How Did Flatfish Develop With Eyes on Same Side of Their Head?

Mystery of the flatfish's head is solved.

By David Alderton | July 2, 2012

Click image to enlarge

This is the skeleton of the primitive flatfish Heteronectes chaneti shown as an x ray image (top), photograph before preparation (middle), and photograph after preparation (bottom). Photo credit: M. Friedman.

It has been one of the great puzzles in the field of piscine development. How did flatfish, such as soles, develop their highly unusual asymmetrical bodies, and end up with both eyes on the same side of their head?

Oxford University researcher Dr. Matt Friedman has unlocked this mystery. He has identified a fossilized fish in rocks made up of marine sediment that shows the transformation in action. Originating in what is now northern Italy, this primitive flatfish has been called Heteronectes, which means “different swimmer.” It has a flattened body, like such fish in the ocean today, but in its case, the second eye had not yet crossed to the other side of the skull.

An Important Site
Dating back 50 million years, Heteronectes lived on what was a coral reef. "This fossil comes from Bolca, a site in Italy that has literally been mined for hundreds of years for its fossil fishes. This remarkable site provides a snapshot of an early coral reef assemblage. Reefs are well known as biodiversity hotspots, so it is perhaps not surprising that Bolca provides us with the first evidence of many modern fish groups," says Friedman.

"Our understanding of the relationships of some of these groups is in a state of change with the increasing influx of molecular genetic studies. Fossils have not contributed very much to this debate, but specimens like that of Heteronectes reveal the superb level of detail that can be extracted from extinct species."

Hidden away
Friedman notes that "The specimen itself was discovered—with no identification—in a museum collection in Vienna. It just goes to show that even well-known fossil sites can yield important surprises, and that not all new discoveries take place in the field."

"This is a profound discovery which clearly shows that intermediate fossil forms are regularly turning up as scientists keep looking for them," says Dr. John Long of the Natural History Museum of L.A. County, an expert in fossil fishes who was not involved in the study.

Reference: Friedman, M. Osteology of Heteronectes chaneti (Acanthomorpha, Pleuronectiformes), an Eocene stem flatfish, with a discussion of flatfish sister-group relationships. Journal of Vertebrate Paleontology, 2012; (32)4: 735-756