Orthoceratoidea


Orthoceratoidea is a subclass, formerly considered an infraclass or a superorder, that comprises Cephalopoda orders that have orthoconic to slightly cyrtoconic shells and central to subcentral siphuncles in which there may be internal deposits. Currently, Orthoceratoidea comprises the orders Dissidocerida, Ascocerida, Pseudorthocerida, Lituitida and Orthocerida.

Taxonomy

As a superorder, Orthoceratiodea was one of six superorders within the Nautiloidea, the others being the Plectronoceratoidea from which the others are derived, the Endoceratoidea, Actinoceratoidea, Discosoratoidea, and Nautilitoidea.
Current classifications separate orthoceratoids, endocerids and actinocerids as co-equal taxa to the Nautiloidea, which becomes much reduced in scope. Wade instead proposed separating the Nautiloidea into phylogenetically related superorders while retaining the basic concept of the subclass: externally shelled cephalopods with simple concave septa and retrochoanitic siphuncles from which the convexly septate Ammonoidea with prochoanitic siphuncles are distinguished.
In Kröger 2008, Orthoceratoidea is used as a subclass, referring to McCoy, revising his perception of the order Orthocerida Kuhn as part of the Nautiloidea, in the sense of Sweet 1964 in the Treatise on Invertebrate Paleontology.
In the classification of Teichert, the subclass Orthoceratoidea is expanded to include the orders found in the Plectronoceratoidea and Orthoceratoidea of Wade, combining forms with thick-ringed ventral siphuncles with those with thin-ringed central siphuncles.
The Orthoceratoidea in Kröger includes the Orthocerida, Ascocerida, Pseudorthocerda as in Wade plus the Dissidocerida and Lituitida. The Lituitida, or Lituitiae, has been shown to have evolved from within the Tarphycerida by a straightening of the adult shell, a common character of many tarphycerids. The Dissidocerida, separated from the Orthocerida by Zhuravleva, includes the Troedssonellidae and two small monogeneric orders, the Polymeridae and Rangeroceratidae, both established by Evans.