Started Out as a Fish. How Did It End Up Like This?

Tiktaalik first became known to humans in 2004, after skulls and other bones of at least 10 specimens turned up in ancient stream beds in the Nunavut Territory of the Arctic. The discoverers, a team of paleontologists including Neil Shubin of the University of Chicago, Ted Daeschler at the Academy of Natural Sciences in Philadelphia, and Farish Jenkins of Harvard University, described their findings in two Nature papers in 2006.

A local council of elders known as the Inuit Qaujimajatuqangit Katimajiit were consulted, and they gave Tiktaalik its name, which translates to a large freshwater fish that lives in the shallows, in Inuktitut. The fossils have since been returned to Canada.

Scientists had been searching for a fossil like Tiktaalik, a creature on the cusp of limbs, for decades. And where other fossils required a bit of explanation, Tiktaalik’s obvious anatomy — a fish with (almost) feet — made it the perfect icon of evolution, situated squarely between water and land.

Even then, the fossil fish struck a popular nerve, arriving on the heels of the case of a trial in Pennsylvania that ruled against teaching creationism as an alternative to evolution in high school biology. To Dr. Shubin, society’s collective desire to throw Tiktaalik back into the water is a bit of a relief: You would want to chuck the fish only if you believed in evolution, “which to me is a beautiful thing,” he said.

When Ms. Deretsky illustrated Tiktaalik, she portrayed it with its derrière submerged in water, as the fossil’s back half was a mystery at the time. But in the years since, scientists have amassed more than 20 specimens and seen more of its anatomy, including its pelvis, hind fin and the joints of its skull.

In particular, computed tomography scans taken by Justin Lemberg, a researcher in Dr. Shubin’s lab, have allowed scientists to peer inside rock to see the bones within. The scans spawned 3-D models of Tiktaalik’s unseen parts. Some scans revealed that Tiktaalik had unexpectedly massive hips (more like Thicctaalik) and a surprisingly big pelvic fin. The fish, instead of dragging itself with only its fore-fins, like a wheelbarrow, appeared to use all four fins to get around, like a jeep.

Other scans revealed the delicate bones of its pectoral fin. Unlike the symmetrical rays of fish fins, Tiktaalik’s fin bones were noticeably asymmetrical, which allowed the joints to bend in one direction. “We think that was because these animals were interacting with the ground,” said Thomas Stewart, an incoming evolutionary and developmental biologist at Penn State University.

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