Apocrinoceratidae constitutes a family of 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. ...

nautiloid cephalopods characterized by straight or slightly curved, transversely ribbed shells having

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

s composed of expanded segments, short recurved septal necks, and thick connecting rings. Derivation is from the Protocycloceratidae, a family of ellesmerocerids, which differ in having straight or concave siphuncle segments, but are otherwise similar in form. Apocrinoceratidae was established by

Rousseau Flower Rousseau Hayner Flower (1913–1988) was an extremely prolific 20th century paleontologist, known for his eccentric personality. Career Although trained as an entomologist, and a specialist in dragonflies and orthopterans, Flower began st ...

(in Flower & Teichert, 1957) repeated in Flower (1964) for the genus '' Apocrinoceras'' (Teichert & Glenister, 1954). Four additional genera are included; '' Desioceras'' and ''

Glenisteroceras ''Glenesteroceras'', which is of questionable validity, is based on a single specimen from the Lower Ordovician of New York state, described by Rousseau Flower, 1957. ''Glenestoceras'' is described as being a weakly annulate orthocone with a cir ...

'' (Flower and Teichert 1957, Flower 1964),'' Bakeroceras'' (Hook and Flower 1977), and ''

Paldoceras ''Paldoceras'' is a genus of nautiloid cephalopods found in the middle Ordovician of Estonia and Sweden, included in the discosorid family Apocrinoceratidae. Two species are recognized; ''Paldoceras paldiskense'' found northwest of Paldiski in ...

'' (Kröger ''et al'' 2009). Apocrinoceratidae was removed from the Ellesmerocerida to the

Discosorida Discosorida are an order of cephalopods that lived from the beginning of the Middle Ordovician, through the Silurian, and into the Devonian. Discosorids are unique in the structure and formation of the siphuncle, the tube that runs through and c ...

by Kröger ''et al'' (2009) based in the inclusion of ''Paldoceras'' interpreted as a morphologic intermediary between ''Apocrinoceras'' and '' Reudemannoceras''. ''Paldoceras'' however is rather dissimilar, being more strongly cyrtoconic (curved) and lacking the characteristic annulations.

References

* Rousseau H Flower and Curt Teichert, 1957. The Cephalopod Order Discosorida. University of Kansas Paleontological Contributions. Mollusca Article 6. July 1, 1957. * Rousseau H Flower 1964. The Nautiloid Order Ellesmeroceratida (Cephalopoda). New Mexico Bureau of Mines and Mineral Resources Memoir 12. * Björn Kröger, Yunbai Zhang, & Mare Isakar 2009. Discosorids and Oncocertids (Cephalopoda) of the Middle Ordovician Kunda and Aseri Regional Stages of Baltoscandia and the early evolution of these groups. ''Geobios'', Vol. 42, No. 3 (May 2009) pp. 273–292. {{Taxonbar, from=Q4780135 Prehistoric nautiloid families Middle Ordovician extinctions Middle Ordovician first appearances Taxa named by Rousseau H. Flower Discosorida