East Riding Museums

Ten examples of fossil ammonites (Lyficoceras Regale), grey to pale pink. From Speeton Clay Cliffs. 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.