July 11, 2008

Fish Fossil May Resolve Questions On Natural Selection

A researcher from the University of Chicago said newly identified fish fossils discovered in several European museums might resolve a long-standing question about evolutionary theory.

The 50 million-year-old fossils fill in a "missing link" in the evolution of flatfishes and explain one of nature's most extraordinary phenomena, namely how flatfish such as sole, flounder halibut developed the bizarre but useful trait of having both eyes on one side of their head.

Even more extraordinary is the fact that every flatfish is born symmetrical, with one eye on each side of its skull. But as the flatfish develops from a larva to a juvenile, one eye gradually "migrates" up and over the top of the head, coming to rest in its adult position on the opposite side of the skull.

For flatfish, which lie on their sides at the bottom of the sea, unique specialization provides a clear survival advantage in allowing the fish to use both of their eyes to look up. But until now, scientists have not understood how evolutionary forces gave rise to this structural adaptation because there had not been a discovery of any so-called transitional fossils that showed the intermediate steps in the trait's evolution.

"This problem of the evolution of asymmetrical flatfishes was particularly puzzling to biologists because it was very hard to explain what evolutionary forces might have led to this transition," said Matt Friedman, the study's author and a graduate student at the University of Chicago.

"How can you arrive at the pattern seen in living flatfishes via gradual evolution? There seems to be no adaptive reason to start down the gradual evolutionary path toward the flatfish condition, because surely these intermediates would not have any kind of evolutionary advantage," said Friedman, who also serves as a research associate at The Field Museum.

Some view this missing link as a flaw in the theory of natural selection. They argue that intermediate, transitional forms of the fish could not exist because there would be no survival benefit from having one eye that was slightly off center on the opposite side of the head.

Biologists subscribe to the "hopeful monster" theory, which says these changes occurred all at once through a large-scale mutation, which fortunately turned out to be very useful.

But Friedman's find indicates 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 during a Reuters telephone interview.

More than 500 species of modern flatfishes live in fresh and salt water. All have an uncommon flattened body shape well suited to life at the bottom of the sea. Some flatfishes have both eyes on the left side of their head while other families have both eyes on the right side.

The flatfish typically have white or pale undersides, with camouflaged uppersides to blend in with their surroundings.  Some species are even able to change the color of their upperside. Weighing as much as 720 pounds, these carnivorous bottom-feeders vary considerably in size from 4 inches to 7 feet.  

Friedman's study examined several specimens of two kinds of fossil fishes from the Eocene (about 50 million years ago) of northern Italy. One of these is a newly described genus that Friedman named Heteronectes, which means "different swimmer", which he discovered in a museum drawer at the Naturhistorisches Museum in Vienna, Austria.

The other fossil, called Amphistium, is known from several specimens from the same Italian site as Heteronectes and a single fossil from France.  Although it has been known to science, it has been incorrectly classified for more than 100 years. All prior studies of Amphistium erroneously concluded that it had a symmetrical skull.

Examining the anatomy of Amphistium and Heteronectes with a wide range of techniques, including CAT scanning and chemical preparation, which dissolves the rock surrounding the fossil skeleton, Friedman discovered that both represent primitive flatfishes with a somewhat asymmetrical skull.

However, in both species one eye had begun migrating, but had not quite crossed the middle of the head, displaying an intermediate condition between what is found in ordinary symmetrical fishes and extraordinary asymmetrical flatfishes.  The discovery disproves the notion that flatfishes arose suddenly as "hopeful monsters".    

"It's not quite in the Cyclops position," Friedman told Reuters.

The find provides evidence of two steps in the gradual assembly of one of the most bizarre body structures found among vertebrates. But it also raises the question of why this bizarre intermediate form developed.

"It turns out they [flatfish] 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, it would have been advantageous to have a slightly asymmetrical eye arrangement, he said.

Interestingly, Friedman found that while the right eye had migrated in some specimens of Amphistium, it was the left eye that had moved in other specimens. This is different than most living flatfish species, where individuals are always either left or right eyed. It also indicates that mixed 'handedness' is primitive for flatfishes.

"There is a broad implication for the tempo and mode of evolution here," Friedman said.

"Scientists had long assumed flatfishes must have arisen suddenly because they could not imagine the adaptive significance of intermediates, but this work delivers clear evidence that such intermediates did exist, and therefore, that flatfish asymmetry arose gradually."

The fossils provide clues to the lifestyle of these primitive fish, and provide evidence of the evolutionary forces that might have led to their strange anatomy. 

"It is certain that these extinct fishes were predators," Friedman said, noting that one of the Amphistium specimens had preserved the remains its last meal in its stomach: a fish nearly half its own length.

"Many flatfishes lie in wait on their sides to ambush unsuspecting prey, but they don't always lie flat"”they often prop themselves above the seafloor with their fins. It's possible that Amphistium and Heteronectes did the same, and that even incomplete asymmetry would have given them a better view of things above and around them than no asymmetry at all."

But Friedman was cautious about making too many inferences from his discovery.

"Our inability to imagine is what got us into this predicament," he said, in reference to the long-standing flatfish debate.

The new discoveries were published in the journal Nature July 10, 2008. A summary can be viewed here.