| Arctoceras Temporal range: | |
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| Fossil of Arctoceras at the New Mexico Museum of Natural History and Science | |
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| Genus: | Arctoceras Hyatt, 1900 |
Arctoceras is a genus of ceratitid ammonoids from the Lower Triassic with a moderately narrow discoidal shell and ceratitic suture. [1] [2]
The shell of Arctoceras is generally involute with compressed whorls that are much higher than wide, flattened sides, narrowly rounded ventral rim, and sutures with wide low saddles and simple serrated lobes. [1]
Arctoceras was named by Alpheus Hyatt in 1900. [1] The genotype Arctoceras polaris (Mojsisovics, 1865) was described from the Smithian (Early Triassic) of Spitsbergen (Svalbard). Up to seven species were described from the Vikinghøgda Formation of Spitsbergen, though a restudy concluded that only one species, Arctoceras blomstrandi, is valid, with A. blomstrandi var. costatus representing a morphologically divergent end member of the lineage. [2]
Arctoceras is the type genus of the Arctoceratidae which is a family in the Meekocerataceae, a superfamily within the order Ceratitida. [3] Ceratitids are ammonoid cephalopods, mostly from the Triassic but with ancestral forms in the upper Permian. Meekocerataceae sometimes appears as Meekoceratoidea to conform with a recent ruling of the ICZN regarding superfamily endings.
Apart from Svalbard, Arctoceras has also been reported from Canada, the United States, Russia, Australia, Afghanistan, and Malaysia. [3]
Ammonoids are extinct spiral shelled cephalopods comprising the subclass Ammonoidea. They are more closely related to living coleoids than they are to shelled nautiloids. The earliest ammonoids appeared during the Devonian, with the last species vanishing during or soon after the Cretaceous–Paleogene extinction event. They are often called ammonites, which is most frequently used for members of the order Ammonitida, the only living group of ammonoids from the Jurassic up until their extinction.
In the geologic timescale, the Olenekian is an age in the Early Triassic epoch; in chronostratigraphy, it is a stage in the Lower Triassic series. It spans the time between 251.2 Ma and 247.2 Ma. The Olenekian is sometimes divided into the Smithian and the Spathian subages or substages. The Olenekian follows the Induan and is followed by the Anisian.
Xenoceltites is an extinct genus of ceratite ammonoid found worldwide in the Lower Triassic.
Anasirenites is a genus of extinct ammonoid cephalopods within the family Trachyceratidae which is part of the ceratitid superfamily Clydonitaceae; sometimes considered a subgenus of Sirenites. Anasirenites is characterized by a ventral furrow bordered on either side by a continuous keel. The shell, as for Sirenites, is compressed with whorls flattened-convex on the sides. Sigmoidal ribs on the flanks bifurcate near the ventro-lateral shoulder on tubercles and project sharply adorally. Whorl sides are tuberculate with tubercles arranged in spiral lines. The suture is ammonitic. Anasirenites, first described by Mojsisovics in 1893 is known from the Upper Triassic of the Alps, Sicily, and the Himalaya.
Arcestes is a genus of extinct ceratitid ammonites found in Triassic-aged marine strata.
Ussuria is a genus of Lower Triassic ammonites with a smooth, involute discoidal shell with submonophyllic sutures, belonging to the ceratitid family Ussuriidae.
Ceratitida is an order that contains almost all ammonoid cephalopod genera from the Triassic as well as ancestral forms from the Upper Permian, the exception being the phylloceratids which gave rise to the great diversity of post-Triassic ammonites.
Medlicottiidae is a family of ammonoid cephalopods belonging to the Prolecanitida, known from the Upper Carboniferous (Pennsylvanian) to the Early Triassic.
Sassenia is an extinct genus of prehistoric coelacanth lobe-finned fish that lived during the Early Triassic epoch in what is now East Greenland and Svalbard.
Prolecanitida is an order of extinct ammonoid cephalopods, the major Late Paleozoic group of ammonoids alongside the order Goniatitida. Prolecanitids had narrow shells, discoidal (disc-shaped) to thinly lenticular (lens-shaped). They retained a retrochoanitic siphuncle, a simple form with septal necks extending backwards. As is typical for ammonoids, the siphuncle sits along the ventral margin of the shell.
Prolecanitoidea is a taxonomic superfamily of ammonoids in the order Prolecanitida. Prolecanitoidea is one of two superfamilies in the order, along with the younger and more complex Medlicottioidea. The Prolecanitoidea were a low-diversity and morphologically conservative group. They lived from the Lower Carboniferous up to the Middle Permian. Their shells are generally smooth and discoidal, with a rounded lower edge, a moderate to large umbilicus, and goniatitic to ceratitic sutures. Suture complexity varies from 10 up to 22 total lobes ; new lobes are added from subdivision of saddles adjacent to the original main umbilical lobe.
Pleuronautilus is a nautiloid genus; family Tainoceratidae, order Nautilida.
Gymnites is a genus of ammonoid cephalopod from the Middle Triassic belonging to the ceratitid family Gymnitidae. These nektonic carnivores lived during the Triassic period, the Anisian age.
Doulingoceras is a genus of ammonoid within the ceratitid order, found in China, that lived during the Late Permian during the time span from about 260.5 to 254 million years ago. The genus is included in the family Paraceltitidae, which belongs to the superfamily Xenodiscaceae.
Inyoites is an ammonoid genus from the Lower Triassic, included in the ceratitid family Inyoitidae.
Flemingites is an extinct genus of evolute ammonoid from the Smithian with spiral ridges on the shell.
Ophiceras is an extinct genus of smooth, evolute ceratitid ammonites from the Griesbachian, with a rounded venter. Fossils of the genus have been found in Armenia, Azerbaijan, China, Greenland, and India.
Meekoceras is an extinct genus of ceratitid ammonites with a discoidal shell that lived during the Early Triassic Epoch.
The Vikinghøgda Formation is a geologic formation in Svalbard, Norway. It preserves fossils dating back to the Early Triassic (Griesbachian-Spathian) period. It is split into three members, from oldest to youngest: the Deltadalen Member (Induan), Lusitaniadalen Member (Smithian), and Vendomdalen Member (Spathian). The formation can be found in central Spitsbergen, southern Spitsbergen, as well as the smaller islands of Barentsøya and Edgeøya. The type locality is positioned in the vicinity of Vikinghøgda and Sticky Keep, two low peaks along the southeast edge of Sassendalen in Spitsbergen. The two upper members of the Vikinghøgda Formation were previously grouped together as the Sticky Keep Formation.
Choristoceratoidea, formerly Choristocerataceae, is a superfamily of Late Triassic ceratitid ammonites. They can be characterized by their simple, four-lobed suture lines and unusual heteromorph shell shapes, which can resemble straight rods or twisting cones rather than the flat coils typical of other ammonites. The shells are usually small and ornamented with prominent ribbing.
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