Heteronectes

Heteronectes chaneti 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. 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.

The evolutionary transition from a symmetric position of eyes in many fish to the position of both eyes on the same side of the head in flatfish was cited as a transition difficult to imagine by St. George Jackson Mivart. This was presented as a difficulty for gradual evolution. The discovery, in 2008, of Heteronectes and Amphistium was considered a vindication of the viability of such a transition.

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