Temporal range: Middle Eocene
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.
- Matt Friedman (2008-07-10). "The evolutionary origin of flatfish asymmetry". Nature. 454 (7201): 209–212. doi:10.1038/nature07108. PMID 18615083.
- Matt Friedman (2012). "Osteology of Heteronectes chantey (Acanthomorpha, Pleuronectiformes), an Eocene stem flatfish, with a discussion of flatfish sister-group relationships". Journal of Vertebrate Paleontology. 21 (4): 735–756. doi:10.1080/02724634.2012.661352.
- Zimmer, Carl (2008). "The Evolution of Extraordinary Eyes: The Cases of Flatfishes and Stalk-eyed Flies". Evolution Education Outreach. 1: 487. doi:10.1007/s12052-008-0089-9.
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