In modern oceans, this genus is represented by two species (Compagno, 1984), the uncommon, Largetooth cookiecutter (Isistius plutodus GARRICK & SPRINGER, 1964) known only from isolated locales (off Alabama and Japan) and the circumtropical species, I. brasiliensis.
Isistius brasiliensis (QUOY & GAIMARD, 1824), Cookiecutter shark
The Cookiecutter is a small (to 50cm), cigar-shaped shark with a disproportionately large reputation. It is primarily known from catches and its geographical distribution is scattered across the warmer waters of the Atlantic and Pacific. According to Compagno, these tend to be over deep basins near islands. The shark moves vertically through the water column, ranging from 85 to 3,500 meters and is known to feed on squid & crustaceans as well as larger marine vertebrates. It is the feeding behavior directed towards these larger animals (tuna, marlin, pinnipeds, cetaceans and even large sharks) that draws attention to the genus. The cookiecutter apparently implants it's small, spike-like upper teeth then rotates its body, using the lower tooth series to cut a plug of flesh from its victim.
Various authors have speculated as to how these small, slow swimming sharks manage to feed on larger and faster pelagic prey. Most concluded that this shark's bioluminescence came into play. Many mesopelagic organisms employ luminescence to hide their silhouettes. Widder (1998) has shown that the non-luminent collar of the cookiecutter, when combined with ventrally directed bioluminescence, creates a small, fish-like silhouette -- a lure to attract prey to predator.
Dentition design. The upper teeth are tall and slender, and the lower, thin and triangular. These lower teeth are strongly interlocked, forming a saw-like tooth-set and a cutting-crasping dentition. Only a single row (of lower teeth) is functional, and shed teeth are lost as a group. There is speculation that shed teeth are consumed for their mineral content. The relative abundance of these teeth at Lee Creek would suggest that this practice, if it exists, was not common in the lower Pliocene of the Albemarle Embayment. According to Compagno, the tooth counts are 31-37 / 25-31.
Fossil teeth are generally ascribed to two extinct species; Isistius trituratus (WINKLER 1874) from the Palaeocene & Eocene of Europe & North Africa and I. triangulus (PROBST 1879) from Miocene & Pliocene of Europe and South America. Purdy, et al (2001) attribute Lee Creek teeth of this design to Isistius sp and note they are present in Yorktown units 1-3.
Teeth of this species are commonly considered rare at Lee Creek. Although undamaged lower teeth may be viewed as rare, specimens can be found regularly when processing basal Yorktown (lower Pliocene) tailings. The upper teeth have not been reported from the mine, and Cappetta (1987) notes that there are no reports from the fossil record.
The teeth are small, generally less than 6.0 mm in height (I have field notes that include a near-perfect specimen, found by Eric Thompsen, as being 7.8 mm high). The upper teeth are slender and may be relegated to an 'unknown symphyseal-type' category in some collections (including mine). The lower's are labio-lingually compressed with triangular crowns and rectilinear roots.
The crowns of the Lee Creek cookiecutter (I. cf triangulus) lower teeth (Fig. ) are weakly serrate (said to be smooth by Purdy, et al), a characteristic not noted in the teeth (I. cf trituratus) found at Muddy Creek (Eocene - Virginia).
The labial and lingual faces of the root have a narrow depression which begins at the base of the root and extends apically, terminating at a "keyhole" foramen near the midpoint of the root. This depression is often open, extending from the labial to lingual face. The lingual face also bears a foramen between the "keyhole" and the base of the crown. The mortise-like grooves which are used to interlock adjacent teeth are clearly visible on a lateral margin of the root.
Widder, Edith A., 1998. A predatory use of counterillumination by the squaloid shark,
Isistius brasiliensis. Environmental Biology of Fishes, 53. pp 267 - 273.