''Heteronectes chaneti'' is a fossil fish which has been identified as a primitive

flatfish A flatfish is a member of the Ray-finned fish, ray-finned demersal fish order (biology), order Pleuronectiformes, also called the Heterosomata, sometimes classified as a suborder of Perciformes. In many species, both eyes lie on one side of the ...

, dating to the early Eocene ( Lutetian stage) of France. ''Heteronectes'' is reported to be a transitional fossil. In a typical modern flatfish, the head is asymmetric with both eyes on one side of the head. In ''Heteronectes'', the transition from the typical symmetric head of a vertebrate is incomplete, with one eye positioned near the top of the head, very similar (but less so) to its Italian relative ''

Amphistium ''Amphistium paradoxum'' (from el, ἀμφί , 'on both sides', el, ιστίον 'sail', and el, παράδοξος 'extraordinary'), the only species classified under the genus ''Amphistium'', is a fossil fish which has been identified as a ...

''. The rest of the skeleton also has some primitive features in common with other Percomorph groups, but absent in living flatfishes.Friedman M (2008). “The evolutionary origin of flatfish asymmetry”, ''Nature'' 454(7201): p. 209-212: The condition in modern, bottom-dwelling flatfish with both eyes on the same side of the head was cited by

St. George Jackson Mivart St. George Jackson Mivart (30 November 1827 – 1 April 1900) was an English biologist. He is famous for starting as an ardent believer in natural selection who later became one of its fiercest critics. Mivart attempted to reconcile D ...

as difficult to imagine how it could have evolved in a gradual fashion by natural selection, as proposed by Charles Darwin. Many evolutionary biologists agreed, and suggested that modern flatfish anatomy arose as a result of saltation. The 2008 discovery of ''Heteronectes'' and ''Amphistium'' was considered a vindication of the viability of a gradual transition.Minard A. “Odd Fish Find Contradicts Intelligent-Design Argument”, ''National Geographic News'' 9/7/2008 Friedman suggested that ''Heteronectes'' and ''Amphistium'' did not rest completely on the sea floor like modern flatfishes. Instead, they might have only held their tail to the sea floor and kept their head lifted into the water above, using one eye to watch for predators, while the other was used to look for prey in the mud below. Janvier P (2008), “Squint of the fossil flatfish”, ''Nature'' 454(7201): p. 169-170 From previous fossil findings, Friedman also notes that several modern families of flatfish seems to have coexisted with ''Heteronectes'' and ''Amphistium'', and speculated that the modern ones eventually outcompeted their primitive relatives.

References

Pleuronectiformes Transitional fossils Prehistoric ray-finned fish genera Eocene fish of Europe Fossil taxa described in 2008 {{paleo-rayfinned-fish-stub