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[[Pleuronectiformes]] (flatfish) are an [[order (biology)|order]] of [[Actinopterygii|ray-finned fish]]. The most obvious characteristic of the modern flatfish is their asymmetry, with both eyes on the same side of the head in the adult fish. In some families the eyes are always on the right side of the body (dextral or right-eyed flatfish) and in others they are always on the left (sinistral or left-eyed flatfish). The primitive [[spiny turbot]]s include equal numbers of right- and left-eyed individuals, and are generally less asymmetrical than the other families. Other distinguishing features of the order are the presence of protrusible eyes, another adaptation to living on the [[seabed]] ([[benthos]]), and the extension of the dorsal fin onto the head.<ref>{{harvnb|Chapleau|Amaoka|1998|pp=223–226}}</ref>
''[[Amphistium]]'' is a 50-million-year-old fossil fish identified as an early relative of the flatfish, and as a transitional fossil.<ref name="NaEvo">{{cite news |last=Minard |first=Anne |date=9 July 2008 |title=Odd Fish Find Contradicts Intelligent-Design Argument |url=http://news.nationalgeographic.com/news/2008/07/080709-evolution-fish.html |work=National Geographic News |___location=Washington, D.C. |publisher=[[National Geographic Society]] |archive-url=https://web.archive.org/web/20080804234422/http://news.nationalgeographic.com/news/2008/07/080709-evolution-fish.html|archive-date=4 August 2008|access-date=2008-07-17}}</ref> In ''Amphistium'', the transition from the typical symmetric head of a vertebrate is incomplete, with one eye placed near the top-center of the head.<ref name="FriedmanFlatfish">{{cite journal |last=Friedman |first=Matt |date=10 July 2008 |title=The evolutionary origin of flatfish asymmetry |journal=Nature |volume=454 |issue=7201 |pages=209–212 |bibcode=2008Natur.454..209F |doi=10.1038/nature07108 |issn=0028-0836 |pmid=18615083 |s2cid=4311712 }}</ref> Paleontologists concluded that "the change happened gradually, in a way consistent with evolution via [[natural selection]]—not suddenly, as researchers once had little choice but to believe."<ref name="NaEvo" />
''Amphistium'' is among the many fossil fish species known from the [[Monte Bolca]] ''[[Lagerstätte]]'' of [[Lutetian]] Italy. ''[[Heteronectes]]'' is a related, and very similar fossil from slightly earlier strata of France.<ref name="FriedmanFlatfish" />
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