| The first sharks appeared in the ancient seas 400 million years ago, about 200 million years before dinosaurs roamed the Earth. At that time there were no reptiles, birds, or mammals. The remains of some of these early sharks were fossilised when they fell to the bottom of the sea and become covered with layers of sand and other sediment. Hard parts, like spines and teeth, fossilised more easily than soft parts, which often rotted away. Sometimes all that is left are impressions of the sharks in rocks. Fossil shark teeth are common because these ancient sharks, like their living descendants, shed many teeth in a lifetime. Shark’s rubbery skeletons, made of cartilage, did not preserve as well as the hard skeletons of bony fish. Shark fossils are often discovered in rocks on land which, in prehistoric times, were under the sea. Scientists can tell how old fossils are from the age of the rocks in which they are found. The earliest groups of sharks become extinct, but the descendants of some groups that first appeared about 200 million years ago - like the bullheads, cat sharks, and cow sharks - are live today. |
