''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-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 Triassic marine strata throughout the world, including California (''A. pacificus''), Nevada (''A (Anisarcestes) mrazici'') and Austria (''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