Mystery of flatfish eyes pursued
Find could be the missing link to resolve a problem of evolutionary biology
Julie Steenhuysen
Reuters
Thursday, July 10, 2008
CREDIT: M. Friedman, Reuters
The skulls of both Heteronectes (left) and Amphistium are asymmetrical, providing a 'missing link' in the evolution of flatfishes from symmetrical to asymmetrical. Citharus and Psettodes are modern flatfishes that, like all living members of the group, have both eyes on one side of the head. The arrow points to the migrated eye.
CHICAGO -- Some odd-looking fish fossils discovered in the bowels of several European museums may help solve a lingering question about evolutionary theory, U.S. researchers said Wednesday.
The 50-million-year-old fossils -- which have one eye near the top of their heads -- help explain how such flatfish as flounder, sole and halibut developed the strange but useful trait of having both eyes on one side. For flatfish, which lie on their sides at the bottom of the sea, this arrangement gives them the use of two watchful eyes. But the trait has posed a problem for evolutionary biologists because no one had found any so-called transitional fossils -- fossils showing intermediate steps in the evolution of this trait. "The important thing about this study is it delivers evidence of those intermediates," said Matt Friedman of the Field Museum and the University of Chicago.
Biologists have theorized that maybe the changes occurred all at once with a large-scale mutation. According to this popular "hopeful monster" theory, flatfishes developed this weird trait, which luckily turned out to be very useful.
Friedman's find now suggests that flatfishes followed a more conventional evolutionary plan. More than 500 species of flatfishes live in fresh and salt water. They are born with eyes in the normal spot, but one eye gradually migrates to the other side of the head.
Friedman examined specimens of two kinds of fossil fishes from the Eocene period in northern Italy. One was a new genus that Friedman named Heteronectes or "different swimmer." The other fossil, Amphistium, has been incorrectly assumed to have a symmetrical skull, but Friedman noticed that in some fossils, the eye was slightly out of place.
That inspired him to use computed tomography or CT scans to get a better look at the skulls of these fossils. What he found is that one eye had begun migrating, but had not quite crossed the middle of the head. "It turns out they don't lie flat and completely prone on the sea floor. They actually will prop themselves up slightly [with their fins]," Friedman said. Once in that position, having a slightly asymmetrical eye arrangement must have proved advantageous, he said.
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