Skeleton of a primitive flatfish Heteronectes shown as an x-ray image. (Photo: M. Friedman)

Flatfish mystery solved

UNITED KINGDOM
Wednesday, June 27, 2012, 02:40 (GMT + 9)

A researcher has discovered why flatfish evolved with both of their eyes on one side of their head.

Oxford University researcher Dr Matt Friedman found a fossil fish, named Heteronectes (meaning 'different swimmer'), in 50 million-year-old marine rocks from northern Italy. The findings are described in the latest issue of the Journal of Vertebrate Paleontology.

Friedman’s study provides the first detailed description of a primitive flatfish; he demonstrates that the migrated eye had not yet moved to the opposite side of the fish’s skull in early members of this group. Heteronectes, with its flattened form, finally offers an image of the perfect intermediate stage between most fish with eyes on each side of the head and specialized flatfish where both eyes are located on the same side.

This is a skull of the primitive flatfish Heteronectes, with views of the left- and right-hand sides. The left-hand side shows an eye that has migrated toward the top of the skull, but has not reached the other side, in this adult specimen. (Image by  M. Friedman)

This fossil was taken from Bolca in northern Italy, a site which has been mined for hundreds of years for its fossil fish. This site offers a snapshot of an early coral reef assemblage, Friedman explained.

"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," he commented.

Dr Matt Friedman (Photo: www.earth.ox.ac.uk)

There are about 600 species of flatfish, including flounder, sole and halibut. As free-swimming larvae, these fish have eyes on both sides of their heads; as they mature, however, they lie on the seafloor and, by shifting one eye up and over their body's midline, they avoid pointing one eye down toward the sediment, LiveScience reports.

The specimen discovered in a museum collection in Vienna is the first to show an intermediate stage.

"This is a profound discovery which clearly shows that intermediate fossil forms, which according to certain creationist theories shouldn't exist, are regularly turning up as scientists keep looking for them," added Dr John Long of the Natural History Museum of LA County, an expert in fossil fishes who was not involved in the study.

By Natalia Real
editorial@fis.com
www.fis.com

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