DID U KNOW ==Flatfish fossils fill in evolutionary link

Amphistium species were early flatfishes The evolution was indeed gradual in nature

Hidden away in museums for more that 100 years, some recently rediscovered flatfish fossils have filled a puzzling gap in the story of evolution and answered a question that initially stumped even Charles Darwin.

Asymmetrical skulls

All adult flatfishes have asymmetrical skulls, with both eyes located on one side of the head. Because these fish lay on their sides at the ocean bottom, this arrangement enhances their vision, with both eyes constantly in play, peering up into the water.

This remarkable arrangement arises during the youth of every flatfish, where the symmetrical larva undergoes a metamorphosis to produce an asymmetrical juvenile. One eye ‘migrates’ up and over the top of the head before coming to rest in the adult position on the opposite side of the skull.

Opponents of evolution, however, insisted that this curious anatomy could not have evolved gradually through natural selection because there would be no apparent evolutionary advantage to a fish with a slightly asymmetrical skull but which retained eyes on opposite sides of the head. No fish — fossil or living — had ever been discovered with such an intermediate condition.

But in the 10 July 2008 issue of Nature, Matt Friedman, graduate student in the Committee on Evolutionary Biology at the University of Chicago draws attention to several examples of such transitional forms that he uncovered in museum collections of underwater fossilized creatures from the Eocene epoch, about 50 million years ago.

A new genus

Friedman examined two primitive flatfishes, Amphistium and a new genus that he named Heteronectes.

Close examination of Amphistium fossils yield clues that they are indeed early flatfishes."

The most primitive flatfishes known, both Amphistium and Heteronectes have many characteristics that are no longer found in modern flatfish.

Partial displacement

But the one that caught Friedman’s attention was the partial displacement of one eye, evident even in the first Amphistium fossil discovered over two centuries ago, according to a University of Chicago press release.

“What we found was an intermediate stage between living flatfishes and the arrangement found in other fishes,” he said.

These two fossil fishes “indicate that the evolution of the profound cranial asymmetry of extant flatfishes was gradual in nature.”