The fastest land animal in North America is the American pronghorn, and previously, researchers thought it evolved its speed because of pressure from the now-extinct American cheetah. But recently, that theory has come under fire. Now, a University of Michigan study examining fossilized ankle bones of ancient relatives of the American pronghorn has shown that the pronghorn was evolving to be faster more than 5 million years before the American cheetah appeared on the continent. The study is published in the Journal of Mammalogy.