A tooth usually lasts about a week before it falls out. When this happens, the tooth behind it moves up to replace it. A new tooth can be replaced in as little as 24 hours. Sharks keep replacing their teeth all their lives. As the shark grows, its new teeth keep pace and grow larger than the ones that are replaced.
Sharks are some of the oldest and most successful vertebrates on the earth.
The first sharks appeared in ancient oceans 400 million years ago. Shark skeletons rarely fossilize. They are made of cartilage which decomposes so fast the tissue is not replaced by minerals. However, shark teeth are made of dentine and enamel and fossilize readily. A single shark may lose thousands of teeth in its lifetime.