Stethacanthus sharks. Artwork of two Stethacanthus sharks swimming. This genus of prehistoric shark existed during the late Devonian and Carboniferous periods (370-345 million years ago). Males had an anvil or hammer shaped dorsal fin which is thought to have been used for courtship display. On the tops of the dorsal fin and the head appear rough, bristle-like scales. Stethacanthus measured 60 centimetres long. Shark skeletons are made of cartilage and do not fossilise well. Most shark fossils consist of teeth and fin parts.