Orthoceras ("straight horn") is a genus of extinct nautiloid cephalopod. This genus is sometimes called Orthoceratites. Note it is sometimes misspelled as Orthocera, Orthocerus or Orthoceros (Sweet 1964:K222). Orthoceras fossils are common and have a global distribution, occurring in any marine rock, especially limestones. 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). 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. 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. Orthoceras grew to a length of 15 centimeters (6 inches) and fed upon small animals. They maybe appeared in the Early to Late Triassic Period.