Listracanthus shark. Artwork of a Listracanthus shark swimming. This genus of prehistoric shark existed during the late Carboniferous period (roughly 300 million years ago) to the late Triassic period (200 million years ago). It had thin, elongated scales (denticles) along it's back which grew up to 10 centimetres long. It had one main spine and numerous feathery secondary spines. Shark skeletons are made of cartilage and do not fossilise well. Most shark fossils consist of teeth and fin parts.