Cephalopoda

The cephalopods are the mollusc class Cephalopoda characterized by bilateral body symmetry, a prominent head, and a modification of the mollusk foot, a muscular hydrostat, into the form of arms or tentacles.

Two important extinct taxa are Ammonoidea, the ammonites, and Belemnoidea, the belemnites.

Evolution
The class developed during the Late Cambrian, and underwent pulses of diversification during the Ordovician period to become diverse and dominant in the Paleozoic and Mesozoic seas. Small shelly fossils such as Tommotia were once interpreted as early cephalopods, but today these tiny fossils are recognized as sclerites of larger animals, and the earliest accepted cephalopods date to the Late Cambrian Period. During the Cambrian, cephalopods are most common in shallow near-shore environments, but they have been found in deeper waters too. Cephalopods were thought to have "undoubtedly" arisen from within the tryblidiid monoplacophoran clade. However genetic studies suggest that they are more basal, forming a sister group to the scaphopoda but otherwise basal to all other major mollusc classes. The internal phylogeny of mollusca, however, is wide open to interpretation - see Mollusca.



The cephalopods are thought to have evolved from a monoplacophoran-like ancestor with a curved, tapering shell, and to be closely related to the gastropods (snails). The development of a siphuncle allowed their shells to become gas-filled (thus buoyant) in order to support them and keep the shells upright while the animal crawled along the floor, and separates the true cephalopods from putative ancestors such as Knightoconus, which lacked a siphuncle. Negative buoyancy (i.e. the ability to float) came later, followed by swimming in the Plectroneocerida and eventually jet propulsion in more derived cephalopods. However, because chambered shells are found in a range of molluscs - monoplacophora and gastropods as well as cephalopods - a siphuncle is essential to ally a fossil shell conclusively to the cephalopoda. :57 The earliest such shells do not have the muscle scars which would be expected if they truly had a monoplacophoran affinity. :57

The earliest cephalopod order to emerge was the Ellesmerocerida, which were quite small organisms; their shells were slightly curved, and the internal chambers were closely spaced. The siphuncle penetrated the septa with meniscus-like holes. Early cephalopods had fine shells which could not cope with the pressures of deep water. In the mid Tremadoc, these were supplemented by larger shells around 20 cm in length; these larger forms included straight and coiled shells, and fall into the orders Endocerida (with wide siphuncles) and Tarphycerida (with narrow siphuncles). By the mid Ordovician these orders are joined by the Orthocerids, whose chambers are small and spherical, and Lituitids, whose siphuncles are thin. The Oncocerids also appear during this time; they are restricted to shallow water and have short conchs which surround the stomach. The mid Ordovician saw the first cephalopods with septa strong enough to cope with the pressures associated with deeper water, and could inhabit depths greater than 100–200 m. The wide-siphuncled Actinocerida and the Discocerida both emerged during the Darriwilian. The direction of coiling would prove to be crucial to the future success of the lineages; endogastric coiling would only permit large size to be attained with a straight shell, whereas exogastric coiling - initially rather rare - permitted the spirals familiar from the fossil record to develop, with their corresponding large size and diversity. (Endogastric means that the tip of the coiling shell points in the same direction as the funnel; exogastric shells coil the other way, allowing the funnel to be pointed backwards beneath the shell.)

Early cephalopods were likely predators near the top of the food chain.



The ancient (cohort Belemnoidea) and modern (cohort Neocoleoidea) coleoids, as well as the ammonoids, all diverged from the external shelled nautiloid during the middle Paleozoic Era, between 450 and 300 million years ago, although the coleoids may be polyphyletic. :289 Unlike most modern cephalopods, most ancient varieties had protective shells. These shells at first were conical but later developed into curved nautiloid shapes seen in modern nautilus species. It is thought that competitive pressure from fish forced the shelled forms into deeper water, which provided an evolutionary pressure towards shell loss and gave rise to the modern coleoids, a change which led to greater metabolic costs associated with the loss of buoyancy, but which allowed them to recolonise shallow waters. :36 However, some of the straight-shelled nautiloids evolved into belemnites, out of which some evolved into squid and cuttlefish. The loss of the shell may also have resulted from evolutionary pressure to increase manoeuvrability, resulting in a more fish-like habit. 289 This pressure may have increased as a result of the increased complexity of fish in the late Palaeozoic, increasing the competitive pressure. :289 Internal shells still exist in many non-shelled living cephalopod groups but most truly shelled cephalopods, such as the ammonites, became extinct at the end of the Cretaceous.

