''Ancistroceras'' is one of the two ancestral lituitids from the late

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. ...

(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 rapidly expanding orthocone with an apical angle of about 30 deg. Growth lines are sinuous, show a pronounced hyponomic sinus in the coiled juvenile portion at the apex but almost none at the mature aperture of the orthcone. ''Holmiceras'' is similar except that its whorls are more loosely coiled and it is known only from the Arenigian whereas ''Ancistroceras'' ranges through most of the Middle Ordovician. ''Ancistroceras'' may have given rise to '' Rhynchorthoceras'' by a loss of the juvenile coiled portion, replacing it with a short semi-tubular cyrtoconic section. ''Ancistroceras'' may also have given rise to ''

Lituites ''Lituites'' is an extinct nautiloid genus from the Middle Ordovician and type for the Lituitidae (a tarphycerid family) that in some more recent taxonomies has been classified with the orthocerids and listed under the order Lituitida. Fossil ...

'' by evolving a narrower, less rapidly expanding, orthocone along with greater ornament and a complex aperture.

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 Part K, Mollusca 3 ...Nautiloidea {{Taxonbar, from=Q4753065 Prehistoric cephalopod genera Early Ordovician first appearances Middle Ordovician extinctions