Tiktaalik Published in Nature

The boundary between fish and land animal was supposed to be a wall. Today it looks more like a hallway, and something with fins and wrists has been found standing in the middle of it.

Two papers published today in Nature describe Tiktaalik roseae, a 375-million-year-old creature pulled from the frozen cliffs of Ellesmere Island in the Canadian Arctic. It has scales. It has fins. It also has a flat, crocodile-like skull, a functional neck, primitive wrist joints, and ribs robust enough to support its body weight out of water. It could, by all indications, do push-ups.

The team — Neil Shubin of the University of Chicago, Edward Daeschler of the Academy of Natural Sciences in Philadelphia, and Harvard's Farish Jenkins Jr. — has been working Arctic summers on Ellesmere since 2000. They went looking for exactly this. Devonian-era freshwater sediments, roughly 375 million years old, exposed at the surface with minimal vegetation. The geological logic said: if a transitional form between lobe-finned fish and the earliest tetrapods exists anywhere in the rock record, it should be here.

In 2004, a fossil snout emerged from a cliff face. Behind it: the rest of the animal. Then a second specimen nearby, twice as large. Specimens range from four to nine feet — a serious predator, flat and wide, built for shallow water or the margins where water meets something else entirely.

"Tiktaalik blurs the boundary between fish and land-living animal both in terms of its anatomy and its way of life," Shubin says. The name comes from the Inuktitut word for large freshwater fish, suggested by elders of the Nunavut Territory where the fossils were found. The popular shorthand is already circulating: fishapod.

What makes Tiktaalik disorienting — and genuinely interesting — is how thoroughly it refuses to be one thing. The major joints of the fin are functional. Shoulder, elbow, parts of the wrist — "already there and working in ways similar to the earliest land-living animals," as Shubin puts it. Jenkins notes that the skeleton could support the creature's body under the force of gravity, whether in very shallow water or on land. It's not a fish that became something else. It's a form that exists in the space between categories.

The discovery was predicted. That's worth pausing on. The researchers didn't stumble across Tiktaalik — they identified the geological window where such a form should appear, traveled to exposed rock from the right era, and found it. The theory said the gap should be occupied. The gap is occupied.

We tend to think of categories as real and transitions as absences — missing links, as if the chain of life has holes where it shouldn't. But what the Devonian keeps demonstrating is that the boundary between states is not empty. It's the most populated territory in evolutionary history. The "gap" between fish and tetrapod was never a gap. It was a habitat.

Tiktaalik lived in a subtropical river delta 375 million years ago, when what's now the Arctic sat near the equator. It breathed air. It looked up. It pushed itself off the bottom of shallow streams with structures that would eventually become your arms.

You are, in the most literal sense, descended from something that couldn't decide whether it was a fish.

Turns out identity has always been a river, not a stone. Tiktaalik just happens to be the fossil that proves it.

Sources:

• The fish that crawled out of the water — Nature News, 2006-04-05
• Newly Found Species Fills Evolutionary Gap Between Fish And Land Animals — ScienceDaily, 2006-04-06