Hybodus shark. Artwork of a Hybodus shark swimming. This genus of prehistoric shark existed during the late Permian period until the beginning of the Cretaceous period (250-130 million years ago). Adult sharks measured up to 2.5 metres long and weighed up to 300 kilograms. The dorsal fin was spiny, probably as a means of defence. It had two types of teeth, one set was sharp, suitable for catching slippery prey such as fish and squid. Another set of teeth were flat and more suited for crushing the casings of shelled animals. Shark skeletons are made of cartilage and do not fossilise well. The only parts that fossilise are teeth and parts of the fins.