''Arnioceras'' is an extinct genus of large, evolute, discoidal

ammonite Ammonoids are extinct, (typically) coiled-shelled cephalopods comprising the subclass Ammonoidea. They are more closely related to living octopuses, squid, and cuttlefish (which comprise the clade Coleoidea) than they are to nautiluses (family N ...

from the Lower

Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately 143.1 Mya. ...

. The shell is normally coiled so that all whorls are exposed. Sides bear strong sharp ribs that are straight until reaching the ventrolateral edge where they swing forward and fade. The rim (venter) is keeled and free of grooves. ''Arnioceras'', named by

Alpheus Hyatt Alpheus Hyatt (April 5, 1838 – January 15, 1902) was an American zoologist and palaeontologist. Hyatt served as the founding president of the American Society of Naturalists from 1883 to 1884 and was the founding editor of the journal '' T ...

, is included in the arietitid family of the

Psiloceratoidea Psiloceratoidea is a superfamily of Early Jurassic Ammonoidea, ammonoid cephalopods proposed by Alpheus Hyatt, Hyatt in 1867, assigned to the order Ammonitida. They were very successful during Hettangian and Sinemurian. Last of them, family Cym ...

, an ammonitid superfamily. Its distribution is fairly worldwide, having been found in Europe, south Asia, and North and South America.

References

;Notes ;Bibliography * Jurassic ammonites Jurassic ammonites of North America Ammonitida genera Early Jurassic ammonites Hasle Formation Taxa named by Alpheus Hyatt Fossil taxa described in 1897 Arietitidae {{Ammonite-stub