Stenopterygii are a

superorder Order () is one of the eight major hierarchical taxonomic ranks in Linnaean taxonomy. It is classified between family and class. In biological classification, the order is a taxonomic rank used in the classification of organisms and recognized ...

ray-finned fish Actinopterygii (; ), members of which are known as ray-finned fish or actinopterygians, is a class of bony fish that comprise over 50% of living vertebrate species. They are so called because of their lightly built fins made of webbings of sk ...

in the

infraclass In biology, taxonomic rank (which some authors prefer to call nomenclatural rank because ranking is part of nomenclature rather than taxonomy proper, according to some definitions of these terms) is the relative or absolute level of a group of or ...

Teleostei Teleostei (; Ancient Greek, Greek ''teleios'' "complete" + ''osteon'' "bone"), members of which are known as teleosts (), is, by far, the largest group of ray-finned fishes (class Actinopterygii), with 96% of all neontology, extant species of f ...

. Their validity is somewhat doubtful, as the group was established to separate, out of a large group of closely related Teleostei, a mere two rather peculiarly

autapomorph In phylogenetics, an autapomorphy is a distinctive feature, known as a derived trait, that is unique to a given taxon. That is, it is found only in one taxon, but not found in any others or outgroup taxa, not even those most closely related to ...

order Order, ORDER or Orders may refer to: * A socio-political or established or existing order, e.g. World order, Ancien Regime, Pax Britannica * Categorization, the process in which ideas and objects are recognized, differentiated, and understood ...

s at best. In some treatments, it is even

monotypic In biology, a monotypic taxon is a taxonomic group (taxon) that contains only one immediately subordinate taxon. A monotypic species is one that does not include subspecies or smaller, infraspecific taxa. In the case of genera, the term "unisp ...

. As originally conceived, the "Stenopterygii" include the

Ateleopodiformes The jellynose fishes or tadpole fishes are the small order Ateleopodiformes. This group of ray-finned fish is monotypic, containing a single family Ateleopodidae. It has about a dozen species in four genera, but these enigmatic fishes are in nee ...

and

Stomiiformes Stomiiformes is an order (biology), order of deep-sea ray-finned fishes of very diverse Morphology (biology), morphology. It includes, for example, Barbeled dragonfish, dragonfishes, lightfishes (Gonostomatidae and Phosichthyidae), loosejaws, ma ...

. Sometimes, the former are removed to form a monotypic superorder Ateleopodomorpha. These lineages are moderately advanced teleosts, but each is uniquely

adapted In biology, adaptation has three related meanings. Firstly, it is the dynamic evolutionary process of natural selection that fits organisms to their environment, enhancing their evolutionary fitness. Secondly, it is a state reached by the p ...

to a deep-water

ocean The ocean is the body of salt water that covers approximately 70.8% of Earth. The ocean is conventionally divided into large bodies of water, which are also referred to as ''oceans'' (the Pacific, Atlantic, Indian Ocean, Indian, Southern Ocean ...

ic environment. Their

plesiomorph In phylogenetics, a plesiomorphy ("near form") and symplesiomorphy are synonyms for an ancestral Phenotypic trait, character shared by all members of a clade, which does not distinguish the clade from other clades. Plesiomorphy, symplesiomorph ...

ies are similar to the

Salmoniformes Salmoniformes (, lit. "salmon-shaped") is an Order (biology), order of Actinopterygii, ray-finned fishes native to the temperate and subarctic Northern Hemisphere. It contains two suborders: Salmonoidei (containing only the Salmonidae) and Esocoi ...

and other

Protacanthopterygii Protacanthopterygii is a superorder of ray-finned fish. They inhabit both Marine (ocean), marine and freshwater habitats. They appear to have evolved in the Cretaceous or perhaps late Jurassic, originating probably roughly 150 million years ago; ...

