Editing Flatfish (section)

==Origin and evolution==
Scientists have been proposing since the 1910s that flatfishes evolved from [[Percoidea|percoid]] ancestors.<ref name="Regan_1910">Regan C.T. (1910). "The origin and evolution of the Teleostean fishes of the order Heterosomata". ''Annals and Magazine of Natural History'' '''6'''(35): p. 484-496. [https://doi.org/10.1080/00222931008692879 doi.org/10.1080/00222931008692879]</ref> There has been some disagreement whether they are a monophyletic group. Some palaeontologists think that some percomorph groups other than flatfishes were "experimenting" with head asymmetry during the [[Eocene]],<ref name="Bannikov&Zorzin_2019"></ref><ref name="Bannikov&Zorzin_2020"></ref> and certain molecular studies conclude that the primitive family of [[Psettodidae]] evolved their flat bodies and asymmetrical head independently of other flatfish groups.<ref name="Campbell_et.al_2013">Campbell M.A., Chen W-J. & López J.A. (2013). "Are flatfishes (Pleuronectiformes) monophyletic?". ''Molecular Phylogenetics and Evolution'' '''69'''(3): p. 664-673. [https://doi.org/10.1016/j.ympev.2013.07.011 doi.org/10.1016/j.ympev.2013.07.011]</ref><ref name="Campbell_et.al_2014">Campbell M.A., López J.A., Satoh T.P., Chen W-J. & Miya M. (2014). "Mitochondrial genomic investigation of flatfish monophyly". ''Gene'' '''551'''(2): p. 176-182. [https://doi.org/10.1016/j.gene.2014.08.053 doi.org/10.1016/j.gene.2014.08.053]</ref> Many scientists, however, argue that pleuronectiformes are [[monophyletic]].<ref name="Duarte-Ribeiro et.al.2024">Duarte-Ribeiro E, Rosas-Puchuri U, Friedman M, Woodruff G.C., Hughes L.C., Carpenter K.E., White W.T., Pogonoski J.J., Westneat M, Diaz de Astarloa J.M., Williams J.T., Santos M.D., Domínguez-Domínguez O, Ortí G, Arcila D & Betancur-R R. (2024). "Phylogenomic and comparative genomic analyses support a single evolutionary origin of flatfish asymmetry". ''Nature Genetics'' '''56''': p. 1069-1072. [https://www.nature.com/articles/s41588-024-01784-w doi.org/10.1038/s41588-024-01784-w]</ref> 

The fossil record indicates that flatfishes might have been present before the [[Eocene]], based on fossil [[otolith]]s resembling those of modern pleuronectiforms dating back to the [[Thanetian]] and [[Ypresian]] stages (57-53 million years ago).<ref name="Schwarzhans1999">Schwarzhans W. (1999). "A comparative morphological treatise of recent and fossil otoliths of the order Pleuronectiformes". ''Piscium Catalogus. Otolithi Piscium 2''. [[doi:10.13140/2.1.1725.5043]]</ref>

Flatfishes have been cited as dramatic examples of evolutionary adaptation. [[Richard Dawkins]], in ''[[The Blind Watchmaker]]'', explains the flatfishes' evolutionary history thus:

<blockquote>...bony fish as a rule have a marked tendency to be flattened in a vertical direction.... It was natural, therefore, that when the ancestors of [flatfish] took to the sea bottom, they should have lain on one ''side''....  But this raised the problem that one eye was always looking down into the sand and was effectively useless. In evolution this problem was solved by the lower eye 'moving' round to the upper side.<ref>{{cite book|last = Dawkins|first = Richard|author-link = Richard Dawkins|title = The Blind Watchmaker|publisher = Penguin Books|year = 1991|location = London|page=92|isbn = 0-14-014481-1}}</ref></blockquote>

