Ammonites bipartitus; Print; Ammonoidea; 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.