The tentacles of the ancestral cephalopod developed from the mollusc's foot; the ancestral state is thought to have had five pairs of tentacles which surround the mouth. Smell-detecting organs evolved very early in the cephalopod lineage.

The earliest cephalopods, like Nautilus and some coeloids, appeared to be able to propel themselves forwards by directing their jet backwards. :289 Because they had an external shell, they would not have been able to generate their jets by contracting their mantle, so must have used alternate methods: such as by contracting their funnels or moving the head in and out of the chamber. :289

The preservation of cephalopod soft parts is not entirely unusual; soft-bodied fossils, especially of coeloids (squid), are relatively widespread in the Jurassic, but phosphatised remains are unknown before this period.

Classification
The classification as listed here (and on other cephalopod articles) follows largely from Current Classification of Recent Cephalopoda (May 2001), plus fossil groups from several sources. The three subclasses are traditional, corresponding to the three orders of cephalopods recognized by Bather. Parentheses indicate extinct groups.

Class Cephalopoda
 * Subclass Nautiloidea: all cephalopods except ammonoids and coleoids
 * (Order Plectronocerida): the ancestral cephalopods from the Cambrian Period
 * (Order Ellesmerocerida): include the ancestors of all later cephalopods
 * (Order Endocerida)
 * (Order Actinocerida)
 * (Order Discosorida)
 * (Order Pseudorthocerida)
 * (Order Tarphycerida)
 * (Order Oncocerida)
 * Order Nautilida: nautilus and its fossil relatives
 * (Order Orthocerida)
 * (Order Ascocerida)
 * (Order Bactritida): include the ancestors of ammonoids and coleoids
 * (Subclass Ammonoidea): extinct ammonites and kin
 * (Order Goniatitida)
 * (Order Ceratitida)
 * (Order Ammonitida): the true ammonites
 * Subclass Coleoidea
 * (Cohort Belemnoidea): extinct belemnites and kin
 * (Genus Jeletzkya)
 * (Order Aulacocerida)
 * (Order Phragmoteuthida)
 * (Order Hematitida)
 * (Order Belemnitida)
 * Cohort Neocoleoidea
 * Superorder Decapodiformes (also known as Decabrachia or Decembranchiata)
 * (?Order Boletzkyida)
 * Order Spirulida: Ram's Horn Squid
 * Order Sepiida: cuttlefish
 * Order Sepiolida: pygmy, bobtail and bottletail squid
 * Order Teuthida: squid
 * Superorder Octopodiformes (also known as Vampyropoda)
 * Order Vampyromorphida: Vampire Squid
 * Order Octopoda: octopus

Other classifications differ, primarily in how the various decapod orders are related, and whether they should be orders or families.

Shevyrev classification
Shevyrev (2005) suggested a division into eight subclasses, mostly comprising the more diverse and numerous fossil forms.

Class Cephalopoda Cuvier 1795
 * Subclass Ellesmeroceratoidea Flower 1950
 * Order Plectronocerida
 * Order Protactinocerida
 * Order Yanhecerida
 * Order Ellesmerocerida
 * Subclass Endoceratoidea Teichert, 1933
 * Order Endocerida
 * Order Intejocerida
 * Subclass Actinoceratoidea Teichert, 1933
 * Order Actinoceratoidea
 * Subclass Nautiloidea Agassiz, 1847
 * Order Basslerocerida
 * Order Tarphycerida
 * Order Lituitida
 * Order Discosorida
 * Order Oncocerida
 * Order Nautilida
 * Subclass Orthoceratoidea Kuhn, 1940
 * Order Orthocerida
 * Order Ascocerida
 * Order Dissidocerida
 * Order Bajkalocerida
 * Subclass Bactritoidea Shimansky, 1951
 * Subclass Ammonoidea Zittel, 1884
 * Subclass Coleoidea Bather, 1888

Cladistic classification
Another recent system divides all cephalopods into two clades. One includes nautilus and most fossil nautiloids. The other clade (Neocephalopoda or Angusteradulata) is closer to modern coleoids, and includes belemnoids, ammonoids, and many orthocerid families. There are also stem group cephalopods of the traditional Ellesmerocerida that belong to neither clade

Monophyly of coeloids
The coeloids may represent a polyphyletic group. <sup:289