Cladoselache shark. Artwork of a Cladoselache shark swimming. 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 metres 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 fossilise well. The only parts that fossilise are teeth and parts of the fins.