face bears one or more grooves. The crown is made-up of a primary cusp and multiple smaller (up to 12) accessory cusps. The basal mesial cutting edge of the primary cusp is serrate. In upper teeth, the root is deeper near the junction with the crown. The mesial cutting edge is similarly serrate but only a couple (one to four), much smaller, accessory cusps (cusplets) are present.

Kent (1994: 20) attributed two of these species to the Chesapeake region, Hexanchus agassizi to the Nanjemoy (Eocene) and H. gigas, to the Calvert Formation (Lower Miocene).

Bakersfield Dentition Edit

From the Middle Miocene of Californian, teeth of the general H. gigas size and design are ascribed to Hexanchus andersoni JORDAN, 1907. These teeth are uncommon in Bakersfield deposits but are still found with much more consistency than many other species. Specimens may measure up to 4-1/2 cm in width and normally have 7-8 conules, but these may (rarely) bear 9 to 10.

Difficulties Identifying Species Edit

Kent (1994: 20),warned of the pitfalls in attempting to identify these teeth to species. They included: ontogenetic variations (an increase in accessory cusps with age), conservative design (with minor changes, stratigraphic data important) and sexual dimorphism (primary cusp is relatively higher and more slender in males). Welton & Farish (1993: 72) included H. microdon as present in the Campanian-Maastrichtian fauna of Texas. The included Aquia specimen, if valid, would prove to be the first report of Hexanchus from the Aquia (Palaeocene). Specimens have been recovered (in situ) from 'zones' 2 and 7.

See Also Edit

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