13-696: Actinoceras is the principal and root genus of the Actinoceratidae , a major family in the Actinocerida , 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 and England during the Paleozoic era . Actinoceras are generally large, with typically straight shells reaching a meter or so in length (about 3 ft), with
26-474: A blunt apex, and usually with a circular to subcircular cross section. 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
39-563: A generally arcuate endosiphucular canal system. Their range is from the upper Middle Ordovician to the Lower Silurian . Actinocerids are generally straight-shelled nautiloid cephalopods with a siphuncle composed of expanded segments, typically with thin connecting rings, in which the internal deposits are penetrated by a system of canals Actinoceratids are derived from Wutinoceras , possibly through an early Armenoceras or through Nybyoceras and give rise to Lambeoceras and to
52-498: Is also found in the derivatives (descendants) of Actinoceras , e.g. Lambeoceras , in some Armenoceras , Nybyoceras , and in Gonioceras . (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. About 45 species have been described from North America, including Greenland and
65-427: Is proportionally much smaller and is removed about one diameter from the ventral wall. The diameter of the siphuncle in the apical part of Kochoceras is 3/4 or more that of the shell. The siphuncle of Kochoceras is similar to that of Actinoceras , the main difference between the two being that Kochoceras is breviconic (short) and has a narrow stratigraphic range, while Actinoceras is longiconic, lengthy, and has
78-581: Is relatively short, breviconic, and grew to be fairly large with a shell more rapidly expanding than in Actinoceras . The ventral, or under, side is strongly flattened with prominent lobes that may give the impression of Lambeoceras . However septa in Kochoceras are more widely spaced and the siphuncle , which is proportionally very large, is in broad contact with the ventral wall. The siphucle in Lambeoceras
91-517: Is strongly flattened on the ventral side causing possible confusion with Lambeoceras to casual observation. Floweroceras is doubtfully distinct from Kochoceras . Leurothoceras is synonymous with Actinoceras ; Saffordoceras and Troostoceras are closely related. Kochoceras Kochoceras is an extinct nautiloid genus from the later Ordovician belonging to the family Actinoceratidae and found in North America. Kochoceras
104-697: Is thought to have given rise to such genera as Kochoceras and Paractinoceras in the Actinoceratidae and to Lambeoceras of the Lambeoceratidae . The derivation of Actinoceras may be in an early Armenoceras or perhaps from an early Nybyoceras , also part of the Armenoceratidae . Actinoceratidae See text 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
117-627: 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 is most similar to Actinoceras centrale from the Chaumont of New York. Actinoceras is one of seven known genera in the Family Actinoceratidae.(Teichert 1964) Actinoceras
130-659: The Huroniidae. Seven genera are included in the Actinoceratidae, Actinoceras , Floweroceras , Kochoceras , Leurorthoceras , Paractinoceras , Saffordoceras , and Troostoceras . Actinoceras is the earliest but with a range that carries it into the Lower Silurian. Troostoceras followed by Saffordoceras are later Middle Ordovician genera related to early Actinoceras . Kochoceras followed by Floweroceras are Upper Ordovian genera related to later Actinoceras . Leurothoceras and Paractinoceras , both from
143-680: The Upper Ordovician, are shown to have their source in Middle Ordovician actinoceratids. Paractinoceras is probably the most distinct actinoceratid from Actinoceras . Paractinoceras has a long, straight, slender shell and a siphuncle that starts off like that in Actinoceras , but becomes narrow like that in Ormoceras in the anterior part of the phragmocone. Kochoceras has a large blunt shell that expands more rapidly than Actinoceras and
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#1732894161777156-505: 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) 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
169-473: The size of the shell among species, is ventral, but not on the ventral margin. (Flower 1957) 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 or Lambeoceras but not as much as in Armenoceras . (Flower 1957) The diameter typically becomes smaller with respect to that of
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