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: 18) 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: 46-48) notes three species that relate to the fauna of the Western Atlantic.
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.
Cione & Reguero (1994) reported a particularly large tooth from the Eocene of Antarctica, but elected not to attribute it to a specific species.
Müller (1999: 31) attributed teeth from Lee Creek to H. aff griseus.
Purdy, et al (2001: 84-86) were also reluctant to validate H. gigas as present in the Lee Creek
fauna noting the similarites with the extant species H. griseus.
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). 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: 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.
In a revisionary study of Eocene members of the family, Ward (1979: 116-17) described a new species from the Eocene of England - H. collinsonae. Differentiating it from H .agassizi 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).
From the Middle Miocene of California, 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.
Cappetta, H., 1987. Chondrichthyes II: Mesozoic and Cenozoic Elasmobranchii. Handbook of Paleoichthyology, 3B. Gustav Fischer Verlag, Stuttgart and New York, 193 pp.
Case, G., and H. Cappetta. 1997. A new selachian fauna from the late Maastrichtian of Texas. Muünchener Geowissenschaften Abhandungen 34:131-189.
Kent, B. 1994. Fossil Sharks of the Chesapeake Region. Egan Rees & Boyer, Maryland, 146 pp.
Müller, A. 1999. Ichthyofaunen aus dem atlantischen Tertiär der USA. Leipziger Geowissenschafteb, Leipzig, 9/10: 1-360.
Purdy, R., Schneider, V., Appelgate, S., McLellan, J., Meyer, R. & Slaughter, R., 2001. The Neogene Sharks, Rays, and Bony Fishes from Lee Creek Mine, Aurora, North Carolina. In: Geology and Paleontology of the Lee Creek Mine, North Carolina, III. C. E. Ray & D. J. Bohaska eds. Smithsonian Contributions to Paleobiology, No 90. Smithsonian Institution Press, Washington D.C. pp. 71-202.
Ward, D., 1979. Additions to the fish fauna of the English Palaeogene. 3. A review of the Hexanchid sharks with a description of four new species. Tertiary Research, 2 (3) pp 111-129.
Welton, B. J. and R. F. Farish 1993. The Collector's Guide to Fossil Sharks and Rays from the Cretaceous of Texas. Before Time, Texas. 204 pp.