''Aphetoceras'' is a genus of tarphycerid

cephalopod A cephalopod is any member of the molluscan class Cephalopoda (Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head ...

within the

Estonioceratidae The Estonioceratidae are a family of loosely coiled tarphycerids in which the inner side of the whorls, which forms the dorsum, is rounded or flat with no impression, and in which the siphuncle, composed of thick tubular segments, is located ve ...

; loosely coiled without an impression along the dorsal margin; early whorls barely reaching, separating then diverging in the final mature whorl; weakly ribbed in some. The cross section of ''Aphetoceras'' is higher than wide, making it compressed in form. The dorsum, along the inner curve, is more broadly rounded than the venter which lies along the outer curve. The siphuncle is relatively large, located near but not at the ventral margin; lined with secondary deposits. Chambers are empty. ''Aphetoceras'' is similar in outline to the related '' Estonioceras'' and ''

Alaskoceras ''Alaskoceras'' is a genus of lower Ordovician coiled nautiloid cephalopods; the shell moderately expanded, ribbed, with a divergent living chamber; whorl section more broadly rounded ventrally than dorsally; siphuncle marginal at maturity, septa ...

'' but most similar in cross section to '' Clytoceras''. ''Aphetoceras'' comes from the Lower Ordovician of North America and Australia and is one of the earlier tarphycerids found in the Lower

Ordovician The Ordovician ( ) is a geologic period and System (geology), system, the second of six periods of the Paleozoic Era (geology), Era. The Ordovician spans 41.6 million years from the end of the Cambrian Period million years ago (Mya) to the start ...

El Paso Group in New Mexico.

References

* Aphetoceras, p K358 in Vol K of the Treatise on Invertebrate Paleontology in the section on the Estonioceratidae, p K357-K359, included in the Chapter on the Tarphycerida by Furnish and Glenister starting with page K343. {{Taxonbar, from=Q4779595 Prehistoric nautiloid genera Ordovician cephalopods Ordovician cephalopods of North America Paleozoic life of Newfoundland and Labrador Paleozoic life of Quebec