Prehistoric megalodon shark tooth fossil. Otodus megalodon (big tooth) is a large ancient shark that appeared about 23 million years ago in the Early Miocene era. This predator, up to 18 metres in length, became a king of the oceans in the middle Miocene era, about 15.9 million years ago. Its massive jaws with 10-centimetre teeth could overpower the huge whales that it hunted. It is thought that megalodon became extinct at the end of the Pliocene era about 2.6 million years ago. Found in the USA.