Arionocerasis an extinct orthocerid genus from the Middle and Upper Silurian

Silurian

The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya , to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya . As with other geologic periods, the rock beds that define the period's start and end are well identified, but the...

, of Europe that is estimated to have lived from 422.9—418.1 mya, existing for approximately .

Taxonomy

Arionoceraswas named by Barskov (1966) and originally assigned to the Michelinoceratidae, but attributed by Serpagli and Gnoli (1977) -ibid to the Geisonoceratidae Zhuravleva (1959) "on the basis of the (admittedly irregular) occurrence of a lining-like endosiphucular deposit." Dzik (1984) assigned Arionoceras to the newly proposed Arionoceratidae as the type genus. Sepkoski (2002) follows previous authors in putting Arionoceras in the Orthoceratida

Orthocerida

Orthocerida is an order of extinct nautiloid cephalopods also known as the Michelinocerda that lived from the Early Ordovician possibly to the Late Triassic . A fossil found in the Caucasus suggests they may even have survived until the Early Cretaceous...

.

Morphology

Arionoceras has a straight or slightly curved shell with a circular or depressed cross section, smooth or transversely sculptured surface, and large apically pointed protoconch . The siphuncle is central with very short suborthochoanitc septal necks and cylindrical connecting rings. Apical chambers in adults have cameral deposits.

Nothing is known of the animal which may have been squid-like, probably with 10 subequal arms, and was probably sexually dimorphic.