A cephalopod is any member of the molluscan class Cephalopoda such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles modified from the primitive molluscan foot. Fishers sometimes call cephalopods "inkfish", referring to their common ability to squirt ink. The study of cephalopods is a branch of malacology known as teuthology.
Ammonoids are extinct spiral shelled cephalopods comprising the subclass Ammonoidea. They are more closely related to living coleoids than they are to shelled nautiloids. The earliest ammonoids appeared during the Devonian, with the last species vanishing during or soon after the Cretaceous–Paleogene extinction event. They are often called ammonites, which is most frequently used for members of the order Ammonitida, the only remaining group of ammonoids from the Jurassic up until their extinction.
Nautilus are the ancient pelagic marine mollusc species of the cephalopod family Nautilidae. This is the sole extant family of the superfamily Nautilaceae and the suborder Nautilina.
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 species rich, with over 2,500 recorded species. They flourished during the early Paleozoic era, when they constituted the main predatory animals. Early in their evolution, nautiloids developed an extraordinary diversity of shell shapes, including coiled morphologies and giant straight-shelled forms (orthocones). No orthoconic and only a handful of coiled species, the nautiluses, survive to the present day.
Endocerida is an extinct nautiloid order, a group of cephalopods from the Lower Paleozoic with cone-like deposits in their siphuncle. Endocerida was a diverse group of cephalopods that lived from the Early Ordovician possibly to the Late Silurian. Their shells were variable in form. Some were straight (orthoconic), others curved (cyrtoconic); some were long (longiconic), others short (breviconic). Some long-shelled forms like Endoceras attained shell lengths close to 6 metres (20 ft). The related Cameroceras is anecdotally reported to have reached lengths approaching 9 metres (30 ft), but these claims are problematic. The overwhelming majority of endocerids and nautiloids in general are much smaller, usually less than a meter long when fully grown.
The Bactritida are a small order of more or less straight-shelled (orthoconic) cephalopods that first appeared during the Emsian stage of the Devonian period with questionable origins in the Pragian stage before 409 million years ago, and persisted until the Carnian pluvial event in the upper middle Carnian stage of the Triassic period. They are considered ancestors of the ammonoids, as well as of the coleoids.
Neocephalopods are a group of cephalopod mollusks that include the coleoids and all extinct species that are more closely related to extant coleoids than to the nautilus. In cladistic terms, it is the total group of Coleoidea. In contrast, the palcephalopoda are defined as the sister group to the neocephalopoda.
Cameroceras is an extinct genus of endocerid cephalopod which lived in equatorial oceans during the entire Ordovician period. Like other endocerids, it was an orthocone, meaning that its shell was fairly straight and pointed. It was particularly abundant and widespread in the Late Ordovician, inhabiting the shallow tropical seas in and around Laurentia, Baltica and Siberia.
The Ascocerida are comparatively small, bizarre Orthoceratoids known only from Ordovician and Silurian sediments in Europe and North America, uniquely characterized by a deciduous conch consisting of a longer juvenile portion and an inflated short adult portion that separate sometime in maturity.
Octamerella is an extinct genus of prehistoric nautiloids from the oncocerid family Hemiphragmoceratidae that lived in what is now Europe and North America during the Middle Silurian.
Osbornoceras is a genus of Lower Silurian cyrtoconic nautiloid cephalopods known from Ohio and possibly Manitoba, one of five general currently included in the oncocerid family Karoceratidae.
Streptoceras is a genus in the extinct oncocerid family Acleistoceratidae that plied the shallow sea floor from the Middle Silurian to the Middle Devonian. Streptoceras is characterized by a shell that is large but short in proportion (breviconic) with the ventral profile convex and dorsal profile concave in the posterior part, convex over the body chamber, then concave toward the front, like Amphycertoceras, but with a triangular shaped aperture. Streptoceras is found in the Middle Silurian of North America, in Ontario.
Shuranoceras is a genus in the extinct oncocerid family Karoceratidae that plied the shallow sea floor from the Early Silurian to the Early Devonian. Shuranoceras is characterized by a smooth, compressed, slowly enlarging orthoconic shell with a ventral submarginal siphuncle composed of complex segments. Shuranoceras was found in middle Silurian strata in Ferghana, central Asia.
The Phragmoceratidae is a family of extinct nautiloid cephalopods from the Order Discosorida that lived during the latter part of the Silurian.
Mandaloceratidae is a family in the nautiloid cephalopod order Discosorida, from the Middle and Upper(?) Silurian characterized by short, essentially straight shells referred to as breviconic, typically with a faintly exogastric shape produced by the profile of the body chamber.
The cephalopods have a long geological history, with the first nautiloids found in late Cambrian strata.
Pentameroceras is a straight to slightly exogastric breviconic oncocerid from the middle Silurian of North America and Europe belonging to the Trimeroceratidae.
Ophioceras is a genus of closely coiled tarphycerid nautiloid cephalopods, the sole representatives of the family Ophidioceratidae, characterized by an evolute shell with narrow, subrounded, annulated whorls and a subcentral siphuncle composed of thin connecting rings that show no evidence of layering. The mature body chamber is strongly divergent and is the longest proportionally of any tarphycerid. The aperture has a deep hyponomic sinus and ocular sinuses, and so resembles some lituitids.
Gomphoceras is a questionable nautiloid cephalopod genus assigned to the Oncocerida. The family to which it might belong is undetermined.
Tetrameroceras is a genus of short, essentially straight, breviconic, nautiloid cephalopods from the middle and Upper Silurian of Europe and North America included in the oncocerid family Hemiphragmoceraidae.
