{{Taxobox , name = Slender Oncoceratidae , fossil_range = M

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- U

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, image = Oocerina plebeia.JPG , image_caption = ''Oonocerina plebeia'', Barrande from Slivenec, Prague, (

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) at the

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, regnum = Animalia , phylum = Mollusca , classis =

Cephalopoda A cephalopod is any member of the molluscan class Cephalopoda (Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, ...

, subclassis =

Nautiloidea 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 speci ...

, ordo =

Oncocerida The Oncocerida comprise a diverse group of generally small nautiloid cephalopods known from the Middle Ordovician to the Mississippian (early Carboniferous; one possible member is known from the Early Permian), in which the connecting rings are ...

, familia = Oncoceratidae , familia_authority = Hyatt, 1884 , subdivision_ranks = Genera , subdivision = See text Slender Oncoceratidae are those in the family

Oncoceratidae Oncoceratidae is a family of nauatiloid cephalopods in the order Oncocerida established by Hyatt, 1884, that range from the Middle Ordovician to the Upper Silurian. Diagnosis Oncoceratidae are characterized by generally compressed, cyrtoconi ...

, (

) which have slender, commonly curved, shells. Some like ''Oocerina'' are gently curved, almost straight, and with only slight expansion. Others like ''Dunleithoceras'' are strongly curved with a more notable rate of expansion. Inclusion in this somewhat arbitrary category is based on illustrations in the

Treatise A treatise is a formal and systematic written discourse on some subject, generally longer and treating it in greater depth than an essay, and more concerned with investigating or exposing the principles of the subject and its conclusions." Treat ...

Part K, 1964.

Included genera

''Loganoceras'' and ''Romingoceras'' 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. ...

of North America have strongly curved shells. ''Loganoceras'', named by Foerste in 1921, (K284) has a circular cross section and an empty

siphuncle The siphuncle is a strand of tissue passing longitudinally through the shell of a cephalopod mollusk. Only cephalopods with chambered shells have siphuncles, such as the extinct ammonites and belemnites, and the living nautiluses, cuttlefish, and ...

ventral of the center. The

genotype The genotype of an organism is its complete set of genetic material. Genotype can also be used to refer to the alleles or variants an individual carries in a particular gene or genetic location. The number of alleles an individual can have in a ...

, ''L. regulare'', comes from

Ontario Ontario ( ; ) is one of the thirteen provinces and territories of Canada.Ontario is located in the geographic eastern half of Canada, but it has historically and politically been considered to be part of Central Canada. Located in Central C ...

. ''Romingoceras'', also named by Foerste in 1932, differs in having a depressed, ovoid, cross section. ''Romingoceras'' expands slowly and like ''Loganoceras'' is strongly curved, may even be gyroconic (openly coiled). The ventral siphuncle is small, with oblong segments. The

, ''R. josephianum'' also comes from Ontario. Also from the Middle Ordovician of North America is the long, slightly curved ''Ehlersoceras'' (K284), named by Foerste, 1932. ''Ehlersoceras'' has a small subventral siphuncle, depressed cross section and almost no expansion. As with ''Longanoceras'' and ''Romingoceras'', the genotype of ''Ehlersoceras'', ''E. huronense'' comes from Ontario. ''Ehlersoceras'' differs from ''Longanoceras'' and ''Romingoceras'' in having a much gentler exogastric curvature. ''Richardsonoceras'' (K288), named also by Foerste in 1932, and ''Dunleithoceras'' (K284), named by him in 1924, come from both the Middle and Upper Ordovician of North America. Both are strongly curved with a notable rate of expansion; ventral sides, as with the previous genera, are on the outside, convex curvature, making them, again, exogastric. ''Dunleithoceras'' has a subcircular cross sections with a rounded longitudinal ridge on the venter. ''Richardsonoceras'', has a sharply rounded, keel-like, venter. ''Richardsonoceras'' is less rapidly expanding and not as strongly curved as ''Dunleithoceras''. The genotype of ''Dunleithoceras'', ''D. dunleithense'', comes 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 Illinois, that of ''Richardsonoceras'', ''R. simplex'', is from the Middle Ordovician of Ontario. ''

Oonoceras ''Oonoceras'' is an extinct genus of fossil cephalopods included in the nautiloid order Oncocerida and the family Oncoceratidae from the Middle Ordovician to Middle Silurian of North America and Europe, arbitrarily included in the Slender Oncoc ...

'' (K288), from the Middle Ordovician to Middle

Silurian The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago (Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleozoi ...

of Europe and North America named by Hyatt in 1884, has a slender compressed exogastrically curved shell with a gradual expansion. The genotype, ''O. acinaces'', is from Middle Silurian of central Europe. The sutures in ''

'' form lateral lobes. The camerae (chambers) and body chamber are short. The siphuncle, which is close to the venter, is cyrtochoanitic, empty, with expanded segments. '' Oocerina'' (K288), named by Foerste, 1926, is a slender, exogastric genus, like Oonoceras, from the

Upper Silurian The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the shortest period of the Paleozo ...

of Europe, Russia and possibly North America. Oocerina differs primarily in having an actinosiphonate siphuncle composed of numuloidal (beaded) segments. ''Paroocerina'' (K288), named by Zhurevleva in 1961, is a slender oncoceratid from the Middle and Upper Silurian of eastern Europe and ex-USSR (Russia), similar to ''Oocerina'', but with a hyponomic sinus. The genotypes of ''Oocerina'' and ''Paroocerina'', respectively ''O.lentigratum'' and ''P. podolskensis'', are from the Upper Silurian of central Europe and Russia

Relationships

A close relationship can be inferred between ''Loganoceras'' and ''Romingoceras'' and between ''Dunleithoceras'' and ''Richardsonoceras'' from the Middle and Upper Ordovician of North America as they are respectively from the same area from the same time. ''Ehlersoceras'' seems to stand more alone. A close relationship may also be inferred between the primarily Silurian genera from Europe, including

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; ''Oonoceras'', ''Oocerina'' and ''Paroocerina'', which are very similar in overall form. The slender type oncoceratids give way to the strongly compressed '' Digenuoceras'' with sharply acute dorsum and venter, and to the more conical '' Miamiceras'' and '' Rizoceras''.

''References''

* Walter C.Sweet, Nautiloidea-Oncocerida, in the Treatise on Invertebrate Paleontology, Part K, Teichert C and Moore R.C Eds, Geological Society of America and University of Kansas press, 1964 (K283-K290) Nautiloids Middle Ordovician first appearances Silurian extinctions