The face of a flounder, sole, halibut or other flatfish looks like a hodgepodge of mismatched puzzle pieces forced together, with eyes that don't seem to match one another nor the orientation of the animal's mouth.
This is because, as these fish mature, one eye migrates over the top of the fish's head, coming to rest above the other eye, so both are on the same side of the head. A new fossil discovery has shed light on how this strange trait came about.
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. [Image Gallery: Freaky Fish]
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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. 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.
There are about 600 species of flatfish, many of which are caught and eaten by humans. Scientists know why these fish have two eyes on one side of their faces. As free-swimming larvae, these fish have eyes on both sides of their heads. But as adults, they lie on the seafloor. By shifting one eye up and over their body's midline, flatfish avoid pointing one eye down toward the sediment.
But how this arrangement emerged has caused debate among scientists, since no living fishes show any intermediate form. 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.
"What this fossil does is it provides a clear example of precisely that intermediate morphology," Friedman told LiveScience.
This discovery indicates that asymmetric faces were among the first features associated with living flatfishes to evolve, Friedman writes in the July issue of the journal Vertebrate Paleontology.
Researchers have known about another ancestor to modern flatfish, called Amphistium, for some time, however, its fossils have not been studied in detail, as Friedman has done with Heteronectes.
The Heteronectes skeleton was excavated from Bolca in northern Italy at a fossil fish-rich site that contains the remains of an ancient coral reef.
Follow Wynne Parry on Twitter @Wynne_Parry or Live Science @livescience. We're also on Facebook & Google+.
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