Ancient snails filled their shells with gas to become super-snails - the nautiloids. Later, their cousins rolled up their shells and became the ammonites. Ammonites were so successful that they would become the most numerous fossil for over 300 million years. They ranged from tiny little ones to ones the size of a person. A large shell would have contained a giant, two-metre-long slug with tentacles. The ridges around the edge of the shell worked as stabilisers, keeping it on an even keel as it bobbed along looking for food. A huge fossil graveyard near Lyme Regis was formed by just one shoal yet stretches for several miles inland.