''Armenoceras'' is a genus of

actinocerid The Actinocerida are 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 deposit ...

nautiloid Nautiloids are a group of marine cephalopods (Mollusca) which originated in the Late Cambrian and are represented today by the living ''Nautilus'' and '' Allonautilus''. Fossil nautiloids are diverse and speciose, with over 2,500 recorded specie ...

cephalopods whose fossils ranged from the late Whiterockian Stage in the early Middle

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

, through the remainder of the period and on into the Upper Silurian. It is the

type genus In biological taxonomy, the type genus is the genus which defines a biological family and the root of the family name. Zoological nomenclature According to the International Code of Zoological Nomenclature, "The name-bearing type of a nominal ...

of the family

Armenoceratidae The Armenoceratidae are a family of early Paleozoic nautiloid cephalopods belonging to the order Actinocerida.Flower 1957.Studies of the Actinoceratida.; Memoir 2; New Mexico Bureau of Mines and Mineral Resources, Socorro, NM Teichert 1964. Act ...

. The shells of ''Armenoceras'' are straight and medium to large in size with a circular to subcircular cross section. The siphuncle is large, located subcentrally to resting on the ventral margin. Segments are wider than long, broadly expanded into the camerae. Septal necks short with wide brings that may be in contact with posterior surface of septa. In most the canal system within the siphuncle in is of the double arc type. Cameral deposits are rare. ''Armenoceras'' is derived from ''

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 ...

'' through a thinning of the connecting rings and a simplification of the endosiphuncular canal system. The earliest ''Armenoceras'' known comes from the lower Whiterock equivalent in northern China and Korea but is unknown in North America until the end of the Middle Ordovician when it appears in Red River faunas. Worldwide, in addition to east Asia and North America including Greenland, ''Armenoceras'' has been found in northern Europe, Russia, and Australia. ''Armenoceras'' is thought to have given rise to '' Nybyoceras'' in the

Chazyan The Chazy Reef Formation is a mid-Ordovician limestone deposit in northeastern North America. It consists of some of the oldest reef systems built by a community of organisms rather than the deposit of a limited range of similar organisms, such ...

(Flower 1968), although the reverse was earlier suggested in Teichert (1964), and to '' Selkirkoceras'' in the Eden-Red River Stage in the Upper Ordovician. Possible Silurian derivatives include '' Megadiscosorus'', '' Monocyrtoceras'', and '' Elrodoceras'' although some may turn our to have ''Nybyoceras'' as the progenitor.

References

* Flower, R.H, 1957, Studies of the Actinoceratida, New Mexico Bureau of Mines and Mineral Resources, Memoir 2. * Flower, R.H. 1968, The First Great Expansion of the Actinoceroids; New Mexico Bureau of Mines and Mineral Resources, Memoir 2, PtI. * Flower, R.H, 1978, Ordovician Cephalopod Faunas and Their Role in Correlation; in The *Ordovician System: proceedings of a Palaeontological Association symposium; Bassett, M.G. (Ed) * Teichert, C, 1964, Actinoceratoidea, in the Treatise on Invertebrate Paleontology, pub Univ of Kansas and the GSA, Vol K, p K206-207; Fig 137 p K200-201. {{Taxonbar, from=Q4793380 Prehistoric nautiloid genera Actinocerida Ordovician cephalopods of Asia Silurian cephalopods Silurian animals of Europe Silurian animals of Asia Middle Ordovician first appearances Silurian extinctions Ordovician cephalopods of North America Paleozoic life of Manitoba Paleozoic life of the Northwest Territories Paleozoic life of Nunavut Paleozoic life of Quebec