Actinoceras
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| Actinoceras Fossil range: M Ordovician - L Silurian |
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Actinoceras is the principal and root genus of the Actinoceratidae, a major family in the Actinocerida, that lived during the Middle and Late Ordovician.
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[edit] Shell
Actinoceras are generally large with generally straight shells reaching a meter or so in length (about 3 ft), with a cross section usually circular to subcircular. Shells may be fusiform with the diameter decreasing from the anterior end of the phragmocone toward the aperture. Chambers are short, septa are close spaced and mostly transverse.
[edit] The Shell's Internal Anatomy
The siphuncle which is ventral of the center but away from the ventral margin, is composed of segments expanded into the chambers, more so than in Ormoceras or Lambeoceras but not as much as in Armenoceras. The outline of the siphuncle and the diameter smaller with respect to that of the shell in the forward or anterior part of the phragmocone. Septal necks are cyrtochoanitic to recumbent, connecting rings thin. The endosiphuncular canal system 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.
[edit] Species
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.
[edit] Phylogeny
Actinoceras is one of about five genera of the Family Actinoceratidae. Actinoceras is thought to have given rise to such genera as Kochoceras and Selkirkoceras in the Actinoceratidae and to Lambeoceras of the Lambeoceratidae. The derivation of Actinoceras may be in an early Armenoceras, perhaps through an early Nybyoceras also in the Armenoceratidae.
[edit] 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 -
