Pronghorn antelopes are the fastest land animals in North America, clocking speeds of up to 55 miles per hour for a half mile, and 45 miles per hour for longer. Since they mostly run in response to predators, a longstanding hypothesis is that pronghorn speediness evolved as an adaptation to escape American cheetahs in an evolutionary arms race. Today, you won’t find cheetahs in the range of the pronghorn antelope (Antilocapra americana), but from about 2.5 million until about 12,000 years ago, cheetah-like cats (genus Miracinonyx) roamed the plains of western North America.
A new study published in the Journal of Mammalogy reports, however, that pronghorn antelopes were already speedy before American cheetahs evolved. University of Michigan paleontologists examined fossils from the Mojave Desert’s Dove Spring Formation, which dates from 12.5 to 8 million years ago, during the Miocene. Because the habitat was changing during that time—from continuous forest to patches of forest interspersed with grasslands—the researchers expected to see corresponding changes in the mammal skeletons.
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“From about 30 million years ago to present, we see mammals becoming increasingly cursorial—i.e., adapted to running,” said study author Anne Kort in a statement. Typically, environmental change provokes adaptations in features such as the bone architecture for movement. For example, animals that run faster have shorter “astragalus” bones, which form the hinge between the ankle and foot. “We expected to see longer astragali in the beginning, and then it would transition to more running-adapted astragali in the end,” said Kort.
The researchers compared two fossil astragalus bones from pronghorn antelope ancestors to astragali of 38 modern terrestrial mammals with various running capabilities. As expected, the top running speed of a mammal was correlated with the shape and size of its astragalus bones. But the antelope astragali showed no change during the approximately 5 million years spanned by the fossils. So, fossil anklebones show that antelopes were evolving their impressive speed more than 5 million years before American cheetahs lived on the continent.
The study authors conclude that the evolution of antelope bodies for fast running happened independently of cheetahs, giving them high efficiency to dash between forest patches as the climate became more arid and their habitats became patchier. “Our guess,” explained Kort, “is that they could just move around to compensate for this aridification and landscape change.” 
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Lead image: Tom Reichner / Shutterstock
