Washington, July 10 : Scientists have explained the evolutionary mystery behind the asymmetrical arrangement of eyes on one side of the head of the flatfish.
All adult flatfishes, including the gastronomically familiar flounder, plaice, sole, turbot, and halibut, have asymmetrical skulls, with both eyes located on one side of the head. ecause 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, Matt Friedman, a member of the Department of Geology at the Field Museum in the US, has drawn 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.
The fossils, which Friedman found in museums in England, France, Italy, and Austria, came from limestone quarries in Northern Italy and underneath modern-day Paris.
Friedman examined multiple adult fossil remains of two primitive flatfishes, Amphistium and a new genus that he named Heteronectes.
"Amphistium has been known for quite some time," he said. "The first specimen was described more than 200 years ago, but its placement in the fish evolutionary tree has been uncertain ever since. Close examination of these fossils yield clues that they are indeed early flatfishes," he added.
The most primitive flatfishes known, both Amphistium and Heteronectes, have many characteristics that are no longer found in modern flatfish.
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.
"Most remarkably, orbital migration, the movement of one eye from one side of the skull to the other during the larval stage, was present but incomplete in both of these primitive flatfishes," said Friedman.
"What we found was an intermediate stage between living flatfishes and the arrangement found in other fishes," he added.
These two fossil fishes indicate that the evolution of the profound cranial asymmetry of extant flatfishes was gradual in nature.