Slender Oncoceratidae
Encyclopedia
Genera within the Oncoceratidae
, a family within the nautiloid order Oncocerida
are found in a few basic and distinct forms. among which are those that are slender in proportion. 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 category is somewhat arbitrary, based on illustrations in the Treatise (K285-K288, figs 200-203)
of North America. Loganoceras, named by Foerste in 1921,(K284) has a circular cross section and an empty, probably cyrtochoanitic, siphuncle that is ventral of the center. The genotype, L. regulare, comes from Ontario
. Romingoceras, also named by Foerste in 1932, differs in having a depressed, ovoid, cross section. Romingoceras expands slowely and like Loganoceras is strongly curved, may even be gyroconic. The ventral siphuncle is small, with oblong segments. The genotype, 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. The venters, as with the previous genera, are on the outside, convex curvature, making them, again, exogastric. Dunleithoceras has a subcicular 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 of Illinois, that of Richardsonoceras, R. simplex, is from the Middle Ordovician of Ontario.
Oonoceras (K288), named by Hyatt, 1884, is a slender compressed exogastric cyrtocone with a gradual expansion from the Middle Ordovician to Middle Silurian of Europe and North America. The genotype, O. acinaces, is from the central European Middle Silurian. The sutures in Oonoceras form lateral lobes. The camerae (closed chambers) and body chamber are short. The siphuncle, which is close to the venter, is cyrtochoaniic, empty, and of expanded segments. Oocerina (K288) , named by Foerste, 1926, is a slender, exogastric genus, like Oonoceras, from the Upper Silurian of Europe and ex-USSR 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
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.
Oncoceratidae
Oncoceratidae, a family in the Oncocerida established by Hyatt, 1884, which is characterized by generally compressed, cyrtoconic and breviconic shells with an exogastric curvature such that the ventral profile is convex or more so than dorsal, and in which the siphucle is generally empty and...
, a family within the nautiloid order Oncocerida
Oncocerida
The Oncocerida comprise a diverse group of generally small nautiloid cephalopods known from the Middle Ordovician to the Mississippian ,in which the connecting rings are thin and siphuncle segments are variably expanded...
are found in a few basic and distinct forms. among which are those that are slender in proportion. 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 category is somewhat arbitrary, based on illustrations in the Treatise (K285-K288, figs 200-203)
Included Genera
Loganoceras and Romingoceras are strongly curved exogastric curtocones from the Middle OrdovicianOrdovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...
of North America. Loganoceras, named by Foerste in 1921,(K284) has a circular cross section and an empty, probably cyrtochoanitic, siphuncle that is ventral of the center. The genotype, L. regulare, comes from Ontario
Ontario
Ontario is a province of Canada, located in east-central Canada. It is Canada's most populous province and second largest in total area. It is home to the nation's most populous city, Toronto, and the nation's capital, Ottawa....
. Romingoceras, also named by Foerste in 1932, differs in having a depressed, ovoid, cross section. Romingoceras expands slowely and like Loganoceras is strongly curved, may even be gyroconic. The ventral siphuncle is small, with oblong segments. The genotype, 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. The venters, as with the previous genera, are on the outside, convex curvature, making them, again, exogastric. Dunleithoceras has a subcicular 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 of Illinois, that of Richardsonoceras, R. simplex, is from the Middle Ordovician of Ontario.
Oonoceras (K288), named by Hyatt, 1884, is a slender compressed exogastric cyrtocone with a gradual expansion from the Middle Ordovician to Middle Silurian of Europe and North America. The genotype, O. acinaces, is from the central European Middle Silurian. The sutures in Oonoceras form lateral lobes. The camerae (closed chambers) and body chamber are short. The siphuncle, which is close to the venter, is cyrtochoaniic, empty, and of expanded segments. Oocerina (K288) , named by Foerste, 1926, is a slender, exogastric genus, like Oonoceras, from the Upper Silurian of Europe and ex-USSR 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 Russia; 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)