While researching a dissertation on the diversity of teleosts (a group of fish species that includes flounder and other flatfish), University of Chicago grad student Matt Friedman discovered some excellent examples of “transitional” fossils—fossils that show one species evolving into another. Carl Zimmer has a good article on Friedman’s findings: Dawn of the Picasso Fish.

Nature has just published Friedman’s report on his flatfish research. [Note: it may take a while for this link to go live.] In a common ancestor of Heteronectes, Amphistium, and living flatfish, he argues, one eye began to move upward. Friedman proposes that at this early stage, proto-flatfish were lying on the sea floor at least some of the time. They propped themselves up a bit with their downward-facing fins, so that they could see a little with their downward facing eye. Mutations arose that produced eyes sitting higher on their heads. Natural selection favored them because they gave the fish better vision. Even with one eye midway up their head, the early flatfish thrived as predators. (Friedman found one fossil of Amphistium with the skeleton of another fish in its stomach.)

But flatfish evolution did not grind to a halt once both eyes ended up on the same side of the head. It turns out there’s a living fossil flatfish on Earth today, known as Psettodes. (There are three species in this genus, found in the Indian and Pacific Oceans.) The ancestors of Psettodes branched off from all other living flatfish long ago. Intriguingly, some Psettodes put both eyes on their left side and some on their right–the same loose variations found in fossil flatfish. They even swim vertically like other teleosts, because they have fins on both sides of their bodies. Friedman argues that the full-blown flatfish body did not emerge until after Psettodes branched off–more evidence of the steps by which this weird kind of creature evolved.

Amphistium and Heteronectes now join the transitional fossil hall of fame, along with a fish with limbs, Tiktaalik, and the limbed cousin of whales, Indohyus. They’re also a reminder that the argument, “It can’t possibly have evolved because I can’t imagine it evolved” is not an argument at all. It may be hard to imagine Amphistium and Heteronectes, but they are real. In fact, they’ve been sitting around in museums for centuries, waiting for someone to recognize their true wonder.

Read the whole thing…