Actinoceras
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Actinoceras Temporal range: M Ordovician - L Silurian |
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| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Mollusca |
| Class: | Cephalopoda |
| Subclass: | Nautiloidea |
| Order: | Actinocerida |
| Family: | Actinoceratidae |
| Genus: |
Actinoceras Bronn, 1885 |
Actinoceras is the principal and root genus of the Actinoceratidae, a major family in the Actinocerida, that lived during the Middle and Late Ordovician.
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. .
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)
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 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 singe 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. 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 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.
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