Sharks are fishes whose skeleton is composed of cartilage rather than bony tissue. Their teeth, however, consist of resistant calcium phosphate minerals. Consequently, the fossil record of sharks largely comprises their teeth, which can be found in great variety and abundance all over the world, especially in Mesozoic and Cenozoic rocks.
Given that an individual great white shark has more than 300 teeth in its jaws at any one time, and teeth are shed regularly during the life of the shark, it is not surprising that sharks' teeth can be abundant fossils.
Fossil sharks' teeth were popularly known as tongue stones or 'glossopetrae ' before their origin became fully understood.