Orthoceras

For other uses, see Orthoceras (disambiguation).

Orthoceras
Temporal range: 488 Ma
↓
Scientific classification
Kingdom: Animalia
Phylum: Mollusca
Class: Cephalopoda
Order: †Orthocerida
Family: †Orthoceratidae
Genus: †Orthoceras
Bruguière, 1789
Species

Orthoceras ("straight horn") is a genus of extinct nautiloid cephalopod restricted to Ordovician-aged marine limestones of the Baltic States and Sweden.[1] This genus is sometimes called Orthoceratites. Note it is sometimes misspelled as Orthocera, Orthocerus or Orthoceros (Sweet 1964:K222).

Orthoceras was once 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. Now, Orthoceras sensu stricto refers to O. 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.

Artist's reconstruction of O. regulare

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 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 (Schlotheim 1820) from the Middle Ordovician of Estonia, Lithuania, Sweden and parts of the former Soviet Union such as Ukraine and Belarus.[1] The genus might include a few related species.

Confusion with Baculites

Orthoceras and related orthoconic nautiloid cephalopods are often confused with the superficially similar Baculites and related Cretaceous orthoconic ammonoids. Both are long and tubular in form, and both are common items for sale in rock shops (often under each other's names). Both lineages evidently evolved the tubular form independently of one another, and at different times in earth history. Orthoceras lived much earlier (Middle Ordovician) than Baculites (Late Cretaceous). 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.

See also

References

  1. 1 2 3 Paleontological Institute. "Part K, Mollusca 3". Treatise on Invertebrate Paleontology. Retrieved 5 December 2013.
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