"Sharks have dominated our oceans for over 400 million years, nearly unchanged in form over the last 100 million years. Unlike other vertebrates, sharks' teeth are not set in sockets, but are loosely attached by tendons to their jaws and are continuously being replaced. When one tooth falls out, another takes its place. It has been estimated that a single tooth may only be used for 20 to 40 days. In its lifetime, a shark may shed as many as 20,000 teeth. Fossilized shark teeth are considered the most prevalent of all fossils.
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