[[File:Amphistium.JPG|thumb|left|Fossil of ''[[Amphistium]]''.]]
The origin of the unusual morphology of flatfishes was enigmatic up to the 2000s, and early researchers suggested that it came about as a result of [[saltation (biology)|saltation]] rather than gradual evolution through natural selection, because a partially migrated eye were considered to have been maladaptive. This started to change in 2008 with a study on the two fossil genera ''[[Amphistium]]'' and ''[[Heteronectes]]'', dated to about 50 million years ago. These genera retain primitive features not seen in modern types of flatfishes. In addition, their heads are less asymmetric than modern flatfishes, retaining one eye on each side of their heads, although the eye on one side is closer to the top of the head than on the other.<ref name="Friedman_2008">Friedman M. (2008). "The evolutionary origin of flatfish asymmetry". ''Nature'' '''454'''(7201): p. 209–212. [[doi:10.1038/nature07108]]</ref><ref name="NaEvo">{{cite news | url=http://news.nationalgeographic.com/news/2008/07/080709-evolution-fish.html  | archive-url=https://web.archive.org/web/20080711035445/http://news.nationalgeographic.com/news/2008/07/080709-evolution-fish.html  | url-status=dead  | archive-date=July 11, 2008  | title=Odd Fish Find Contradicts Intelligent-Design Argument  | publisher=[[National Geographic Society|National Geographic]] |date= July 9, 2008 | access-date =2008-07-17}}</ref> The more recently described fossil genera ''[[Quasinectes]]''<ref name="Bannikov&Zorzin_2019">Bannikov A.F. & Zorzin R (2019). [https://museodistorianaturale.comune.verona.it/media/_Musei/_StoriaNaturale/_Allegati/Biblioteca/Studi%20Bolca/Vol.%2019(2019)/5-15_Bannikov-Zorzin.pdf "A new genus and species of ''incertae sedis'' percomorph fish (Perciformes) from the Eocene of Bolca in northern Italy, and a new genus for ''Psettopsis latellai'' Bannikov, 2005"]. ''Studi e ricerche sui giacimenti terziari di Bolca'': p. 5-15.</ref> and ''[[Anorevus]]''<ref name="Bannikov&Zorzin_2020">Bannikov A.F. & Zorzin R. (2020). [https://www.researchgate.net/publication/344613829 "A new genus and species of percomorph fish ("stem pleuronectiform") from the Eocene of Bolca in northern Italy"]. ''Miscellanea Paleontologica'' '''17''': p. 5–14</ref> have been proposed to show similar morphologies and have also been classified as "stem pleuronectiforms". Suchs findings lead Friedman to conclude that the evolution of flatfish morphology "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"/> 

To explain the survival advantage of a partially migrated eye, it has been proposed that primitive flatfishes like ''Amphistium'' rested with the head propped up above the seafloor (a behaviour sometimes observed in modern flatfishes), enabling them to use their partially migrated eye to see things closer to the seafloor.<ref name="Janvier 2008 p169-170">Janvier P. (2008). "Squint of the fossil flatfish". ''Nature'' '''454'''(7201): p. 169–170</ref>

While known basal genera like ''Amphistium'' and ''Heteronectes'' support a gradual acquisition of the flatfish morphology, they were probably not direct ancestors to living pleuronectiforms, as fossil evidence indicate that most flatfish lineages living today were present in the [[Eocene]] and contemporaneous with them.<ref name="Friedman_2008"></ref> It has been suggested that the more primitive forms were eventually outcompeted.<ref name="NaEvo"/>

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<gallery mode="packed">
File:Pleuronectes platessa.jpg|The [[European plaice]] is the principal commercial flatfish in Europe.
File:Lined sole.jpg|[[American sole]]s are found in both freshwater and marine environments of the Americas.
File:Alaska 2007 071.jpg|[[Halibut]] are the largest of the flatfishes, and provide lucrative fisheries.
File:Psetta maxima Luc Viatour.jpg|The [[turbot]] is a large, left-eyed flatfish found in sandy shallow coastal waters around Europe.
File:Flatfish-lefteyed-flounder.jpg|Flatfish (left‐eyed flounder)
</gallery>