Nybyoceras is an actinocerid

Actinocerida

The Actinocerida comprise an order of generally straight, medium to large cephalopods that lived during the early and middle Paleozoic, distinguished by a siphuncle composed of expanded segments that extend into the adjacent chambers, in which deposits formed within contain a system of radial...

 genus assigned to the Armenoceratidae

Armenoceratidae

Armenoceratidae is a family of early Paleozoic nautiloid cephalopods belonging to the Order Actinocerida.The Armenoceratidae, established by Troedsson is characterized by large, straight or slightly curved shells and large siphuncles with strongly expanded segments between the septa. Septal necks...

 and similar to Armenoceras

Armenoceras

Armenoceras is the principal genus of the Armenoceratidae, ranging from the late Whiterockian Stage in the early Middle Ordovician, through the remainder of the period and on into the Upper Silurian....

except for having a siphuncle close to the ventral side of the shell.

Morphology

As with Armenoceras, Nybyoceras has a medium to large, straight shell.(Teichert 1964) Its primary diagnostic feature is its ventral siphuncle which the septa across obliquely.(Flower 1957,Teichert 1964) Ventrally, the septal necks are short and free, or recumbent for a short distance. Dorsally the brims are definitely recumbent.(Flower 1957) The connecting rings are broadly adnate to the septa ventrally at the posterior ends of the segments and dorsally at the anterior (adoral) ends.(Teichert 1964) Flower (1957) described the canal system within the siphuncle as being primarily of the single arc type, like Armenoceras, and with interconnecing tubes. Teichert (1964) describes the canal system as being reticulate to curved and branching.

Phylogengy and Distribution

Nybyoceras is derived from Wutinoceras

Wutinoceras

Wutinoceras, orthoconic actinocerids with ventral siphuncles composed of broadly expanded segments.and the type genus of the Wutinoceratidae. -Distinguishing Characters:...

according to Flower (1976) although there is a possibiliy of its being derived from an early Armenoceras (Flower 1968) .

Nybyoceras ranges from the Middle Ordovician (Chazyan equivalent) in northeast China to the early Upper Ordovician Cobourg-Eden-Red Stage tn North America, (Flower 1976) and according to Flower (1968, 1976) left no descendant genera. Teichert on the other hand, earlier (1964) indicated that Nybyoceras gave rise to the Upper Silurian

Silurian

The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...

 Selkiroceras

Armenoceratidae

Armenoceratidae is a family of early Paleozoic nautiloid cephalopods belonging to the Order Actinocerida.The Armenoceratidae, established by Troedsson is characterized by large, straight or slightly curved shells and large siphuncles with strongly expanded segments between the septa. Septal necks...

 and to the Silurian Megadiscosorus

Armenoceratidae

Armenoceratidae is a family of early Paleozoic nautiloid cephalopods belonging to the Order Actinocerida.The Armenoceratidae, established by Troedsson is characterized by large, straight or slightly curved shells and large siphuncles with strongly expanded segments between the septa. Septal necks...

.

References

• Flower, R.H, 1957, Studies of the Actinoceratida, Memoir 2. New Mexico Bureau of Mines and Mineral Resources
• Flower, R.H. 1968, The First Great Expansion of the Actinoceroids; Memoir 2, Pt I. ibid
• Flower, R.H, 1976, Ordovician Cephalopod Faunas and Their Role in Correlation; in The Ordovician System: Palaeontological Association; Bassett, M.G. (Ed)
• Teichert, C, 1964, Actinoceratoidea, Treatise on Invertebrate Paleontology,Park K, Geological Society of America and University of Kansas Press.