Even though it had been in a museum in Vienna for years, Heteronectes had not been recognized for what it was—an intermediate form between "normal" fishes, with eyes on opposite sides of the head, and modern flatfish such as halibut and flounder, with both eyes on the same side.

The left and right views of the skull illustrate the asymmetrical placement of Heteronectes's eyes. Colored dots indicate their locations: The right eye is in a normal place for a fish but the left eye socket is very high, almost on top of the head.

Matthew Friedman, a paleobiologist at the University of Oxford, found the 50-million-year-old fish fossil in museum archives on a research visit and extracted it from the surrounding rock. He has been studying flatfish evolution since graduate school.

Friedman says that one of the interesting things about this discovery is that it was hiding at the museum for so long. "Museums are wonderful places where discoveries continue to happen. They're not just archives where things that we've figured out are deposited to molder away," he says.

—Evelyn Lamb