Lambeoceras
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| Lambeocears Fossil range: M-U Ordovician |
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Lambeoceras is a unique actinocerid characteristic of Red Riveran faunas in North America. 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.
The Treatise (Teichert 1964) includes Lambeoceras with Gonioceras in the Gonioceratidae, both being depressed in section. Flower ( 1968, 1976 ) recognizing important distinctions having to do with the sutures and the siphuncle placed Lambeoceras in its own family, the Lambeocertidae.
Lambeoceras is likely derived from Actinoceras near the end of the Middle Ordovician, although consideration has been given to Armenoceras at about the same time.
Lambeoceras is found with Actinoceras, Armenoceras, and Nybyoceras in the Second Value Formation in New Mexico; with Armenoceras and Selkirkoceras in the Burnam Limestone in central Texas; with Actinoceras, Paractinoceras, and Kochoceras in the Lander Sandstone in Wyoming, 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 and with Actinoceras and Kochoceras in the Mt. Silliman beds on Baffin Island.
Speculatively, Lambeoceras may have had a similar life style to that of the earlier Gonioceras, only Lambeoceras may have hunted more actively over the sea floor, rather than waiting in ambush on the sea floor.
[edit] 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
