Orthoceratoidea, from

Ancient Greek Ancient Greek (, ; ) includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Greek ...

ὀρθός (''orthós''), meaning "straight", and κέρας (''kéras''), meaning "horn", is a major subclass of

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

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 An orthocone is the long, cone-shaped shell belonging to several species of ancient nautiloid cephalopod—the prehistoric ancestors of today's marine cephalopod mollusks, including the cuttlefishes, nautiluses, octopuses and squids.; During the ...

(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 Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on the Mohs scale of mineral hardness, based on scratc ...

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 depo ...

Pseudorthocerida Pseudorthocerida is an order of generally straight longiconic (consisting of longicones - long, thin conic shells) orthoceratoids with a subcentral to marginal cyrtochoanitic siphuncle composed of variably expanded segments which may contain in ...

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 related ...

and

Orthocerida Orthocerida, from Ancient Greek ὀρθός (''orthós''), meaning "straight", and κέρας (''kéras''), meaning "horn", also known as the Michelinocerida, is an order of extinct Orthoceratoidea, orthoceratoid cephalopods that lived from the ...

. Orthocerida is a noteworthy

paraphyletic Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...

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

ammonoids Ammonoids are extinct, (typically) coiled-shelled cephalopods comprising the subclass Ammonoidea. They are more closely related to living octopuses, squid, and cuttlefish (which comprise the clade Coleoidea) than they are to nautiluses (family N ...

and living

coleoids Coleoidea or Dibranchiata is one of the two subclasses of cephalopod molluscs containing all the various taxa popularly thought of as "soft-bodied" or "shell-less" (i.e. octopus, squid and cuttlefish). Unlike its extant sister group Nautiloidea ...

(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, from Ancient Greek ἔνδον (''éndon''), meaning "inside", and κέρας (''kéras''), meaning "horn", is an extinct Nautiloidea, nautiloid order, a group of cephalopods from the Lower Paleozoic with cone-like deposits in their s ...

, 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 extinct, (typically) coiled-shelled cephalopods comprising the subclass Ammonoidea. They are more closely related to living octopuses, squid, and cuttlefish (which comprise the clade Coleoidea) than they are to nautiluses (family N ...

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,'' published from 1953–2007 by the Geological Society of America and the University of Kansas, then 2009–present by the University of Kansas Paleontological Institute, is a definitive multi-authore ...

. 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

. 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 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