Cephalopod shells are very well represented in the fossil record as far back as the Upper Cambrian period, about 505 million years ago. Soft-tissue fossils, however, are extremely rare. Two very different subclasses of cephalopods live in modern seas: (1) the nautilids (subclass Nautiloidea), represented by two genera and about six species that have two pairs of gills and external shells into which they can withdraw; and (2) the neocoleoids (a division of subclass Coleoidea), sometimes called dibranchiates because they have a single pair of gills, comprising all other living species of cephalopods (fewer than a thousand). The neocoleoids include the familiar squids, cuttlefishes, and octopods.
There are about 44 families of extant neocoleoids, plus one family of nautilids. Only eight families have more than 20 species; many families consist of only one species or one genus. Although the familial relationships of living cephalopods are fairly stable, researchers have not completely resolved many relationships at higher and lower levels of classification (orders and genera). Eight distinctive groups of neocoleoids can be defined, however: Incirrata (common octopods); Cirrata (finned octopods); Vampyromorpha (the vampire squid, one species); Sepiida (cuttlefishes), Spirulida (the ram's horn squid, one species); Sepiolida (bobtail and bottle squids); Myopsida
(inshore squids), and Oegopsida (oceanic squids). There is no consensus regarding the taxonomic level of these groups, and some families cannot be assigned with confidence to any of them. The first three groups listed comprise the Octopodi-formes; the remaining five are grouped together as the De-capodiformes, commonly called decapods.
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