Link to full storyPalaeontologists didn't previously have a decent fossil representing the intermediate between finned fish and four-footed land animals, or tetrapods.
The important part:
Hoping to understand this key period better, Shubin and his colleague Ted Daeschler of the Academy of Natural Sciences in Philadelphia, together with Farish Jenkins of Harvard, began searching for fossil-bearing sediments of the right age. After five years of digging on Ellesmere Island, in the far north of Nunavut, they hit pay dirt: a collection of several fish so beautifully preserved that their skeletons were still intact. As Shubin's team studied the species they saw to their excitement that it was exactly the missing intermediate they were looking for. "We found something that really split the difference right down the middle," says Daeschler.
They predicted such a fossil would exist. They predicted where such a fossil might be found. They looked and there it was.
A theory is tested by using it to make predictions and then testing them. Evolution has had some claim it is untestable. This qualifies as such a test.
The Image from the article: