''Lambeoceras'' is a genus of large

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 ...

s with a convexly lenticular cross section from the Upper

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 ...

of North America and the sole representative of the family Lambeoceratidae.

Morphological description

''Lambeoceras'' is of medium to moderately large size with a long, straight, depressed shell, broad in cross section with the dorsum and venter both about equally convex, meeting acutely along the sides. Chambers are short, and septa are closely spaced, forming broad lobes on the upper and lower sides, which meet in sharp saddles along the sides. The siphuncle is submarginal, near the ventral side and relatively narrow. Septal necks are extremely long, brims short and recumbent. Segments are broadly expanded, connecting rings thin. Radial canals within the siphuncle from broad arcs that may bifurcate close to the parispatium.

Derivation and phylogeny

''Lambeoceras'' is derived from the same stock in '' Actinoceras'' that produced ''Kochoceras'' according to Flower (1968), although consideration had been given to '' Armenoceras'' in Flower (1957). Teichert (1964) on the other hand, in the Treatise, thought that ''Lambeoceras'' was derived from '' Gonioceras'' and included it in the Gonioceratidae. ''Kochoceras'' differs from ''Lambeoceras'' in having a flattened venter and a siphuncle that rests on the ventral side and is closely allied with ''Actinoceras'' (Flower 1957)

Distribution

''Lambeoceras'' is found with ''Actinoceras'', ''Armenoceras'', and '' Nybyoceras'' in the Second Value Formation in

New Mexico ) , population_demonym = New Mexican ( es, Neomexicano, Neomejicano, Nuevo Mexicano) , seat = Santa Fe , LargestCity = Albuquerque , LargestMetro = Tiguex , OfficialLang = None , Languages = English, Spanish ( New Mexican), Navajo, Ke ...

; with ''Armenoceras'' and '' Selkirkoceras'' in the Burnam Limestone in central

Texas Texas (, ; Spanish: ''Texas'', ''Tejas'') is a state in the South Central region of the United States. At 268,596 square miles (695,662 km2), and with more than 29.1 million residents in 2020, it is the second-largest U.S. state by ...

; with ''Actinoceras'', ''Paractinoceras'', and ''Kochoceras'' in the Lander Sandstone in

Wyoming Wyoming () is a state in the Mountain West subregion of the Western United States. It is bordered by Montana to the north and northwest, South Dakota and Nebraska to the east, Idaho to the west, Utah to the southwest, and Colorado to the s ...

, and with ''Armenoceras''. ''Actinoceras'', and ''Selkirkoceras'' in the overlying dolomite – all of Red River age, marking the transition between the Middle and Upper Ordovician. ''Lambeoceras'' is also found with ''Actinoceras'' in the Dog Head member of the Red River Series in

Manitoba , image_map = Manitoba in Canada 2.svg , map_alt = Map showing Manitoba's location in the centre of Southern Canada , Label_map = yes , coordinates = , capital = Winn ...

and with ''Actinoceras'' and ''Kochoceras'' in the Mt. Silliman beds on Baffin Island.

Paleoecology

While the earlier ''Gonioceras'' no doubt rested on the sea floor in ambush, perhaps half buried or more in sediment, ''Lambeoceras'' may have more actively hunted in the water close to but above. On the other hand, ''Lambeoceras'' ' shape may have had more to do with stabilization in shallow surge ridden waters than with hunting methods.

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, Pt i. * 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 K208-210 {{Taxonbar, from=Q6481197 Ordovician cephalopods of North America Prehistoric cephalopod genera Paleozoic life of Manitoba Paleozoic life of the Northwest Territories Paleozoic life of Nunavut Paleozoic life of Quebec