| Arcestes Temporal range: | |
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| A. intuslabiatus and A. binacostomus | |
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| Genus: | Arcestes Suess, 1865 |
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Arcestes is a genus of extinct ceratitid ammonites found in Triassic-aged marine strata.
Their shells were broad and rounded, giving them an almost spherical appearance. Unlike many other ammonites, the shells of Arcestes lack keels that would otherwise stabilize them while swimming. Because of this, some paleontologists have suggested that they were bottom-dwelling crawlers.
The shell of Arcestes is globular or subglobular with periodic narrow transverse constrictions in internal molds due to periodic internal transverse ridges (varices) in the shell. The suture is ammonitic with complexly subdivided elements. The ventral lobe is subdivided by a low median saddle. Lateral lobes and saddles have generally triangular outlines but are deeply embayed by strong projections forming tree-like patterns. They form a series diminishing in size and complexity in a rather straight line going from the venter to the umbilicus in the middle of the shell.
Fossils of Arcestes are found in Mid to Upper Triassic marine strata throughout the world, including California (A. pacificus), Nevada (A (Anisarcestes) mrazici) and Austria (A. intuslabiatus and A. binacostomus). [2]
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 remaining group of ammonoids from the Jurassic up until their extinction.
Goniatites is a genus of extinct cephalopods belonging to the family Goniatitidae, included in the superfamily Goniatitaceae. Hibernicoceras and Hypergoniatites are among related genera.
Stephanoceras is an extinct genus of Stephanoceratoid ammonite which lived during the Bajocian. It is the type genus of the family Stephanoceratidae.
Ancyloceras is an extinct genus of heteromorph ammonites found throughout the world during the Lower Cretaceous, from the Lower Barremian epoch until the genus extinction during the Lower Aptian.
Audaxlytoceras is an extinct genus of lytoceratid ammonites.
Ceratites is an extinct genus of ammonite cephalopods. These nektonic carnivores lived in marine habitats in what is now Europe, during the Triassic, from the upper-most Anisian to the lower Ladinian age.
Nipponites is an extinct genus of heteromorph ammonites. The shells of Nipponites form "ox-bow" bends, resulting in some of the most bizarre shapes seen among ammonites.
Psiloceras is an extinct genus of ammonite. Psiloceras is among the earliest known Jurassic ammonites, and the appearance of the earliest Psiloceras species form the definition for the base of the Jurassic. Unlike most earlier ammonites, which had complex shell shapes and ornamentation, Psiloceras had a smooth shell.
Sphenodiscus is an extinct genus of acanthoceratacean ammonite. The genus has been found from many continents and is thought to have had a large global distribution during the Maastrichtian stage of the Late Cretaceous. It was one of the last ammonoids to have evolved before the entire subclass became extinct during the Paleocene, which was directly after the Cretaceous–Paleogene extinction event.
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.
Episageceratinae is a subfamily of the Medlicottiidae, a family of prolecanitid ammonites. The Episageceratinae, proposed by Ruzhencev, 1956, is based on the genus Episageceras previously included in the subfamily Medlicottiinae and lived during Late Permian and Early Triassic times. So far only three confirmed genera are included: Episageceras, Latisageceras, and Nodosageceras.
Bochianites is a straight shelled ammonite which lived from the Upper Jurassic, Tithonian, to the Lower Cretaceous, Hauterivian in what is now Europe, Greenland, Africa, North America and Asia. The shell is long, narrow, moderately expanding; smooth or with weak to strong oblique annular ribs. Sutural elements are short and boxy. The umbilical lobe, which lies between the lateral lobe and dorsal lobe, on either side, is about the same size as the lobule dividing the first lateral saddle.
Craspedites is an ammonoid cephalopod included in the Perisphinctoidea that lived during the Late Jurassic and Early Cretaceous, found in Canada, Greenland, Poland, and the Russian Federation.
Hypophylloceras is a Cretaceous ammonite with a finely ribbed, compressed, involute shell; some having periodic stronger ribs or folds. The suture is complex, with large, asymmetric and finely divided lobes; the 1st lateral being much larger than the external (=ventral) and 2nd lateral lobes. Saddle endings commonly not phylloid.
Calliphylloceras is an ammonite belonging to the Phylloceratidae.
Arcestidae is an extinct family of ammonite cephalopods.
Parapopanoceras is a ceratitid ammonite with a small, smooth, very involute and moderately globose shell that lived during the middle Triassic.
Monophyllites is an extinct cephalopod genus belonging to the family Ussuritidae that lived during the Triassic period, from Anisian to Ladinian age.
Rhacophyllites is an extinct genus of cephalopods belonging to the family Discophyllitidae. These nektonic carnivores lived during the Triassic period, from Carnian to Rhaetian age.
Leiophyllites is a genus of early to middle Triassic ammonites belonging to the family Ussuritidae, possibly forming an evolutionary link between Lower Triassic and later members of the family.
