Oncocerida

Systematic Paleontology

Class: Cephalopoda

Subclass: Nautiloidea

Order: Oncocerida

The Oncocerida is an order of nautiloid cephalopods, also derived from the Bassleroceratidae, whose evolutionary sequence parallels that of the Tarphycerida and Barrandeocerida. Shells (conchs) range in form from simple, short curved, breviconic cyrtocones, to elongate cyrtocones and straight orthocones, and include openly coiled gyrocones, close coiled nautilicones, and trochoidal torticones. Siphuncles vary nature. Some are simple with straight segments. Others expand into the adjacent chambers. And some have longitudinal blade-like structures that project inwardly, referring them as actinosiphonate.

Oncocerida made their first appearance is in the middle Ordovician, their last in the lower Carboniferous (Mississippian). Within, sixteen families have been named and described. From the middle and upper Ordovician, the Graciloceratidae, Tripteroceratidae and Valcouroceratidae. From the middle Ordovician and extending throughout most of the Silurian, the Oncoceratidae. From the middle Silurian to the lower Devonian, the Jovellanidiae and throughout most of the Silurian and Devonian, the Nothoceratidae. From the lower Silurian, extending possibly to the lower Devonian, the Karoceratidae. First appearing next in the middle Silurian, the Trimeroceratidae, Hemiphragmoceratidae, Acleistoceratidae, Polyelasmoceratidae and Brevicoceratidae. Last came the Archiacoceratidae from the middle Devonian the the Poterioceratidae and Tripleuroceratidae from the Devonian and reaching into the lower Carboniferous.

Most families upon becoming extinct left no progeny. However the Brevicoceratidae is thought to have given rise to the Nautilida early in the Devonian, represented by today's living Nautilus and Allonautilus, ultimately tying the primitive ellesmerocerids of the Cambro-Ordovician to the present.