Published online 9 July 2008 | Nature | doi:10.1038/news.2008.946

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Fossilized flatfish settle evolutionary conundrum.

A trio of fossilized fish has finally settled an evolutionary conundrum that once puzzled Charles Darwin.

Fossilized fish provide a snapshot of evolution in action. Nature

The flatfish has always been regarded as an oddity. Although the immature fish has eyes on opposite sides of its head, one of the eyes migrates around its skull before it reaches maturity. Yet there was no evidence for this development process in the fossil record.

Some evolutionary biologists, including Darwin, have argued that the trait evolved gradually over many generations of flatfish. If true, intermediate flatfish with partially offset eyes would once have lived — but no such fossils have ever been identified, giving succour to both creationists and those arguing for sudden jumps in evolution.

But Matt Friedman, a PhD student studying evolutionary biology at the University of Chicago in Illinois, has now found three examples of these transitional forms. In the process, he unearthed an entirely new species of ancient flatfish in Vienna and re-interpreted already known fossil fish in London. His work is reported in this week's Nature1 (see video).

Fossil power

Friedman says that the fossils are important because "they help to settle a long-standing evolutionary debate and shed light on the mode and tempo of evolutionary change". "From my standpoint as a palaeontologist and evolutionary biologist, I"m pleased that I've been able to showcase the power of fossil data in solving a problem that seemed so baffling from examining living diversity alone," he adds.

The new species, named Heteronectes chaneti, was unearthed in the Natural History Museum in Vienna.

"When I first noticed the fossil, it was sitting unidentified in a drawer of indeterminate fossil fish pieces from Monte Bolca [in Italy]," he says. "And believe me, it didn’t look like much at the time — it was an incomplete specimen covered with dust and soot."

After applying weak acid to the rock and exposing the fossil it became clear that it was in fact a new species, and one with a clearly asymmetrical skull.

Friedman also used an X-ray technique called computed tomography to analyse two specimens of a known species called Amphistium residing in the Natural History Museum in London. Although palaeontologists had noticed that this animal was 'flatfish-like', Friedman says that they had thought the fossil was symmetrical and had been distorted in the fossilization process.

Friedman's scans show that the degree of asymmetry of Amphistium is the same in both specimens, suggesting that it is not due to such distortion.

Flatfish eyes move early during their development but Friedman says the specimens he describes are clearly adult, as they are too large to be very young fish. And the bones of living flatfish do not ossify until after metamorphosis — in these fossils, the bones are present and fully formed.

"There are, as we sadly know, people who are uncomfortable about the idea of evolution," says Per Ahlberg of Uppsala University in Sweden. "It's always nice to demonstrate that even for major morphological transitions, the steps are in the evolutionary record."

"From a more scientific point of view, these are the changes that are most difficult to understand," he adds. "It's all very well knowing it must have happened, but that’s still not a complete picture of how this works."

It's possible that the asymmetrical eyes may have allowed the creatures to bottom-feed, watching for predators above while lifting themselves up on their fins to look for prey on the sea floor.

Basement discoveries

Friedman is making something of a career finding things that others have missed in dusty collections. In 2006 he authored a paper with his colleague Michael Coates which showed a fossil previously thought to be a ray-finned fish was in fact a coelacanth2.

"Many of my most interesting discoveries have been made in museum basements rather than in a quarry at some exotic field site," he says.

"I suppose there is a general perception that museum collections are dusty, static archives, and that everything in them has been carefully studied and precisely identified. But the truth is that they are much more than just long-term storage, because as our interpretive framework matures, we can begin to make sense of specimens that evaded or baffled earlier generations of researchers, or draw new conclusions about materials we mistakenly thought we had figured out ages ago."

Zerina Johanson, curator for fossil fishes at London's Natural History Museum, agrees: "There are definitely many more discoveries to be made in fossil fish collections."