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 Nautiloids are a group of marine cephalopods (Mollusca) which originated in the Late Cambrian and are represented today by the living ''Nautilus'' and ''Allonautilus''. Fossil nautiloids are diverse and speciose, with over 2,500 recorded species. ...

, whose members have a rigid outer shell for protection, the coleoids have at most an internal

cuttlebone Cuttlebone, also known as cuttlefish bone, is a hard, brittle internal structure (an internal shell) found in all members of the family Sepiidae, commonly known as cuttlefish, within the cephalopods. In other cephalopod families it is calle ...

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 Some species have lost their cuttlebone altogether, while in some it has been replaced by a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 Ribonucleic acid (RNA) is a polymeric molecule essential in various biological roles in coding, decoding, regulation and expression of genes. RNA and deoxyribonucleic acid ( DNA) are nucleic acids. Along with lipids, proteins, and carbohydra ...

. 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 Belemnoids are an extinct group of marine cephalopod, very similar in many ways to the modern squid and closely related to the modern cuttlefish. Like them, the belemnoids possessed an ink sac, but, unlike the squid, they possessed ten arms o ...

, presumably had ten equally-sized arms in five pairs numbered

dorsal Dorsal (from Latin ''dorsum'' ‘back’) may refer to: * Dorsal (anatomy), an anatomical term of location referring to the back or upper side of an organism or parts of an organism * Dorsal, positioned on top of an aircraft's fuselage * Dorsal co ...

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

as I, II, III, IV and V. More modern species either modified or lost a pair of arms. The

superorder Order ( la, ordo) is one of the eight major hierarchical taxonomic ranks in Linnaean taxonomy. It is classified between family and class. In biological classification, the order is a taxonomic rank used in the classification of organisms and ...

Decapodiformes Decapodiformes is a superorder of Cephalopoda comprising all cephalopod species with ten limbs, specifically eight short arms and two long tentacles. It is hypothesized that the ancestral coleoid had five identical pairs of limbs, and that one ...

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 Octopodiformes is a superorder of the subclass Coleoidea, comprising the octopuses and the vampire squid. All living members of Octopodiformes have eight arms, either lacking the two tentacles of squid (as is the case in octopuses) or modifying t ...

has modifications to arm pair II; it is significantly reduced and used only as a sensory filament in the

Vampyromorphida Vampyromorphida is an order of cephalopods comprising one known extant species (''Vampyroteuthis infernalis'') and many extinct taxa. Physically, they somewhat resemble octopuses (their closest relatives), but the eight arms are united by a web ...

, 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 ''Carbonifero ...

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, whe ...

, 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 Paleozo ...

. It is possible that the Early–Middle

Cambrian The Cambrian Period ( ; sometimes symbolized C with bar, 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 ...

fossil ''

Nectocaris ''Nectocaris'' is a genus of squid-like animal of controversial affinities known from the Cambrian period. The initial fossils were described from the Burgess Shale of Canada. Other similar remains possibly referrable to the genus are known from ...

'' 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 to ...

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

: nautilus **Subclass †

Ammonoidea Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid and cuttle ...

: ammonites **Subclass Coleoidea ***Division †

: 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 In zoology, a ...

'' ****Order †

Hematitida Hematitida is a group of coleoid cephalopods known from the early Carboniferous Period. They are the oldest definite coleoids, although there are controversial claims for even older coleoids from the Devonian. Fossil hematitidans have so far been ...

****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 ar ...

****Order † Donovaniconida ****Order †

Aulacocerida Aulacocerida is an order of primitive coleoid cephalopods, possibly derived from michelinoceraitids ( Orthocerida) early in the Devonian, which in turn gave rise to the Belemnites. Aulococerids are characterized by a straight, slim, conical sh ...

****Order †

Belemnitida 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 to ...

***Division

Neocoleoidea Neocoleoidea is a large group of marine cephalopods. This cohort contains two extant groups: Decapodiformes (squid, cuttlefish, and relatives) and Octopodiformes (octopuses and the vampire squid). Species within this group exist in all major hab ...

****Superorder

*****Order

Spirulida Spirulida is an order of cephalopods comprising one extant species ('' Spirula spirula'') and several extinct taxa. Fossil record * Oldest representative: Carboniferous, though contested: see '' Shimanskya'' * Oldest uncontested representativ ...

: 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 bu ...

: cuttlefish *****Order

Sepiolida Bobtail squid (order Sepiolida) are a group of cephalopods closely related to cuttlefish. Bobtail squid tend to have a rounder mantle than cuttlefish and have no cuttlebone. They have eight suckered arms and two tentacles and are generally quite ...

: bobtail squid *****Order

Myopsida Myopsida is one of the four orders of squid. It consists of two families: the monotypic ''Australiteuthis'' and the diverse and commercially important Loliginidae (~50 species). Some taxonomists classify this taxon as a suborder of the order Te ...

: 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. This ...

: neritic squid ****Superorder

*****Family † Trachyteuthididae (''

incertae sedis ' () or ''problematica'' is a term used for a taxonomic group where its broader relationships are unknown or undefined. Alternatively, such groups are frequently referred to as "enigmatic taxa". In the system of open nomenclature, uncertainty ...

'') *****Order

: vampire squid *****Order Octopoda: octopus ****Superorder

Palaeoteuthomorpha Palaeoteuthomorpha is a superorder of primitive coleoid cephalopods, containing the Boletzkyida, characterized by an orthoceroid-like phragmocone early in ontogeny and a teuthid True squid are molluscs with an elongated soft body, large ...

*****Order †

Boletzkyida Boletzkyida is a primitive order (biology), order of teuthid coleoidea, coleoid cephalopoda, cephalopod: the boletzkyids are thought to be the earliest forms of coleoid cephalopods, and appear to form a link between nautiloidea, nautiloid orthoce ...

* (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 Subclass (biology), Subclass Coleoidea, or Dibranchiata, is the grouping of cephalopods 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, ...

Mollusc subclasses Carboniferous first appearances