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 species ...

cephalopods. Members of this subclass usually have

orthoconic An orthocone is an unusually long straight shell of a nautiloid cephalopod.; During the 18th and 19th centuries, all shells of this type were named '' Orthoceras'', creating a wastebasket taxon, but it is now known that many groups of nautiloids ...

(straight) to slightly cyrtoconic (curved) shells, and central to subcentral

siphuncles 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 ...

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 Riocerida, originally named as Rioceratida, is an extinct order of Ordovician nautiloid cephalopods. They were the earliest-diverging members of the major group Orthoceratoidea, with which they share dorsomyarian muscle scars (a few large muscle ...

Dissidocerida Dissidocerida is an order of Early Ordovician to the Early Silurian orthoceratoid cephalopods in which the siphuncle has a continuous lining or a longitudinal rod-like structure within. The order Dissidocerida was proposed by Zhuravleva (1994) ...

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 p ...

Lituitida Lituitida is an order of orthoceratoid cephalopods. They correspond to the family Lituitidae of the Treatise (Furnish & Glenister, 1964), reranked as an order and combined with other orthoceratoids. They are considered to be more closely relate ...

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 ...

. 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 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 cuttlefish) ...

and living coleoids (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 Plectronoceratoidea is a superorder or subclass containing primitive nautiloids from the Late Cambrian and Early Ordovician. This group is best considered a paraphyletic grade of early cephalopods, as it contains the ancestors of subsequent post ...

(= Ellesmeroceratoidea of some) from which the others are derived, the

Endoceratoidea Endocerida is an extinct nautiloid order, a group of cephalopods from the Lower Paleozoic with cone-like deposits in their siphuncle. Endocerida was a diverse group of cephalopods that lived from the Early Ordovician possibly to the Late Silurian ...

, Actinoceratoidea, Discosoratoidea, and Nautilitoidea. 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 cuttle ...

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

and

. 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 gyroconi ...

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 Troedssonellidae is a family of orthoceroid cephalopods from the Ordovician, derived from rod-bearing Baltoceratidae, that have a continuous lining within the siphuncle that resembles very thin and slender endocones. Shells are generally slend ...

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