Pyritised ammonite (Crucilobiceras densinodulum) fossils from the Densinodulum Subzone of the Jurassic Lower Lias cliffs of Stonebarrow, near Charmouth, Lyme regis, Dorset, UK. Ammonites are a type of extinct cephalopod mollusc, resembling a squid-like animal within a coiled spiral shell. The many chambered shell protected the delicate organs of the mollusc, and was also used for buoyancy when some of the chambers were filled with air. Ammonites lived from the Devonian period (approximately 400 million years ago) to the end of the Cretaceous period (64 million years ago). The original calcium carbonate shell has been fossilised in limestone and the mineral pyrite (iron sulphide) which results in its golden metallic sheen.