''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 cephalopods (Mollusca) which originated in the Late Cambrian and are represented today by the living ''Nautilus'' and ''Allonautilus''. Fossil nautiloids are diverse and species rich, with over 2,500 recorded species. Th ...

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 ...

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

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 ...

, through the remainder of the period and on into the Upper

Silurian The Silurian ( ) is a geologic period and system spanning 23.5 million years from the end of the Ordovician Period, at million years ago ( Mya), to the beginning of the Devonian Period, Mya. The Silurian is the third and shortest period of t ...

. It is the

type genus In biological taxonomy, the type genus (''genus typica'') 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-bearin ...

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 Nautiloidea, nautiloid cephalopods of the Wutinoceratidae family. It exhibits orthoconic Actinocerida, actinocerids with ventral siphuncles composed of broadly expanded segments.Memoir 2, Studies of the ...

'' 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 ''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 ...

'' 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 ''Selkirkoceras'' is a genus of armonoceratid cephalopods similar to ''Kochoceras'' of the Actinoceratidae, but with recumbent septal necks. The first siphuncular segment is large, broad, and blunt. ''Selkirkoceras'', which is known from the Mid ...

'' in the Eden-Red River Stage in the Upper Ordovician. Possible

derivatives include '' Megadiscosorus'', ''

Monocyrtoceras ''Monocyrtoceras'' is a genus of armenoceratids (Cephalopoda) from the Middle Silurian of North America (Wisconsin), with a gently curved shell and a siphuncle like that of ''Elrodoceras''. According to Teichert, 1964, in the Treatise part K, ' ...

'', and ''

Elrodoceras ''Elrodoceras'' is a genus of armenoceratids, (Cephalopoda, Actinocerida), that grew fairly large, characterized by a straight shell, slightly curved at the apex, a siphuncle narrower than that of ''Armenoceras'' but with segments still wider th ...

'' 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 The ''Treatise on Invertebrate Paleontology,'' published from 1953–2007 by the Geological Society of America and the University of Kansas, then 2009–present by the University of Kansas Paleontological Institute, is a definitive multi-authore ...

, 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