''Eburoceras'' is an early

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 from the Upper Yenchau and Wanwankau, Upper Cambrian Trempealeauan of China, assigned to the

Ellesmeroceratidae The Ellesmeroceratidae constitute a family within the cephalopod order Ellesmerocerida. They lived from the Upper Cambrian to the Lower Ordovician. They are characterized by straight and endogastric shells, often laterally compressed, so the d ...

. The shell of ''Eburoceras'' is long and narrow, curved in a broad arc of near constant curvature, interpreted as endogastric, assuming the small

siphuncle The siphuncle is a strand of biological tissue, tissue passing longitudinally through the mollusc shell, shell of a cephalopod mollusc. Only cephalopods with chambered shells have siphuncles, such as the extinct ammonites and belemnites, and the li ...

on the inner margin to be ventral. Known for sure dorsal siphuncles do not appear until later in the Early Ordovician with the

Trocholitidae The Trocholitidae are Tarphycerida with Whorl (botany), whorls in close contact as with the Tarphyceratidae, but in which the siphuncle, similar in structure, becomes dorsal. The Trocholitidae are derived from the Tarphyceratidae, perhaps from di ...

, which if so would make it (''Eburoceras'') exogastric. ''Eburoceras'' is the most strongly curved of the Cambrian ellesmeroceratids. Eburoceras is the earliest ellesmeroceratid except for the short lived '' Hunyuanocers'', from the lower part of the Upper Yenchou, that proceeded it. ''Eburoceras'' died out at the end of Wanwankau time, along with the other Late Cambrian ellesmeroceratid genera, except for ''

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. ...

'' and ''

Clarkoceras ''Clarkoceras'' is a genus of breviconic ellesmerocerid cephalopods, one of only two genera known to have crossed from the Late Cambrian, Trempealeauan, into the Early Ordovician, Gasconadian. (Flower 1964, Teichert 1988); the other being ''Ect ...

'', which persisted into the

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 ...

. Late Cambrian '' Tanycamerocers'' (Ellesmeroceratidae) is similar in being long and narrow and having a small marginal siphuncle, but is straight. Late Cambrian '' Huaiheceras'' (Hauiheceratidae) is also similar in being long with a small marginal siphuncle, but is exogastric and less strongly curved.

References

* Chen Jun-yuan & Curt Teichert. Cambrian Cephalopods. Geology v.11 Nov. 1983 * Curt Teichert. Main features of Cephalopod Evolution. the Mollusca Volume 12. Paleontology and Neontology of Cephalopods. Academic Press Inc. 1988. {{Taxonbar, from=Q18389096 Ellesmerocerida