Orthoceras

Orthoceras was formerly thought to have had a worldwide distribution due to the genus' use as a wastebasket taxon for numerous species of conical-shelled (orthocone) nautiloids throughout the Paleozoic and Triassic. Since this work was carried out and re-cataloging of the genus, Orthoceras sensu stricto refers to Orthoceras regulare, of Ordovician-aged Baltic Sea limestones of Sweden and neighboring areas.

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. Many other very similar species are included under the genus Michelinoceras.

History of the name

Originally Orthoceras referred to all nautiloids with a straight-shell, called an "orthocone". But later research on their internal structures, such as the siphuncle, cameral deposits, and others, showed that these actually belong to a number of groups, even different orders.

According to the authoritative Treatise on Invertebrate Paleontology, the name Orthoceras is now only used to refer to the type species O. regulare from the Middle Ordovician of Sweden, Russia, Ukraine, Belarus, Estonia and Lithuania. The genus might include a few related species.