| Coleoidea | |
|---|---|
 | |
| Acuttlefish | |
 | |
| Acommon octopus | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Mollusca |
| Class: | Cephalopoda |
| Clade: | Neocephalopoda |
| Subclass: | Coleoidea Bather, 1888 |
| Divisions | |
| |
Coleoidea[1][2] orDibranchiata is one of the twosubclasses ofcephalopods containing all the varioustaxa popularly thought of as "soft-bodied" or "shell-less" (i.e.octopus,squid andcuttlefish). Unlike its extant sister groupNautiloidea, whose members have a rigidouter shell for protection, thecoleoids have at most an internal shell calledcuttlebone orgladius that is used forbuoyancy or as muscle anchorage. Some species, notablyincirrate octopuses, have lost their internal shell altogether, while in some it has been replaced by achitinous support structure.
The major divisions of Coleoidea are based upon the number ofarms ortentacles and their structure. The extinct and most primitive form, theBelemnoidea, presumably had ten equally-sized arms in five pairs numbereddorsal toventral as I, II, III, IV and V. More modern species either modified or lost a pair of arms. ThesuperorderDecapodiformes has arm pair IV modified into long tentacles with suckers generally only on the club-shapeddistal end. SuperorderOctopodiformes has modifications to arm pair II; it is significantly reduced and used only as a sensory filament in theVampyromorphida, whileOctopoda species have totally lost that arm pair. The inner surface of the suckers (acetabulum) are reinforced with rigid sucker rings which are smooth inSepiolida, have blunt teeth inSepiida and sharply pointed teeth inLoliginidae andOegopsida. The arms and/or tentacles of some oegopsid families have also evolved claw-like hooks.[3]
The earliest certain coleoids are known from theMississippian sub-period of theCarboniferous Period, about 330 million years ago. Some older fossils have been described from theDevonian,[4]but paleontologists disagree about whether they are coleoids.[5] Other cephalopods with internal shells, which could represent coleoids but may also denote the independent internalization of the shell, are known from theSilurian.[6] It has been hypothesized that the Early–MiddleCambrian fossilNectocaris represents a coleoid (or other cephalopod) that lost its shell, possiblysecondarily,[7][8] although it is more likely thatNectocaris represents an independent lineage within theLophotrochozoa.[9]
By the Carboniferous, coleoids already had a diversity of forms, but the major radiation happened during theTertiary.[10] Although most of these groups are traditionally classified as belemnoids, the variation among them suggests that some are not closely related tobelemnites.[11]