Belemnite fossils. Belemnites are an extinct group of molluscs that are thought to be related to modern cephalopods such as octopus, squid and cuttlefish. They were plentiful during the Jurassic Period (about 200-145 million years ago) but became extinct 65 million years ago, at the end of the Cretaceous period. These belemnites are thought to have been regurgitated by a predator such as an ichthyosaur, a type of marine reptile. Fossils found in Saltwick Bay in Yorkshire, UK.