Slender Oncoceratidae
| Slender Oncoceratidae Temporal range: M Ord- U Sil |
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| Oonocerina plebeia, Barrande from Slivenec, Prague, (Czech Republic) at the National Museum (Prague) | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Mollusca |
| Class: | Cephalopoda |
| Subclass: | Nautiloidea |
| Order: | Oncocerida |
| Family: |
Oncoceratidae Hyatt, 1884 |
| Genera | |
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See text |
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See text
Slender Oncoceratidae are those in the family Oncoceratidae, (Nautiloidea, Oncocerida) 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 Part K, 1964.
Loganoceras and Romingoceras from the Middle Ordovician of North America have strongly curved shells. Loganoceras, named by Foerste in 1921, (K284) has a circular cross section and an empty siphuncle 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 (openly coiled). 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; 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 of Illinois, that of Richardsonoceras, R. simplex, is from the Middle Ordovician of Ontario.
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