Ammonite shells. Computer-enhanced engraving of an assortment of ammonite species. Side and front views of the spiral shells are seen, each with many chambers. Ammonites are a type of extinct cephalopod mollusc that resembled a squid-like animal within a coiled shell. The many chambered shell protected the delicate organs of the mollusc, and was also used for buoyancy when some of the chambers were pumped full of air. Ammonites lived from the Devonian period (approximately 400 million years ago) to the end of the Cretaceous period (64 million years ago). Engraving taken from Ernst Haeckel's famous treatise: insArt Forms of Natureins (1904).