''Actinoceras'' is the principal and root genus of the

Actinoceratidae The Actinoceriatidae are a family of actinocerids named by Saemann in 1853 for those that grew to have large shells with blunt apices and large siphuncles with widely expanded segments and a generally arcuate endosiphucular canal system.Teicher ...

, a major family in the

Actinocerida The Actinocerida are an order of generally straight, medium to large cephalopods that lived during the early and middle Paleozoic, distinguished by a siphuncle composed of expanded segments that extend into the adjacent chambers, in which deposit ...

, that lived during the Middle and Late Ordovician. It is an extinct genus of nautiloid cephalopod that thrived in the warm waters of the

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territorie ...

during the Paleozoic era.

Morphology

''Actinoceras'' are generally large, with typically straight shells reaching a meter or so in length (about 3 ft), with a blunt apex, and usually with a circular to subcircular cross section. .

Shell characteristics

The shells of ''Actinoceras'' are generally straight and long, although some are breviconic. Some are fusiform with the diameter decreasing from the anterior end of the phragmocone toward the aperture. Chambers are short and contain cameral deposits which are more concentrated apically and ventrally. Septa are close spaced, sutures are mostly transverse. The siphuncle, which varies in proportion to the size of the shell among species, is ventral, but not on the ventral margin. (Flower 1957)

Siphucle

The siphuncle, which is ventral of the center but away from the ventral margin, is generally large and composed of segments that are expanded into the chambers, more so than in ''

Ormoceras ''Ormoceras'' is an actinocerid nautiloid genus and type for the family Ormoceratidae, found in North America from the late Chazyan through the early Cincinnatian of the Middle and Upper Ordovician, but which continued through the Devonian worl ...

'' or ''

Lambeoceras ''Lambeoceras'' is a genus of large actinocerids with a convexly lenticular cross section from the Upper Ordovician of North America and the sole representative of the family Lambeoceratidae. Morphological description ''Lambeoceras'' is of mediu ...

'' but not as much as in ''

Armenoceras ''Armenoceras'' is a genus of actinocerid nautiloid cephalopods whose fossils ranged from the late Whiterockian Stage in the early Middle Ordovician, through the remainder of the period and on into the Upper Silurian. It is the type genus of th ...

''. (Flower 1957) The diameter typically becomes smaller with respect to that of the shell in the forward or anterior part of the phragmocone. Septal necks are long with wide cyrtochoanitic to recumbent brims. Connecting rings are thin.(Teichert 1964)

Canal system

The canal system within the siphuncle in ''Arctinocers'' is of the single arc type wherein the radial canals branch off the central canal near the septal openings and sweep back and out, connecting to the parispatium in the preceding segments at their broadest expansion. This type is also found in the derivatives (descendants) of ''Actinoceras'', e.g. ''

'', in some ''

'', ''

Nybyoceras ''Nybyoceras'' is an actinocerid genus assigned to the Armenoceratidae and similar to ''Armenoceras'' except for having a siphuncle close to the ventral side of the shell. Morphology As with ''Armenoceras'', ''Nybyoceras'' has a medium to large ...

'', and in ''

Gonioceras ''Gonioceras'' is an extinct genus of actinocerid nautiloidean cephalopods typified by a broad, low shell; flattened ventrally, convexly rounded dorsally; top and bottom meeting at an acute angle along the sides. In most the shell is rather thin ...

''. (Flower 1957). The parispatium is a narrow opening or seam that forms between the inside of the connecting rings and the endosiphunclar deposits that grow forward and back from the region of the septal foremina.

Species

About 45 species have been described from North America, including

Greenland Greenland ( kl, Kalaallit Nunaat, ; da, Grønland, ) is an island country in North America that is part of the Kingdom of Denmark. It is located between the Arctic and Atlantic oceans, east of the Canadian Arctic Archipelago. Greenland is t ...

and the Canadian Arctic with ''Actinoceras margaretae'', ''A. aequale'', and ''A. gradatum'' the earliest known, coming from the lower Blackriveran Loweville fm of Ottawa. ''Actinoceras concavum'' from the Ssuyan of southern

Manchuria Manchuria is an exonym (derived from the endo demonym " Manchu") for a historical and geographic region in Northeast Asia encompassing the entirety of present-day Northeast China (Inner Manchuria) and parts of the Russian Far East (Outer Manc ...

is most similar to ''Actinoceras centrale'' from the Chaumont of New York.

Phylogeny

''Actinoceras'' is one of seven known genera in the Family Actinoceratidae.(Teichert 1964) ''Actinoceras'' is thought to have given rise to such genera as ''Kochoceras'' and ''Paractinoceras'' in the Actinoceratidae and to ''Lambeoceras'' of the

Lambeoceratidae ''Lambeoceras'' is a genus of large actinocerids with a convexly lenticular cross section from the Upper Ordovician of North America and the sole representative of the family Lambeoceratidae. Morphological description ''Lambeoceras'' is of mediu ...

. The derivation of ''Actinoceras'' may be in an early ''Armenoceras'' or perhaps from an early ''Nybyoceras'', also part of the

Armenoceratidae The Armenoceratidae are a family of early Paleozoic nautiloid cephalopods belonging to the order Actinocerida.Flower 1957.Studies of the Actinoceratida.; Memoir 2; New Mexico Bureau of Mines and Mineral Resources, Socorro, NM Teichert 1964. Act ...

References

*Flower, R,H, 1957, Studies of the Actinoceratida, New Mexico Bureau of Mines and Mineral Resources, Memoir 2. *Teichert, C, 1964, Actinoceratoidea, in the Treatise on Invertebrate Paleontology, pub Univ of Kansas and the GSA, Vol K, p K190 - {{Taxonbar, from=Q4676916 Actinocerida Prehistoric nautiloid genera Ordovician cephalopods of North America Silurian cephalopods of North America Middle Ordovician first appearances Silurian extinctions Paleozoic life of Ontario Verulam Formation Paleozoic life of Manitoba Paleozoic life of Newfoundland and Labrador Paleozoic life of the Northwest Territories Paleozoic life of Nunavut Paleozoic life of Quebec