Helicoprion prehistoric shark, illustration. This shark lived during the late Carboniferous period (280 million years ago) until the early Triassic period (225 million years ago). Helicoprion is known only from fossilised sets of teeth that form circular coils. It is thought that the 'tooth-whorl' formed on the lower jaw and developed as newer teeth grew and pushed the older teeth further forward. Shark skeletons are made of cartilage and do not fossilise well. The only parts that fossilise are teeth and parts of the fins.