''Clarkoceras'' is a genus of breviconic ellesmerocerid

cephalopod A cephalopod is any member of the molluscan class Cephalopoda ( Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, ...

s, one of only two genera known to have crossed from the Late

Cambrian The Cambrian Period ( ; sometimes symbolized Ꞓ) was the first geological period of the Paleozoic Era, and of the Phanerozoic Eon. The Cambrian lasted 53.4 million years from the end of the preceding Ediacaran Period 538.8 million years ag ...

, Trempealeauan, into the Early

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. ...

, Gasconadian. (Flower 1964, Teichert 1988); the other being ''

Ectenolites ''Ectenolites'' is a genus of small, slender, cylindrical Ellesmeroceratids that resemble '' Ellesmeroceras'' but are smaller and proportionally narrower. Septa, as typical for ellesmerocerids, are close spaced with shallow lobes on either flank ...

''.

Description

Clarkoceras has a rapidly expanding, laterally compressed, relatively short, endogastrically cyrtoconic shell; The upper or dorsal side is more strongly convex longitudinally than the lower or ventral side is concave. Sutures are essentially straight and close spaced indicating very short camerae (chambers). The siphuncle is relatively large, 0.3 the dorsoventral dimension and is ventral, although not necessarily marginal. (Flower 1964, Furnish and Glenister 1964). Septal necks are straight, reaching about half way back to the previous septum and the connecting rings are thick and layered

Taxonomy

''Clarkoceras'' was first thought by Clarke to be a pilocerid, who in 1897 give it the name ''Piloceras newton-winchelli''. Ruedemann in 1905 recognized this as belonging to the Ellesmerocerida and renamed it ''Clarkoceras newton-winchelli'' (Clarke), which became the genotype.

Distribution and Range

The earliest known ''Clarkoceras'' comes from the upper Trempealeauan Wanwankou Member of the Fengshan Formation in northern China, one of twelve ellesmeroceratid genera from the upper Late Cambrian. (Chen and Teichert 1983, Teichert 1988). In North America ''Clarkoceras'' is found in early Lower Ordovician, Gasconadian age shallow marine sediments near Smith Basin, State of New York, the Llano Uplift of Central Texas, and the El Paso Group of the eastern Rocky Mountains of

Trans-Pecos The Trans-Pecos, as originally defined in 1887 by the Texas geologist Robert T. Hill, is the portion of Texas that lies west of the Pecos River. The term is considered synonymous with Far West Texas, a subdivision of West Texas. The Trans-Pe ...

Texas and Southern New Mexico.

References

* *Flower, R.H., 1964, The Nautiloid Order Ellesmerocerida (Cephalopoda). New Mexico Bureau of Mines and Mineral Resources Memoir 12, Socorro, NM *Furnish and Glenister, 1964, Nautiloidea-Ellesmerocerida, in Teichert and Moore (Eds.), The

Treatise on Invertebrate Paleontology The ''Treatise on Invertebrate Paleontology'' (or ''TIP'') published by the Geological Society of America and the University of Kansas Press, is a definitive multi-authored work of some 50 volumes, written by more than 300 paleontologists, and co ...

Partl K, Nautiloidea. GSA and University of Kansas Press *Ruedemann, R., 1905, The Structure of Some Primitive Cephalopods, New York State Museum Bulletin 80, pp. 296–341 *Teichert, C. 1988, Main Features of Cephalopod Evolution, in M.R Clarke and E.R.Trueman (Eds.), The Mollusca. Academic Press {{Taxonbar, from=Q5127571 Prehistoric nautiloid genera Cambrian molluscs Ordovician cephalopods Cambrian animals of North America Ordovician animals of North America Cambrian animals of Asia Ordovician animals of Asia Cambrian first appearances Early Ordovician extinctions Ellesmerocerida