''Cyrtonybyoceras'' is a genus of slightly exogastric members of the

Wutinoceratidae The Wutinoceratidae are a family of early actinocerids defined by Shimazu and Obata in 1938Flower 1976. New American Wutinoceratidae with Review of Actinoceroid Occurrences in Eastern Hemisphere;PartI, Memoir 28; New Mexico Bureau of Mines and M ...

, a family of actinocerids and probably derived from an earlier ''

Wutinoceras ''Wutinoceras'' is a genus of now extinct nautiloid cephalopods of the Wutinoceratidae family. It exhibits orthoconic actinocerids with ventral siphuncles composed of broadly expanded segments.Memoir 2, Studies of the Actinocerida, New Mexic ...

'' . The shell of ''Cyrtonybyoceras'' is curved slightly upwardly and is slightly compressed. Sutures slope toward the aperture, from the dorsum to the venter. The siphuncle is ventral but not marginal, in general form like that of ''

Nybyoceras ''Nybyoceras'' is an actinocerid genus assigned to the Armenoceratidae and similar to ''Armenoceras'' except for having a siphuncle close to the ventral side of the shell. Morphology As with ''Armenoceras'', ''Nybyoceras'' has a medium to large ...

''. Upper and lower septal necks are recumbent or narrowly free. The canal system is reticular, characteristic of the Wutinoceratidae.Flower R.H.,1968;The First Great Expansion of the Actinoceroids, New Mexico Bureau of Mines and Mineral Resources, Memoir 19 Part I, Socorro NM ''Cyrtonybyoceras'' and ''Wutinoceras'' are similar, except for ''Cyrtonybyoceras'' being slightly cyrtoconic, and almost intergrade. ''Cyrtonybyoceras'' is so far known only from the

Whiterock Stage The Whiterockian, often referred to simply as the Whiterock, is an earliest or lowermost stage of the Middle Ordovician. Although the Whiterockian or Whiterock Stage refers mainly to the early Middle Ordovician in North America, it is often used in ...

in North America. The genotype, ''Cyrtonybyoceras haesitans'', was named by Teichert in 1933, based on ''Orthoceras haesitans'' Billings 1865, from the Tablehead formation in Newfoundland. It has also been found in the Antelope Valley Limestone in the

Toquima Range The Toquima Range is a mountain range, located primarily in Nye County with a small extension into Lander County, in Nevada, United States. Most of the range is included within the Humboldt-Toiyabe National Forest. The range reaches a maximu ...

in Nevada.

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