, but plesiomorphic traits are no reliable indicator of a close relationship. Nonwithstanding, the "Stenopterygii" appear to be close relatives of the Protacanthopterygii. Some

cladistic Cladistics ( ; from Ancient Greek 'branch') is an approach to biological classification in which organisms are categorized in groups ("clades") based on hypotheses of most recent common ancestry. The evidence for hypothesized relationships is ...

analyses find at least the Stomiiformes deep within the latter superorder.Nelson (2006): pp.207-208,212-213, Diogo (2008) It thus cannot be ruled out that the "Stenopterygii" are better included in the Protacanthopterygii. An alternative approach is to consider the entire group – "Stenopterygii", Protacanthopterygii, the monotypic superorders " Cyclosquamata", and perhaps the monotypic superorders " Lampridiomorpha" and " Scopelomorpha" – an unranked

clade In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...

named Euteleostei. But this would probably require splitting up the Protacanthopterygii, so that each euteleost superorder would at most contain 2 orders (though most would be monotypic), as the relative placement of all these groups is not very well resolved. Such a profusion of monotypic

taxa In biology, a taxon (back-formation from ''taxonomy''; : taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular name and ...

is generally rejected by modern

taxonomy image:Hierarchical clustering diagram.png, 280px, Generalized scheme of taxonomy Taxonomy is a practice and science concerned with classification or categorization. Typically, there are two parts to it: the development of an underlying scheme o ...

, if it is not absolutely required to adequately reflect

phylogeny A phylogenetic tree or phylogeny is a graphical representation which shows the evolutionary history between a set of species or Taxon, taxa during a specific time.Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, M ...

.Diogo (2008) Moreover, owing to the uncertain relationships of the euteleosts to the less advanced

Otocephala Otocephala is a clade of ray-finned fishes within the infraclass Teleostei that evolved some 230 million years ago. It is named for the presence of a hearing (otophysic) link from the swimbladder to the inner ear. Other names proposed for the gro ...

(which include the superorders

Clupeomorpha Clupeomorpha is a superorder of ray-finned fish which belongs to the clade Otocephala. Represented today only by the diverse, economically-important order Clupeiformes (containing herrings, anchovies and allies), it was formerly even more diverse ...

and

Ostariophysi Ostariophysi is the second-largest superorder of fish. Members of this superorder are called ostariophysians. This diverse group contains 10,758 species, about 28% of known fish species in the world and 68% of freshwater species, and are present ...

) and the more advanced teleosts (e.g.

Acanthopterygii Acanthopterygii (meaning "spiny-finned one") is a superorder of teleost, bony fishes in the class Actinopterygii. Members of this superorder are sometimes called ray-finned fishes for the characteristic sharp, bony rays in their fins; however th ...

), it may well be that the supposed "clade" is merely an

evolutionary grade A grade is a taxon united by a level of morphological or physiological complexity. The term was coined by British biologist Julian Huxley, to contrast with clade, a strictly phylogenetic unit. Phylogenetics The concept of evolutionary grades ...

. Such a

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 ...

group would according to modern understanding not warrant a taxon name at all. In this case, a possible solution would be to expand the Protacanthopterygii to subsume all the small "superorders", and to treat the Euteleostei as including the Protacanthopterygii and their more "modern" relatives.

Footnotes

References

* (2008): On the cephalic and pectoral girdle muscles of the deep sea fish ''Alepocephalus rostratus'', with comments on the functional morphology and phylogenetic relationships of the Alepocephaloidei (Teleostei). '' Anim. Biol.'' 58(1): 23-29. * (2006): ''

Fishes of the World ''Fishes of the World'' is a standard reference for the systematics of fishes. It was first written in 1976 by the American ichthyologist Joseph S. Nelson (1937–2011). Now in its fifth edition (2016), the work is a comprehensive overview of t ...

'' (4th ed.). John Wiley & Sons, Inc. {{Taxonbar, from=Q616727 Euteleostei Fish superorders Polyphyletic groups