''Rhynchorthoceras'' is a

Middle 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. The ...

genus characterized by a rapidly expanded, weakly annulate

orthocone An orthocone is an unusually long straight shell of a nautiloid cephalopod.; During the 18th and 19th centuries, all shells of this type were named ''Orthoceras'', creating a wastebasket taxon, but it is now known that many groups of nautiloids d ...

, like the orthoconic section of ''

Ancistroceras ''Ancistroceras'' is one of the two ancestral lituitids from the late Early Ordovician (Arenigian). The other being '' Holmiceras''. The shell is weakly annulate, starts off with 1.5 to 2 contiguous or slightly separated whorls followed by a rap ...

'', but with only a curved, cyrtoconic apex instead of juvenile whorls. ''Rhynchorthoceras'' is probably derived from ''

'' by a loss of the tarphycerid type apex, although it has been included with the orthocerid Sinoceratidae.

References

*Flower, R. H. 1950. A Classification of the Nautiloidia. Jour Paleontology, V.24, N.5, pp 604–616, Sept. *Furnish & Glenister, 1964. Nautiloidea -Tarphycerida.

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 ...

Part K, Mollusca 3 ...Nautiloidea {{Taxonbar, from=Q7321718 Nautiloids Paleozoic life of Newfoundland and Labrador