Paleontologists working in the Canadian Arctic have discovered the fossilized remains of an animal that elucidates one of evolution's most dramatic transformations: that which produced land-going vertebrates from fish. Dubbed Tiktaalik roseae, the large, predatory fish bears a number of features found in four-limbed creatures, a group known as tetrapods.

Neil Shubin of the University of Chicago and his colleagues found Tiktaalik on Ellesmere Island, some 600 miles from the North Pole, in deposits dating to 375 million years ago. Like all fish, Tiktaalik possesses fins and scales. But it also has a number of distinctly un-piscine characteristics, including a neck, a flat, crocodilelike skull, and robust ribs. As such Tiktaalik neatly fills the gap between previously known tetrapodlike fish such as Panderichthys, which lived some 385 million years ago, and the earliest tetrapods, Ichthyostega and Acanthostega, which lived about 365 million years ago. "Tiktaalik blurs the boundary between fish and land animals," Shubin observes. "This animal is both fish and tetrapod; we jokingly call it a 'fishapod.'"

Especially significant is the anatomy of Tiktaalik's pectoral fin, which contains the makings of a proper tetrapod arm. Thanks to the spectacular three-dimensional preservation of the bones--many of which were found still articulated--and the discovery of multiple specimens, the researchers were able to estimate the range of motion of the fin bones. "Most of the major joints of the fin are functional in this fish," Shubin notes. "The shoulder, elbow and even parts of the wrist are already there and working in ways similar to the earliest land-living animals." Tiktaalik, the scientists believe, used its fins to support its body on a substrate.

That doesn't mean Tiktaalik was primarily a fish out of water, however. Today Ellesmere Island is the icy dominion of the polar bear. But 375 million years ago, as part of a supercontinent that straddled the equator, it was a subtropical delta. Based on the sedimentological profile of the rock in which Tiktaalik was found, the team posits that it spent most of its time in shallow water. "This kind of shallow stream system seems to be the place where many features of land living animals first arose," comments team member Ted Daeschler of the Academy of Natural Sciences in Philadelphia. ("Tiktaalik" is the local Inuktikuk word for a large freshwater fish seen in the shallows.)

Tiktaalik is already drawing comparisons to the iconic early bird, Archaeopteryx, for its explanatory power as a transitional fossil. But it certainly leaves room for more discoveries, especially those bridging the new gap between it and the first tetrapods, along with those that contain clues to the origin of the tetrapod hindlimb. Two papers detailing the findings, as well as an accompanying commentary, appear today in Nature.