Two extinct primitive pleuronectiformes with incomplete orbital migration have again attracted public attention to the mechanism of eye migration, a subject of speculation and research for over a century. The best-known change is the migration of their eyes: one eye migrates from one side to the other. At one point, flatfishes were among the examples some critics used to attack gradual evolution and natural selection that occurs over many generations, in favor of dramatic changes that could occur within a single generation, Friedman said.The left/right asymmetry of adult flatfishes (Pleuronectiformes) is remarkable given the external body symmetry of the larval fish. By shifting one eye up and over their body's midline, flatfish avoid pointing one eye down toward the sediment.īut how this arrangement emerged has caused debate among scientists, since no living fishes show any intermediate form. As free-swimming larvae, these fish have eyes on both sides of their heads. Scientists know why these fish have two eyes on one side of their faces. There are about 600 species of flatfish, many of which are caught and eaten by humans.

The eye's position places this fish in between the modern, asymmetrical flatfishes and their distant ancestors, which had symmetrical faces with eyes on opposite sides of the face.

The left eye of this single specimen, a 50-million-year-old fish called Heteronectes, has migrated toward the top of the skull, but not all the way over. Matt Friedman, a paleobiologist at University of Oxford in the United Kingdom, found a sort of flatfish missing link in a drawer of unidentified fish fossils in the Natural History Museum of Vienna, Austria.

Flat fish with eyes on one side