Orthoceras

For the orchid genus, see Orthoceras (orchid).

Orthoceras ("straight horn") is a genus of extinct cephalopod. The name (and the similar Orthoceratites, along with misspellings like Orthocera and Orthocerus (Sweet, 1964, p.K222)) was originally used to refer to all straight-shelled nautiloids (Fenton and Fenton 1958 p. 40) ("orthocones"). Research on internal structures (siphuncle, cameral deposits, etc) showed that these actually belonged to a number of groups, even different orders, and in the Treatise on Invertebrate Paleontology, the name Orthoceras is used only to refer to the type species O. regulare (Schlotheim 1820) from the Middle Ordovician of Estonia, Lithuania, "USSR" (perhaps what is now Ukraine, Belorus etc) and Sweden, and perhaps a few related species. These are slender, elongate shells with the middle of the body chamber transversely constricted, and a subcentral orthochoanitic siphuncle. The surface is ornamented by a network of fine lirae (Sweet 1964 p.K224). Many other very similar species are now included under the genus Michelinoceras.

One often finds on eBay highly polished and beautiful looking fossils from Morrocco that are called Orthoceras, almost all of which have been touched up in some way. While these specimens (or rather the original, untouched versions) are indeed members of the order Orthocerida, none can be said to belong to the genus Orthoceras.

References

• Fenton, C. L., and Fenton M. A. (1958) The Fossil Book (Doubleday & Co., Garden City, New York)

• Sweet, Walter C., (1964), Nautiloidea — Orthocerida, in Treatise on Invertebrate Paleontology. Part K. Mollusca 3. (Geological Society of America, and University of Kansas Press, New York, New York and Lawrence, Kansas)

See also

• Baculites – another type of extinct, straight-shelled cephalopod.