''Heteronectes chaneti'' is a fossil fish which has been identified as a primitive flatfish, dating to the early

Eocene The Eocene ( ) Epoch is a geological epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period in the modern Cenozoic Era. The name ''Eocene'' comes from the Ancient Greek (''ēṓs'', " ...

(

Lutetian The Lutetian is, in the geologic timescale, a stage or age in the Eocene. It spans the time between . The Lutetian is preceded by the Ypresian and is followed by the Bartonian. Together with the Bartonian it is sometimes referred to as the ...

stage) of France. ''Heteronectes'' is reported to be a

transitional fossil A transitional fossil is any fossilized remains of a life form that exhibits traits common to both an ancestral group and its derived descendant group. This is especially important where the descendant group is sharply differentiated by gross a ...

. 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''. 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 as difficult to imagine how it could have evolved in a gradual fashion by

natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the heritable traits characteristic of a population over generations. Cha ...

, as proposed by

Charles Darwin Charles Robert Darwin ( ; 12 February 1809 – 19 April 1882) was an English naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all species of life have descended ...

. 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 A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

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