Most recently, the teeth of Scapanorhynchus texanus, also known as the "goblin shark of Texas," were found and dated to between 78.8 and 79.2 million years ago
The teeth were recovered from what is now a vertical cliff next to a creek in western Alabama. The spot was once a seabed below waters teaming with unusual looking fish, including the now-extinct shark.
"Judging from the abundance of shark teeth preserved in the Cretaceous of Alabama, sharks were abundant in Alabama seas during the Cretaceous," Martin Becker, who led the project, told Discovery News.
"Shark teeth are also abundant, however, in Cretaceous deposits of the Gulf Coast and the Atlantic coastal plain, so it's pretty clear that they were abundant up and down the coast of what is now the United States," added Becker, an associate professor of environmental science at William Paterson University.
Becker and colleagues David Seidemann, John Chamberlain, Dieter Buhl and William Slattery studied 15 fossil shark teeth from the vertical cliff created by Trussels Creek in Greene County, Ala. Although the researchers focused on the goblin shark specimen, also nicknamed "spade snout," they believe some of the teeth belonged to different sharks because the deposit concentrated materials coming from various parts of a seabed.
They cut slices of the teeth to study its three different types of tissues: the orthodentine and osteodentine, which form the pulp cavity and root structure of a tooth, and the enameloid, which is the hard outer surface.
The scientists then determined the strontium isotope composition of each of the different types of tooth tissue. Strontium isotopes have a radioactive component that increases incrementally over time as the result of decay. When compared with the non-changing part of the metal's atoms, scientists can date tooth fossils.
They can also reconstruct past environments, since the chemical composition of the teeth usually locks in certain isotopic signatures of the seawater in which the shark lived.
The findings have been accepted for publication in the journal Palaeogeography, Palaeoclimatology, Palaeoecology.
The researchers discovered that tooth enamel provided far more accurate data than the other parts of the tooth, suggesting that future studies should focus only on the hard enamel surfaces.
"We expect that enamel in any creature would be less susceptible to alteration after its formation than the other dental tissues, probably because enamel is well-crystallized and not as porous as dentine," explained Seidemann, a Brooklyn College geochemist.