Orthoceras

Orthoceras Temporal range: Middle Ordovician (Dapingian to Darriwilian), 470–458 Ma PreꞒ Ꞓ O S D C P T J K Pg N [1]
Artist's reconstruction of O. regulare
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Mollusca
Class:	Cephalopoda
Order:	†Orthocerida
Family:	†Orthoceratidae
Genus:	†Orthoceras Bruguière, 1789
Species:	†O. regulare
Binomial name
†Orthoceras regulare (Schlotheim, 1820)

Orthoceras is a genus of extinct nautiloid cephalopod restricted to Middle Ordovician-aged marine limestones of the Baltic States and Sweden. This genus is sometimes called Orthoceratites. Note it is sometimes misspelled as Orthocera, Orthocerus or Orthoceros.

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 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.^[1]

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.