Beautiful Ammonites disc with stand 128.41 g 115x115x6 mm Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, 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.The earliest ammonites appear during the Devonian, and the last species died out in the Cretaceous–Paleogene extinction event. Ammonites are excellent index fossils, and it is often possible to link the rock layer in which a particular species or genus is found to specific geologic time periods.