Ammonites are an extinct group of marine invertebrate animals. The soft body of the creature occupied the largest chamber of the shell, called a phragmocone, which is at the end of the coil. The earlier smaller chambers were sealed off and filled with a gas so the animal could stay buoyant. These molluscs are more closely related to living coleoids (i.e. octopuses, squid, and cuttlefish) than they are to shelled nautiloids such as the living Nautilus species.
Ammonites are excellent index fossils, and it is often possible to link the rock layer in which they are found to specific geological time periods.
The name ammonite, from which the scientific term is derived, was inspired by the spiral shape of their fossilized shells, which somewhat resemble tightly coiled rams' horns. Pliny the Elder called fossils of these animals ammonis cornua ("horns of Ammon") because the Egyptian god Ammon (Amun) was typically depicted wearing ram's horns. Often the name of an ammonite genus ends in -ceras, which is Greek (κέρας) for "horn".