Cleviceras
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Cleviceras Temporal range: Early Jurassic, Toarcian |
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| Cleviceras exaratum | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Mollusca |
| Class: | Cephalopoda |
| Subclass: | Ammonoidea |
| Order: | Ammonitida |
| Suborder: | Ammonitina |
| Superfamily: | Hildoceratoidea |
| Family: | Hildoceratidae |
| Subfamily: | Harpoceratinae |
| Genus: |
Cleviceras Howarth 1992 |
| Type species | |
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Ammonites exaratus Young & Bird, 1828 |
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| Species | |
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Cleviceras is an extinct genus of cephalopod belonging to the family Hildoceratidae. These cephalopods existed in the Jurassic period, during Toarcian and possibly even uppermost Pliensbachian age. Sometimes, it is considered to be a synonym of Eleganticeras.
Genus is named after Cleveland, and area in north England. Fossils of Cleviceras exaratum, type species of this genus are very abundant there.
Fossils of species contained in this genus have been found in the lower to middle Toarcian of Europe, Canada, Japan, Tibet, Siberia, Africa, or South America.
First two described members of this genus (C. exaratum and C. elegans) were described as members of today invalid genus Ammonites. Later, they were reassigned into genus Harpoceras, into which was added third member of genus Cleviceras (C. chrysanthemum), formerly member of Hildoceras. In 1992, due to recognized different evolutionary history of Harpoceras and C. exaratum with C. elegans, genus Cleviceras has been erected, but with only two members, as C. chrysanthemum has been transferred into this genus only in 1994.
Sometimes, this genus is not considered as valid. Reasoning for this is, that when Howarth described it, he considered only specimens from Great Britain. According to him, reason for not adding species of this genus into Eleganticeras are much stronger ribs. But outside of Great Britain, there are some specimens of E. elegantulum, which also have strong ribs and on the other hand, speciemens of C. exaratum are thicker, with bigger ventral area and are also more rounded than E. elegantulum. This opinion is not general and Cleviceras is often considered to be a valid genus.
Shells of Cleviceras show dimorphism in their size. While microconchs reach 16-62 mm in diameter, macroconchs shells width is 85-200 mm. They are moderately involute, compressed and have flat whorl sides. Umbilical walls can be sloping, vertical, or undercut. Keel is strong and floored, but in abnormal specimens might be missing. This kind of abnormality has been described as invalid genus Monestieria. Under this name were added not only specimens of Cleviceras, but also members of genera Protogrammoceras and Harpoceras. Ribs are falcoid and in small sizes of shell are bifurcating, later they are single. On the outer part of whorl, they are broad and flat. In adults, ribs become striate. There are no tubercules.
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Wikipedia