"We do not deny that there is minor variation that occurs within created groups or kinds," he said, adding that he fails to see the new paper as evidence of a progression from one flatfish form to another.
"Fish have always been fish, all the way down to the lower Cambrian [roughly 542 to 488 million years ago]," he added.
"We have no problem with the variation within flatfish. What we're asking is, Show me how a fish came from a nonfish ancestor."
Part of the argument is that the asymmetrical eye configuration can easily be seen as intelligent, because it is advantageous to flatfish survival.
The feature allows flatfishes to use both of their eyes to look up when lying on the seafloor—part of a suite of adaptations that includes a "top" side camouflaged to fit the fishes' surroundings.
Hiding in Plain Sight
Paleontologist Matt Friedman, the new study's author, visited natural history museums in London, Vienna, and elsewhere to study some of the oldest known flatfish fossils.
Using CT scans, he imaged the bone structures around the ancient fishes' eyes.
In more than one specimen, "one side of the skull looked normal," said Friedman, who is affiliated with the University of Chicago and Chicago's Field Museum.
"But on the other side of the head, the eye was moved up."
It's possible that even the intermediate eye position would have provided an evolutionary advantage for the fish, he said.
"Living flatfish often don't lie completely flat on the sea floor," he said—they prop themselves up with their fins.
"Once you get that extra degree of movement, having a slightly shifted eye is going to be a lot better than having no shifted eye at all," said Friedman, whose study will be published tomorrow in the journal Nature.
Fossils from excavations in northern Italy and Paris revealed that the intermediate specimens once lived together with flatfishes having both eyes on one side of the skull, he said.
It's possible that the more modern forms eventually outcompeted the intermediate versions, Friedman added.
More than 500 species of flatfishes now live in fresh and salt water. They range in size from four inches to seven feet and can weigh up to 720 pounds (327 kilograms).
Though known for their odd eye arrangement, no flatfish start life that way. Each is born symmetrical, with one eye on each side of its skull.
As a flatfish develops from a larva to a juvenile, one eye migrates up and over the top of the head, coming to rest in its adult position on the opposite side of the skull.
The change leaves the young fish baffled, and they swim at bizarre angles until they adapt, said evolutionary biologist Richard Palmer of the University of Alberta in Canada.
Palmer added that the new work is "a fantastic paper" that helps resolve a mystery "that's bedeviled evolutionary biologists for more than a century.
"It's really been a major, major puzzle to evolutionary biologists."
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