Heteronectes is a fossil fish which has been identified as a primitive flatfish, dating to the early Eocene (Lutetian stage) of France.

The rest of the skeleton also has some primitive features in common with other Percomorph groups, but absent in living flatfishes.

[1][2] 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, as proposed by Charles Darwin.

Many evolutionary biologists agreed, and suggested that modern flatfish anatomy arose as a result of saltation.

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.