Orthoceratoidea is a major subclass of

nautiloid Nautiloids are a group of marine cephalopods ( Mollusca) which originated in the Late Cambrian and are represented today by the living ''Nautilus'' and '' Allonautilus''. Fossil nautiloids are diverse and speciose, with over 2,500 recorded speci ...

cephalopods A cephalopod is any member of the molluscan Taxonomic rank, class Cephalopoda (Greek language, Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral symm ...

. Members of this subclass usually have orthoconic (straight) to slightly cyrtoconic (curved) shells, and central to subcentral siphuncles which may bear internal deposits. Orthoceratoids are also characterized by dorsomyarian muscle scars (a small number of large scars concentrated at the top of the body chamber), extensive cameral deposits, and calciosiphonate connecting rings with a porous and calcitic inner layer. Currently, Orthoceratoidea comprises the orders Riocerida, Dissidocerida,

Actinocerida The Actinocerida are an order of generally straight, medium to large cephalopods that lived during the early and middle Paleozoic, distinguished by a siphuncle composed of expanded segments that extend into the adjacent chambers, in which deposit ...

Pseudorthocerida Pseudorthocerida is an order of generally straight longiconic orthoceratoids with a subcentral to marginal cyrtochoanitic siphuncle composed of variably expanded segments which may contain internal deposits that may develop into a continuous pa ...

, Lituitida and

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

. Orthocerida is a noteworthy

paraphyletic In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...

order which is ancestral to the major cephalopod groups such as the extinct ammonoids and living

coleoids Subclass Coleoidea, or Dibranchiata, is the grouping of cephalopods containing all the various taxa popularly thought of as "soft-bodied" or "shell-less" (i.e., octopuses, squid and cuttlefish). Unlike its extant sister group, Nautiloidea, whose ...

(cephalopods without external shells, including squids, octopus, cuttlefish, etc.).

Taxonomy

As a superorder, Orthoceratiodea was one of six superorders within the Nautiloidea, the others being the Plectronoceratoidea (= Ellesmeroceratoidea of some) from which the others are derived, the Endoceratoidea, Actinoceratoidea, Discosoratoidea, and

Nautilitoidea The Nautilitoidea is a superorder within the subclass Nautiloidea, comprising the phylogenetically related Nautilida, Oncocerida, and Tarphycerida. The superorder has its roots in the Bassleroceratidae, the ancestral family of the Tarphycerida, ...

. Current classifications (e.g. Kröger 2008 and Teichert 1980) separate orthoceratoids, endocerids and actinocerids as co-equal taxa to the Nautiloidea, which becomes much reduced in scope. Wade (1988) 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 Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid and cuttl ...

with prochoanitic siphuncles are distinguished. In Kröger 2008, Orthoceratoidea is used as a subclass, referring to McCoy (1844), revising his (Kröger 2004) perception of the order Orthocerida Kuhn (1940) as part of the Nautiloidea, in the sense of Sweet 1964 in the

Treatise on Invertebrate Paleontology The ''Treatise on Invertebrate Paleontology'' (or ''TIP'') published by the Geological Society of America and the University of Kansas Press, is a definitive multi-authored work of some 50 volumes, written by more than 300 paleontologists, and co ...

. In the classification of Teichert (1988), the subclass Orthoceratoidea is expanded to include the orders found in the Plectronoceratoidea and Orthoceratoidea of Wade (1988), combining forms with thick-ringed ventral siphuncles with those with thin-ringed central siphuncles. The Orthoceratoidea in Kröger (2008) includes the Orthocerida, Ascocerida, Pseudorthocerda as in Wade (1988) plus the Dissidocerida and Lituitida. The Lituitida, or Lituitiae, has been shown to have evolved from within the

Tarphycerida The Tarphycerida were the first of the coiled cephalopods, found in marine sediments from the Lower Ordovician (middle and upper Canad) to the Middle Devonian. Some, such as ''Aphetoceras'' and ''Estonioceras'', are loosely coiled and gyroconic; ...

by a straightening of the adult shell, a common character of many tarphycerids. The Dissidocerida, separated from the Orthocerida by Zhuravleva (1964), includes the Troedssonellidae and two small monogeneric orders, the Polymeridae and Rangeroceratidae, both established by Evans (2005).

References

External links

*Flower 1950 in Flower and Kummel 1950, A Classification of the Nautiloidea; ''Journal of Paleontology'' 24(5):604-616, Sept 1950 *Kroger 2008, Brief Report, A new genus of middle Tremadocian orthoceratoids and the Early Ordovician origin of orthoceratoid cephalopods; Acta Palaeontologica Polonica 53 (4): 745–749, 2008 *Kroger 2004, Revision of Middle Ordovician orthoceratacean nautiloids from Baltoscandia; Acta Palaeontologica Polonica 49 (1): 57–74. *Stanley & Techert 1976; Lamellorthoceratida (Cephalopoda, Orthoceratoidea) from the Lower Devonian of New York; the University of Kansas Paleontological Contributions, Nov.12, 1976, Paper 86. *Sweet, W..C. 1964. Nautiloidea- Orthocerida; Treatise on Invertebrate Paleontology, Part K, Teichert and Moore (eds) Geological Society of America and University of Kansas Press. *Teichert C, 1988, Main Features of Cephalopod Evolution, Ch 2 in The Mollusca Vol 12 Paleontology and Neontology of Cephalopods, Clarke & Trueman (eds) Academic Press. *Wade, M 1988. Nautiloids and their descendants: cephalopod classification in 1986; New Mexico Bureau of Mines & Mineral Resources Memoir 44, Oct 1988 :15-25. {{Taxonbar, from=Q7104418 Mollusc subclasses Mesozoic cephalopods Paleozoic cephalopods Ordovician first appearances Triassic extinctions Paraphyletic groups