Sharks are one of the oldest animal species on earth having outlived the dinosaurs with fossil records dating them back 400 million years. Most sharks live in saltwater environments, although two species can survive in freshwater and saltwater. In their ecosystems, the carnivorous shark is usually the top of their food chain, but are being threatened by human activities and hunting. The largest fish in the world is the whale shark, capable of growing to 40 feet (12.2 m) in length. Sharks have multiple rows of teeth that are embedded in their gums instead of their jaws, and are constantly losing and replacing their teeth—sharks can lose and replace up to 30,000 teeth in their lifetime.
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