Fish fossils plug hole in evolutionary theory

CHICAGO (Reuters) - 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 on Wednesday.

The 50 million-year-old fossils -- which have one eye near the top of their heads -- help explain how flatfish such 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, whose study appears in the journal Nature.

This missing link in the evolution of flatfishes has been seen as a hole in the theory of natural selection.

The argument is that intermediate forms of these fish could not exist because there would be no survival benefit from having one eye that was slightly off center, but still on the opposite side of the head.

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. "There was no macromutation that all of a sudden gave them both eyes on the same side of the head," he said in a telephone interview.

EYE MIGRATION

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.

"Every flatfish is born symmetrical before it becomes a perversely asymmetrical adult," Friedman said.

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."

"It is a fossil I found unloved in a museum drawer in Vienna covered with about a century of industrial-era soot."

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's not quite in the Cyclops position," Friedman said.

The find raises the question of why this bizarre intermediate form developed. "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.

But Friedman cautioned about making too many inferences.

"Our inability to imagine is what got us into this predicament," he said, referring to the whole flatfish debate.

(Editing by Maggie Fox)