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- 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, Belarus 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.
These orthocone cephalopods are conspicuous in the fossil record for their occasional but persistent appearances in monospecific assemblages dense enough to be rock-forming. These assemblages, referred to colloquially as "orthocone orgies" (cf. "belemnite battlefields"), are known mostly from Ordovician rocks but do occur later as well, at least into the Silurian. Well-known examples occur in Morocco, Scandinavia, the Alps, and Iowa (USA). Based on studies of size distributions of the orthocone shells, some scientists have concluded that these assemblages likely represent post-mating mass deaths, as are common among modern cephalopods (though not modern nautiloids) and indeed among many semelparous species. However, such studies have not been entirely convincing and do leave the door open for alternate interpretations.
One often finds on eBay highly polished and beautiful-looking fossils from Morocco 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. For example, the Baltic island of Öland off the southern coast of Sweden has many quarries that yield orthocone nautiloids of great beauty. For centuries Öland has supplied greater Europe with material for floors, stairs and grave stones. The use of this limestone on the island itself is proof of its popularity. This hard limestone is durable and the fossil inclusions make it very desirable. Occasionally the chambers of the fossil shells are colored differently. The ground water minerals that percolated the strata during diagenesis determines the color. Greens and browns are most common.
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
"Orthoceras"