Ammonitina comprises a diverse suborder of ammonite cephalopods that lived during the Jurassic and Cretaceous periods of the Mesozoic Era. They 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 Ammonitina are derived from the Phylloceratina, another ammonitid suborder which has its origin in the Ceratitida of the Triassic. As with the subclass, the closest living relatives of the Ammonitina are the Coleoidea (octopus, squid, and cuttlefish) and not the superficially similar modern Nautilus.