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The Times The evolutionary origins of flatfish have finally been explained, 150 years after Charles Darwin grappled unsuccessfully with the problem. 
Darwin was troubled by how fish such as flounder and sole had made the seemingly improbable evolutionary leap from being symmetrical to having two eyes on the same side of their face. Now, a collection of ancient fossilised specimens that were halfway to becoming flatfish has offered evidence that the unusual skull anatomy evolved over millions of years. "Flatfishes and their profoundly asymmetrical skulls have been enlisted in many arguments against gradual evolutionary change precisely because it is difficult to imagine how intermediate forms might have been adaptive," said Matt Friedman, a palaeobiologist at the University of Chicago. The previous absence of any intermediate fossils led some to conclude that the peculiar body plan arose suddenly through large-scale genetic mutations. But speaking yesterday at the annual meeting of the Society of Vertebrate Palaeontology at the University of Bristol, Dr Friedman said that his findings showed that flatfish evolved through tiny adaptive steps. The 50-million-year-old fossils have asymmetrical skulls but retain eyes on both sides of the head. One of the eyes is higher than the other and, in some specimens, approaches the ridge of the face, showing how one eye gradually migrated across the head and on to the other side. The specimens were found in the 1700s in northern Italy but previous studies concluded that the strange skull anatomy was due to the skull having been crushed. However, using high-resolution computer imaging, Dr Friedman was able to show that the skull asymmetry and the higher position of one eye were genuine. Asymmetric eyes could have made spotting and catching prey easier, according to Dr Friedman. Flatfish hunt by lying in the sand and waiting for smaller fish to pass overhead. Another explanation is that having asymmetric eyes would allow the fish to lie on the seabed tilted to one side and avoid getting sand in the lower eye and damaging it. Dr Friedman's work has been met with a mixed reaction from creationists. "I've made some enemies online," he said. "But someone else told me, 'We've never had a problem with evolution within fishkind'." Dr Friedman's research has also helped to place the acanthomorph fossils squarely on the same evolutionary branch as flat fish. The fossils, which belong to two different species called Amphistium and Heteronectes had perplexed taxonomists who over the years had placed them in a diverse range of groups on the evolutionary tree. "It was like having a fossil mammal and not knowing whether it is more closely related to a platypus, a kangeroo or you," said Dr Friedman. In related research Dr Friedman looked at how the eye moves from one side of the head to the other in embryonic flatfish. A time-lapse video collating images from over a two-week period during the late stages of development shows one eye migrating over the skull and coming to rest on the other side of the face. Once the migration is complete the skull hardens into a settled form. Dr Friedman was one of sixteen scientists nominated for the Society of Vertebrate Palaeontology's Romer Prize, which recognises outstanding scientific contributions of doctoral students. The winner of the prize will be announced at an awards ceremony in Bristol on Saturday evening.
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