Cephalopoda is an extremely diverse class of mollusks that has been evolving since the Cambrian. The extant lineages arose in the late Silurian and diverged into Nautiloidea and Coleoidea in the mid-Palaeozoic. Nautiloidea is represented by a handful of Recent species only. In contrast, Coleoidea has diverged into two superorders, Decapodiformes and Octopodiformes, which together comprise around 800 Recent species. The relationships among orders of Decapodiformes are not well understood, and molecular systematics has failed to provide much resolution, although there is some evidence for a sister-taxon relationship between Spirulida, the ram's horn squid, and Sepiida, the cuttlefishes. A sister-taxon relationship between Bathyteuthida and Oegopsida is well established. The relationships among Octopodiformes are better understood. The vampire squid is placed in a separate order, and all other octopods are placed in Octopoda. Within Octopoda there are well-understood clades: Octopoda is divided into Cirrata and Incirrata; Incirrata is further divided into Argonautoidea and Octopodoidea. Several lineages of cephalopods have been evolving independently for a long time: for example, Spirulida, represented by a single extant species, appears to have diverged from other groups 150 million years ago, nautiloids appear little changed since 300 million years ago, and Vampyromorphida, also represented by a single extant species, appears little changed since 200 million years ago. In contrast, several groups, for example, the sepiids, appear to have undergone recent radiations.