Heteronectes is a fossil fish which has been identified as an early relative of the Flatfish

Flatfish

The flatfish are an order of ray-finned fish, also called the Heterosomata, sometimes classified as a suborder of Perciformes. In many species, both eyes lie on one side of the head, one or the other migrating through and around the head during development...

, and as a Transitional fossil

Transitional fossil

A transitional fossil is any fossilized remains of a lifeform that exhibits characteristics of two distinct taxonomic groups. A transitional fossil is the fossil of an organism near the branching point where major individual lineages diverge...

. 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 than its Italian relative, Amphistium

Amphistium

Amphistium is a fossil fish which has been identified as an early relative of the Flatfish, and as a Transitional fossil. In a typical modern flatfish, the head is asymmetric with both eyes on one side of the head...

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Heteronectes is found in the early to middle Eocene of France.