Subclass Coleoidea, or Dibranchiata, is the grouping of

cephalopod A cephalopod is any member of the molluscan class Cephalopoda ( Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head ...

s containing all the various taxa popularly thought of as "soft-bodied" or "shell-less" (i.e., octopuses, squid and cuttlefish). Unlike its extant sister group, Nautiloidea, whose members have a rigid outer shell for protection, the coleoids have at most an internal cuttlebone,

gladius ''Gladius'' () is a Latin word meaning "sword" (of any type), but in its narrow sense it refers to the sword of ancient Roman foot soldiers. Early ancient Roman swords were similar to those of the Greeks, called '' xiphe'' (plural; singular ''xi ...

, or shell that is used for buoyancy or support. Some species have lost their cuttlebone altogether, while in some it has been replaced by a

chitin Chitin ( C8 H13 O5 N)n ( ) is a long-chain polymer of ''N''-acetylglucosamine, an amide derivative of glucose. Chitin is probably the second most abundant polysaccharide in nature (behind only cellulose); an estimated 1 billion tons of chit ...

ous support structure. A unique trait of the group is the ability to edit their own RNA. The major divisions of Coleoidea are based upon the number of arms or tentacles and their structure. The extinct and most primitive form, the Belemnoidea, presumably had ten equally-sized arms in five pairs numbered dorsal to ventral as I, II, III, IV and V. More modern species either modified or lost a pair of arms. The superorder Decapodiformes has arm pair IV modified into long tentacles with suckers generally only on the club-shaped

distal Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...

end. Superorder Octopodiformes has modifications to arm pair II; it is significantly reduced and used only as a sensory filament in the Vampyromorphida, while Octopoda species have totally lost that arm pair.

Evolutionary history

The earliest certain coleoids are known from the Mississippian sub-period of the

Carboniferous The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carboniferou ...

Period, about 330 million years ago. Some older fossils have been described from the

Devonian The Devonian ( ) is a geologic period and system of the Paleozoic era, spanning 60.3 million years from the end of the Silurian, million years ago (Mya), to the beginning of the Carboniferous, Mya. It is named after Devon, England, wh ...

, but paleontologists disagree about whether they are coleoids. Other cephalopods with internal shells, which could represent coleoids but may also denote the independent internalization of the shell, are known from the

Silurian The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleoz ...

. It is possible that the Early–Middle

Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ago ...

fossil '' Nectocaris'' represents a coleoid (or other cephalopod) that lost its shell, possibly secondarily. By the Carboniferous, coleoids already had a diversity of forms, but the major radiation happened during the

Tertiary Tertiary ( ) is a widely used but obsolete term for the geologic period from 66 million to 2.6 million years ago. The period began with the demise of the non-avian dinosaurs in the Cretaceous–Paleogene extinction event, at the start ...

.Cephalopods: Ecology and Fisheries
/ref> Although most of these groups are traditionally classified as belemnoids, the variation among them suggests that some are not closely related to

belemnite Belemnitida (or the belemnite) is an extinct order of squid-like cephalopods that existed from the Late Triassic to Late Cretaceous. Unlike squid, belemnites had an internal skeleton that made up the cone. The parts are, from the arms-most ...

Classification

*Class

Cephalopod A cephalopod is any member of the molluscan class Cephalopoda ( Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head ...

a **Subclass Nautiloidea: nautilus **Subclass † Ammonoidea: ammonites **Subclass Coleoidea ***Division † Belemnoidea: extinct belemnoids ****Genus †''

Jeletzkya ''Jeletzkya douglassae'' is a fossil coleoid from the early Pennsylvanian Mazon Creek lagerstätten and represents the earliest known crown-group squid. Non-mineralized anatomy is preserved and comprises ten hooked tentacles and a radula T ...

'' ****Order † Hematitida ****Order †

Phragmoteuthida Phragmoteuthida is an order of extinct coleoid cephalopod A cephalopod is any member of the molluscan class Cephalopoda (Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals a ...

****Order † Donovaniconida ****Order † Aulacocerida ****Order † Belemnitida ***Division Neocoleoidea ****Superorder Decapodiformes *****Order Spirulida: ram's horn squid *****Order

Sepiida Cuttlefish or cuttles are marine molluscs of the order Sepiida. They belong to the class Cephalopoda which also includes squid, octopuses, and nautiluses. Cuttlefish have a unique internal shell, the cuttlebone, which is used for control of b ...

: cuttlefish *****Order Sepiolida: bobtail squid *****Order Myopsida: coastal squid *****Order

Oegopsida Oegopsida is one of the two orders of squid in the superorder Decapodiformes, in the class Cephalopoda. Together with the Myopsina, it was formerly considered to be a suborder of the order Teuthida, in which case it was known as Oegopsina. Th ...

: neritic squid ****Superorder Octopodiformes *****Family † Trachyteuthididae ('' incertae sedis'') *****Order Vampyromorphida: vampire squid *****Order Octopoda: octopus ****Superorder Palaeoteuthomorpha *****Order † Boletzkyida * (uncertain order) ** family †

Ostenoteuthidae Ostenoteuthidae is an extinct family of cephalopods from Lower Jurassic of Italy. They had ten arms with unusual structure. Both known species are from Lower Sinemurian. The type locality for up to date known two species is near village Osteno, ...

References

External links

*
Tree of Life web project: Coleoidea

"Octopuses Do Something Really Strange to Their Genes"
{{Taxonbar , from=Q749814 Coleoidea Mollusc subclasses Carboniferous first appearances