Lambeoceras is a rather large 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...

 with a convexly lenticular cross section from the Upper Ordovician

Ordovician

The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...

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

Morphological Description

Lambeoceras is of medium to moderately large 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, septa close 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

Actinoceras

Actinoceras is the principal and root genus of the Actinoceratidae, a major family in the Actinocerida, that lived during the Middle and Late Ordovician.- Morphology:...

that produced Kochoceras according to Flower (1968), although consideration had been given 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....

in Flower (1957). Teichert (1964) on the other hand, in the Treatise, thought that Lambeoceras was derived from Gonioceras

Gonioceras

Gonioceras is an actinocerid with a broad, low shell; flattened ventrally, convexly rounded dorsally; top and bottom meeting at an acute angle along the sides. In most the shell is rather thin, especially along the lateral portion. The aperture is contracted...

and included it in the Gonioceratidae

Gonioceras

Gonioceras is an actinocerid with a broad, low shell; flattened ventrally, convexly rounded dorsally; top and bottom meeting at an acute angle along the sides. In most the shell is rather thin, especially along the lateral portion. The aperture is contracted...

. Kochoceras differs from Lambeocernas 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

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:...

in the Second Value Formation in New Mexico

New Mexico

New Mexico is a state located in the southwest and western regions of the United States. New Mexico is also usually considered one of the Mountain States. With a population density of 16 per square mile, New Mexico is the sixth-most sparsely inhabited U.S...

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

Texas

Texas is the second largest U.S. state by both area and population, and the largest state by area in the contiguous United States.The name, based on the Caddo word "Tejas" meaning "friends" or "allies", was applied by the Spanish to the Caddo themselves and to the region of their settlement in...

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

Wyoming

Wyoming is a state in the mountain region of the Western United States. The western two thirds of the state is covered mostly with the mountain ranges and rangelands in the foothills of the Eastern Rocky Mountains, while the eastern third of the state is high elevation prairie known as the High...

, 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

Manitoba

Manitoba is a Canadian prairie province with an area of . The province has over 110,000 lakes and has a largely continental climate because of its flat topography. Agriculture, mostly concentrated in the fertile southern and western parts of the province, is vital to the province's economy; other...

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

Baffin Island

Baffin Island in the Canadian territory of Nunavut is the largest island in the Canadian Arctic Archipelago, the largest island in Canada and the fifth largest island in the world. Its area is and its population is about 11,000...

.

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