Skull of heteronectes chaneti, showing incomplete orbital migration intermediate between generalized fishes and living flatfishes.
(Credit:
Matt Friedman,
University of Chicago)
Flatfish Fossils Fill In Evolutionary Missing Link
ScienceDaily
(July 10, 2008)
— 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.
Opponents of evolution have insisted that adult flatfishes, which have both eyes on one side of the head,
could not have evolved gradually. A slightly asymmetrical skull offers no advantage. No such fish -- fossil or living -- had ever been discovered, until now.
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. 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 under
goes 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 and a member of the Department of Geology at the Field Museum, draws attention to several examples of such transitional forms that he uncovered in museum collections of under
water fossilized creatures from the Eocene epoch--about 50 million years ago.
"We owe this discovery, in part, to the European fondness for limestone," said Friedman. The fossils, which
he 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."
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," he said, "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." For both sets of fossils, the eye had begun the journey but had not crossed the midline from one side of the fish to the other.
"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."
The Amphistium fossils were known and previously analyzed but not definitively linked to flatfish. Previous studies, relying on conventional techniques, did not detect the oddly shaped skull, but by performing CT scans on the fossils Friedman "unequivocally" demonstrated the cranial asymmetry.
