Shark teeth are relics of shark evolution an biology. Shark skeletons are composed entirely of cartilage. Often the only parts of the shark to survive as fossils are teeth. Fossil shark teeth have been dated back hundreds of millions of years. The most ancient types of sharks date back to 450 million years ago, and they are mostly known from their fossilized teeth. The most common, however, are from the Cenozoic Era (65 million years ago).