Halibut, sole, and flounder may be renowned in culinary circles, but they're also famous among evolutionary biologists. Unlike most vertebrates, the flatfish is profoundly asymmetrical, with both eyes on one side of its head. Scientists think the asymmetry arose as an adaptation to living on the sea floor, with one side of the body constantly exposed to potential predators. But how it happened has stumped researchers for decades. Did a single severe mutation misplace an eye, or did one eye migrate gradually over time? The answer may lie with Heteronectes, a 50-million-year-old fossilized flatfish (inset) whose discovery is reported online today in the Journal of Vertebrate Paleontology. Heteronectes has one eye on one side of its head (right), and the other near the top of its skull on the other side (left). Along with Amphistium, a contemporary of Heteronectes with a similar intermediately placed eye, Heteronectes seems to represent a transitional stage between symmetry and the lopsidedness of the modern flatfish. And that suggests that the flatfish's eye wandered over time.
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