A class in the phylum Mollusca that is exclusively marine and contains the extant members—squids, cuttlefishes, octopuses, and the chambered nautiluses—and the fossil taxa—ammonoids, nautiloids, and belemnoids. The earliest known cephalopod fossils date to roughly 500 million years ago from the Upper Cambrian of northeast China. Fossilized external shells of the extinct subclass Ammonoidea and the nearly extinct subclass Nautiloidea are interpreted as indicating that these animals were shallow-living and slow-moving. Despite the thousands of species of such shelled cephalopods that have been recognized, all are extinct except for six species in two surviving genera, Nautilus and Allonautilus. Other living cephalopods, all predators in the subclass Coleoidea with internal shells, number over 725 species, with many more likely to be recognized.