Scientists have found the missing link in the evolution of modern-day flatfish, fish such as halibut and flounder that lie flat on the bottom of the sea and have both eyes on one side of their heads.
In an examination of fossils 45 million years old, scientists found specimens with one eye that had started moving but had not quite reached the top of the head.
The report in the journal Nature on Thursday suggests flatfish evolved gradually and not, as some suspected, in one radical, skull-morphing mutation, answering a question that has stumped biologists for decades.
Flatfish are among the few examples of vertebrates that aren’t symmetrical in appearance.
Young flatfish begin their lives with their eyes normally placed, but as they mature, one eye moves over the head to the other side. Thus the adult fish has two eyes looking upward.
Using imaging technology to examine the fossils, study author Matt Friedman, a graduate student from the University of Chicago, showed the odd location of the eye was not a result of distortions caused by the process of fossilization, as was thought when the fossils were discovered in Italy in the late 19th century.
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