Gli squali fossili
The cow sharks are generally a deepwater-genus, which inhabits the outer shelf and upper slope in boreal, cold temperate and tropical waters, worldwide. Although generally a deepwater bottom dweller, these sharks may be found near the surface and can range from near shore to submarine canyons (1800 meters). The genus feeds on sharks, other fishes, squid, crab, and marine mammals. 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) H. 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.
For some reason (unknown to the author), the terminology commonly employed for hexanchid teeth differs from that used for most sharks. Mammal tooth terms such as cone and conule are employed for descriptions. I will use primary cusp rather than primary cone or acrocone, and accessory cusps instead of conules or accessory cones.
The anterior and lateral teeth of this family are quite unique, particularly the lower laterals. Differentiating teeth between genus is more difficult and between species - hair splitting. As Kent (1994) suggests, it's real good to know the specimen's stratigraphic origin.
The heterodont Hexanchus dentition serves a grasping-cutting function. 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.
Cappetta (1987) notes three species that relate to the fauna of the Western Atlantic. Hexanchus agassiz 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.
Kent (1994) attributed two of these species to the Chesapeake region, Hexanchus agassiz to the Nanjemoy (Eocene) and H. gigas, to the Calvert Formation (Lower Miocene). He 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) included H. microdon as present in the Campanian-Maastrichtian fauna of Texas.
In a revisionary study of Eocene
members of the family, Ward (1979) described a new species from the Eocene
of England - H. collinsonae. Differentiating it from H .agassiz were: the
width to height ratio of the root (4:1 vs 2.5:1), the strength of the mesial
serrations (medium vs fine) and size (width 1.5 vs 2.5 cm).
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