There are many misconceptions regarding the evolution of sharks. Contrary to popular belief they have not remained unchanged for 300 million years. However, many of the families we have today have been in existence for about the last 150 million years. Compared to our own puny 3.5 million years this makes the elasmobranch lineage very ancient indeed. The fossil record is sketchy at best when it comes to sharks. Cartilage is preserved very poorly so the body structures of many early sharks are purely speculative. Fortunately for the paleontologists, sharks discard their teeth on a regular basis and these teeth which fossilize well are often enough to allow accurate identification of individual groups and help place them correctly in the evolutionary time line. One of the downfalls of using fossilized teeth for identification is that the teeth of sharks sometimes vary significantly depending on which area of the jaw they have come from. In the past this has led to paleontologists inventing multiple species of extinct sharks that were actually the same animal. In recent years there has been a revision of extinct species that paid more attention to the dental nature of modern species, and also the list of extinct species has been paired down.