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 . Many other very similar species are included under the genus Michelinoceras. Orthoceras's closest living relative is the Nautilus.
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.
Originally Orthoceras referred to all nautiloids with a straight-shell , called an ("orthocone"). 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 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.
Orthoceras and related orthocone nautiloid cephalopods are often confused with the superficially similar Baculites and related Cretaceous orthocone ammonite cephalopods. Both are long and tubular in form, and both are common items for sale in rock shops (often under each others' names). Both lineages evidently evolved the tubular form independently of one another, and at different times in earth history. Orthoceras lived much longer ago (Middle Ordovician Period) than Baculites (Late Cretaceous Period). The two types of fossils can be distinguished by many features, most obvious among which is the suture line: simple in Orthoceras, intricately foliated in Baculites and related forms.