Shark teeth are relics of shark evolution and biology. Shark skeletons are composed entirely of cartilage. Often the only parts of the shark to survive as fossils are teeth. Fossil shark teeth have been dated back hundreds of millions of years. The most ancient types of sharks date back to 450 million years ago, and they are mostly known from their fossilized teeth. The most common, however, are from the Cenozoic Era (65 million years ago).
The teeth of sharks are not attached to the jaw, but embedded in the flesh, and in many species are constantly replaced throughout the shark's life. All sharks have multiple rows of teeth along the edges of their upper and lower jaws. New teeth grow continuously in a groove just inside the mouth and move forward from inside the mouth on a "conveyor belt" formed by the skin in which they are anchored. In some sharks rows of teeth are replaced every 8–10 days, while in other species they could last several months. The lower teeth are primarily used for holding prey, while the upper ones are used for cutting into it. The teeth range from thin, needle-like teeth for gripping fish to large, flat teeth adapted for crushing shellfish.
Source: Wikipedia.Org
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