Actinoceras

Taxobox
fossil_range = M Ordovician - L Silurian
name = "Actinoceras"


regnum = Animalia
phylum = Mollusca
classis = Cephalopoda
subclassis = Nautiloidea
ordo = Actinocerida
familia = Actinoceratidae
genus = "Actinoceras "
genus_authority = 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.

hell

"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.

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.

pecies

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.

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.

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 -