Xenacanthus sharks. Artwork of two Xenacanthus sharks swimming. This genus of prehistoric shark existed during the late Devonian through to the Triassic period (410-220 million years ago). This species lived in freshwater and had a long spine protruding from the back of their head. It had unusual double forked front teeth and a ribbon-like dorsal fin giving it an eel-like appearance. Shark skeletons are made of cartilage and do not fossilise well. Most shark fossils consist of teeth and fin parts.