The Evolution of Sharks, Rays, and Chimaeras
The ancestors of modern sharks started showing up in the fossil record around 400 million years ago. Over the millennia, they have undergone considerable evolutionary changes, but some shark families we see today have existed relatively unchanged for the last 150 million years. By contrast, humanity has not yet to celebrate its first megaannum (million years), so the elasmobranch lineage is 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 palaeontologists, 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 pitfalls 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 palaeontologists inventing multiple species of extinct sharks that were actually the same animal. During a shark’s lifetime its dentition also may change which only adds to the confusion. In recent years there has been a revision of extinct species that paid more attention to the hetero-dental nature of modern species, and consequently the list of extinct species has been paired down.