Illustration of a Cladoselache shark. This genus of prehistoric shark existed during the late Devonian period (roughly 370 million years ago). The long, streamlined body (adults measured up to 2 meters in length), and deep forked tail enabled it to swim quickly. Unlike modern sharks and most prehistoric sharks, Cladoselache lacked scales on its skin, and males lacked sexual claspers. Modern sharks use their claspers to grip onto the female during sexual intercourse. Shark skeletons are made of cartilage and do not fossilize well. The only parts that fossilize are teeth and parts of the fins.