Orthoceras ("straight horn") is a genus of extinct cephalopod. This genus is sometimes called Orthoceratites. Note it is sometimes misspelled as Orthocera or Orthocerus (Sweet 1964:K222). Fossils are common and have been found on many continents, including the Americas, Africa, Europe, and Asia. They are dated to the lower Ordovician to Triassic ages (500 to 190 million years ago). 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:K224). Many other very similar species are 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.
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. These assemblages, are known mostly from Ordovician rocks but do occur later as well, at least into the Devonian. Well-known examples occur in Morocco, Scandinavia, the Alps, and Iowa (USA).
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.
The Baltic island of Íland off the southern coast of Sweden has many quarries that yield orthocone nautiloids of great beauty. For centuries Íland has supplied greater Europe with material for floors, stairs and grave stones. 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.
History of the Name
Originally Orthoceras referred to all nautiloids with a straight-shell , called an ("orthocone") (Fenton & Fenton 1958:40). But later research on their internal structures, such siphuncle, cameral deposits and others, showed that these actually belong to a number of groups, even different orders.
In the authoritative Treatise on Invertebrate Paleontology, the name Orthoceras is now only used to refer to the type species O. regulare (Schlotheim 1820) from the Middle Ordovician of Estonia, Lithuania, Sweden and parts of the former Soviet Union such as Ukraine and Belarus. The genus might include a few related species.
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