''Arcestes'' is a genus of extinct

ceratitid 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 ammoni ...

ammonite Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid and cuttlefish) ...

s found in

Triassic The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest period ...

-aged marine strata.

Description

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.

Distribution

Fossils of ''Arcestes'' are found in Mid to Upper

marine strata throughout the world, including

California California is a U.S. state, state in the Western United States, located along the West Coast of the United States, Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the List of states and territori ...

(''A. pacificus''),

Nevada Nevada ( ; ) is a U.S. state, state in the Western United States, Western region of the United States. It is bordered by Oregon to the northwest, Idaho to the northeast, California to the west, Arizona to the southeast, and Utah to the east. N ...

(''A (Anisarcestes) mrazici'') and

Austria Austria, , bar, Östareich officially the Republic of Austria, is a country in the southern part of Central Europe, lying in the Eastern Alps. It is a federation of nine states, one of which is the capital, Vienna, the most populous ...

(''A. intuslabiatus'' and ''A. binacostomus'').

Gallery

File:Arcestes 1.JPG, Museum specimen of ''Arcestes'' species File : Arcestidae - Arcestes sp.JPG, Museum specimen of ''Arcestes'' species

References

* Arkell et al. Mesozoic Ammonoidea. Treaatise on Invertebrate Paleontology, Part L, 1957 Geological Society of America and University of Kansas press, R.C.Moore ed. * Johnston, F.N. Trias of New Pass Nevada (New lower Karnic ammonoids); Journal of Paleontology V.15 No 5 p 447-491, Sept 1941 {{Taxonbar, from=Q4785273 Triassic ammonites of North America Ceratitida genera Late Triassic ammonites Carnian genus first appearances Norian genera Rhaetian genus extinctions Fossil taxa described in 1865 Arcestidae