''Aphetoceras'', from

Ancient Greek Ancient Greek (, ; ) includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Greek ...

ἄφετος (''áphetos''), meaning "loose", and κέρας (''kéras''), meaning "horn", is a genus of tarphycerid

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

within the Estonioceratidae; 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'' 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, and the second of twelve periods of the Phanerozoic Eon (geology), Eon. The Ordovician spans 41.6 million years f ...

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