CHRISTIAN DARKIN / SCIENCE PHOTO LIBRARY CHRISTIAN DARKIN / SCIENCE PHOTO LIBRARY
Helicoprion. Artwork of the prehistoric shark Helicoprion swimming among ammonites. This genus of shark existed from the late Carboniferous period (280 million years ago), until the early Triassic period (225 million years ago). This shark 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. Ammonites are an extinct group of molluscs that are thought to be related to modern cephalopods such as octopus, squid and cuttlefish.
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