Ammonites gervillii. 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. octop

Ammonites gervillii. 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.. (Photo by: Sepia Times/Universal Images Group via Getty Images)