Aphetoceras
Encyclopedia
Aphetoceras is a genus of Tarphycerida
cephalopod
within the Estonioceratidae
; loosely coiled without an impression along the dorsal margin; early whorls barely teaching, 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
El Paso Group in New Mexico.
Tarphycerida
The Tarphycerida were the first of the coiled cephalopods. They are found in marine sediments from the Lower Ordovician to the Middle Devonian. Some like Aphetoceras and Estonioceras are loosely coiled, gyroconic, others like Campbelloceras, Tarphyceras, and Trocholites are tightly coiled, but...
cephalopod
Cephalopod
A cephalopod is any member of the molluscan class Cephalopoda . These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles modified from the primitive molluscan foot...
within the Estonioceratidae
Estonioceratidae
The Estonioceratidae is 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 ventrally...
; loosely coiled without an impression along the dorsal margin; early whorls barely teaching, 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
Estonioceras
Estonioceras is an extinct genus of tarphyceridan nautiloid from the Ordovician of Europe.-Sources:* Dinosaur Encyclopedia by Jayne Parsons* Fossils by David Ward-External links:* in the Paleobiology Database...
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
Ordovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...
El Paso Group in New Mexico.