East Riding Museums

Bag of various fossils including ammonites and belemnites. Ammonites were sea dwelling creatures that lived during the Mesozoic period (about 248-66 million years ago). They inhabited coiled shells, which expanded during the creature's lifetime. The creature itself lived in the outermost chamber, with others filled with liquid or gas to give the creature buoyancy in the water. Ammonite shells were made of a chalky substance called aragonite, but this was often dissolved or mineralised during the process of fossilisation. There were numerous species of ammonite (all now extinct) ranging from a few centimetres to two metres in diameter. Belemnites were small tentacled sea creatures, (a member of the cephalopod group) which lived during the Mesozoic period (about 248-66 million years ago). In antiquity these fossils were popularly known as "thunderbolts", since it was thought they had been flung out of the sky during thunderstorms. The long, bullet-shaped shell of the creatures was made of calcite (calcium carbonate).