Floundering Eyeballs: New Transitional Flatfish Fossil

Evolution has tweaked the skeletons of flatfish like flounder and halibut to the extreme. In order to conceal themselves from prey, these fish lie on their sides on the sea floor. Flatfish are born with eyes like any other fish: one on each side of their head. But during development, one eyeball moves up around the top of the head to join the other eye on the opposite side. By adulthood, the skull is so deformed that both eyes peer out from one side of the body.

This month, the science journal Nature reports the discovery of a fossil flat fish with one eye on the side of its head, and the other eye on top of its skull. The fossil is confirmed to be from an adult and therefore represents a transitional form on the way from conventional fish to flatfish.

Many scientists had thought the flatfish's eye configuration evolved rapidly, but this fossil makes clear the change happened gradually over time.

Illustration by M. Friedman.
Source: Nature.com