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| Percopsiformes Temporal range:
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|---|---|
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| Northern cavefish | |
Scientific classification 
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| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Actinopterygii |
| Superorder: | Paracanthopterygii |
| Order: | Percopsiformes L. S. Berg, 1940 |
| Type species | |
| Percopsis omiscomaycus (Walbaum, 1792)
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| Families | |
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Amblyopsidae (cavefishes) | |
| Synonyms | |
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The Percopsiformes /pɜːrˈkɒpsɪfɔːrmiːz/ are a small order of freshwater teleost fishes measuring less than 20 cm in length, comprising the trout-perch and its allies.[2]: 319 It contains just ten extant species, grouped into seven genera and three families. Five of these genera are monotypic.[3]
They inhabit freshwater habitats in North America, and fossil evidence indicates that they have inhabited this region since the Late Cretaceous, with both suborders having diverged by the Maastrichtian.[4][1] Most species in this order are known from the eastern and central regions of North America, although the two Percopsis species have a primarily boreal and western distribution, with P. omiscomaycus reaching as far north as the Arctic Circle and P. transmontana being restricted to the Pacific Northwest.
They are generally small fish, ranging from 5 to 20 cm (2.0 to 7.9 in) in adult body length. They are grouped together because of technical characteristics of their internal anatomy, and the different species may appear quite different externally.[5] Despite their scientific name and the common names for some taxa, they are not closely related to actual perches in the order Perciformes, and rather represent a freshwater lineage of the otherwise almost entirely marine superorder Paracanthopterygii. They are more closely related to the cods, dories, and the deep-sea tube-eye, and fossil evidence suggests that their closest relative was the extinct order Sphenocephaliformes, comprising two enigmatic genera of Late Cretaceous marine fish, as well as Omosomopsis, another Cretaceous marine fish from Morocco.[4]
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