''Orthoceras'' ("straight horn") is a

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...

of extinct

nautiloid Nautiloids are a group of marine cephalopods (Mollusca) which originated in the Late Cambrian and are represented today by the living ''Nautilus'' and '' Allonautilus''. Fossil nautiloids are diverse and speciose, with over 2,500 recorded specie ...

cephalopod restricted to

Middle Ordovician The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya. Th ...

-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 Wastebasket taxon (also called a wastebin taxon, dustbin taxon or catch-all taxon) is a term used by some taxonomists to refer to a taxon that has the sole purpose of classifying organisms that do not fit anywhere else. They are typically defined ...

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 ''O. regulare'', of Ordovician-aged Baltic Sea limestones of Sweden and neighboring areas. These are slender, elongate shells with the middle of the body chamber

transverse Transverse may refer to: *Transverse engine, an engine in which the crankshaft is oriented side-to-side relative to the wheels of the vehicle *Transverse flute, a flute that is held horizontally * Transverse force (or ''Euler force''), the tangen ...

ly 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

s 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 ''Treatise on Invertebrate Paleontology'' (or ''TIP'') published by the Geological Society of America and the University of Kansas Press, is a definitive multi-authored work of some 50 volumes, written by more than 300 paleontologists, and co ...

, the name ''Orthoceras'' is now only used to refer to the

type species In zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the species that contains the biological type specime ...

''O. regulare'' from the Middle

Ordovician The Ordovician ( ) is a geologic period and system, the second of six periods of the Paleozoic Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start of the Silurian Period Mya. T ...

of Sweden and parts of the former

Soviet Union The Soviet Union,. officially the Union of Soviet Socialist Republics. (USSR),. was a List of former transcontinental countries#Since 1700, transcontinental country that spanned much of Eurasia from 1922 to 1991. A flagship communist state, ...

such as

Russia Russia (, , ), or the Russian Federation, is a transcontinental country spanning Eastern Europe and Northern Asia. It is the largest country in the world, with its internationally recognised territory covering , and encompassing one-eig ...

Ukraine Ukraine ( uk, Україна, Ukraïna, ) is a country in Eastern Europe. It is the second-largest European country after Russia, which it borders to the east and northeast. Ukraine covers approximately . Prior to the ongoing Russian inv ...

Belarus Belarus,, , ; alternatively and formerly known as Byelorussia (from Russian ). officially the Republic of Belarus,; rus, Республика Беларусь, Respublika Belarus. is a landlocked country in Eastern Europe. It is bordered by ...

Estonia Estonia, formally the Republic of Estonia, is a country by the Baltic Sea in Northern Europe. It is bordered to the north by the Gulf of Finland across from Finland, to the west by the sea across from Sweden, to the south by Latvia, a ...

and Lithuania. The genus might include a few related species.

Confusion with ''Baculites''

''Orthoceras'' and related orthoconic

cephalopods are often confused with the superficially similar ''

Baculites ''Baculites'' ("walking stick rock") is an extinct genus of cephalopods with a nearly straight shell, included in the heteromorph ammonites. The genus, which lived worldwide throughout most of the Late Cretaceous, and which briefly survived the ...

'' and related

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...

orthoconic

ammonoids Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid and cuttlefish) ...

. 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

) than ''

'' (Late

). The two types of fossils can be distinguished by many features, most obvious among which is the suture line: simple in ''Orthoceras'' (see image), intricately foliated in ''

'' and related forms.

References

# #

External links

PhotoArticle
english.fossiel.net–"Explanation of Fossil Types". {{Taxonbar, from=Q5605054 Prehistoric cephalopod genera Orthocerida Ordovician cephalopods Paleozoic cephalopods of Europe Fossil taxa described in 1789 Taxa named by Jean Guillaume Bruguière

History of the name

Confusion with ''Baculites''

See also

References

External links