Pronghorn are among the fastest animals on Earth. Often ranked second to the cheetah for mammalian land speed records, America’s peculiar giraffoid has been said to hit top speeds over 50 miles per hour and maintain their sprints for much longer than quick carnivores. The exact figure has been difficult to pin down, but the swiftness of pronghorn in full sprint leaves no doubt that these herbivores are easily capable of outpacing coyotes and other potential predators. But why should pronghorn be so much faster than North America’s carnivores? The answer, some researchers have speculated, lies in prehistory.

Today’s pronghorn species – Antilocapra americana – is the last survivor of a deeper, disparate, and more diverse family that was almost extinguished by the end-Pleistocene extinction about 10,000 years ago. As a group, their record goes back about 17 million years, although when pronghorn of modern aspect evolved is a trickier question. Some researchers have proposed that Antilocapra originated around three million years ago, with Antilocapra americana itself being a late arrival during the Pleistocene, but the scant and neglected record of fossil pronghorn has given researchers cause to be tentative. All we know for sure is that the only surviving pronghorn species evolved sometime during the past two million years, part of the wonderful, mostly-lost megafauna that roamed North America. False cheetahs were among those charismatic, recently-extinct mammals, and have been implicated as the reason pronghorn are so speedy.

Paleontologists started cataloging the remains of North America’s cheetah-wannabes in the late 19th century. Researchers regularly regarded their bones as similar to those of cougars, but distinct enough to merit new species names. Slowly, as paleontologists accumulated additional remains of these felids from places like Natural Trap Cave in Wyoming, the cheetah-like nature of these cats started to come into focus. Although their skeletons still recalled those of cougars, these were long-limbed cats with shortened skulls and enlarged nasal openings – a constellation of traits that hinted at a fast-running lifestyle.

Experts disagreed about exactly what the cats were. Some considered them to be unusual cousins of cougars. University of California, Berkeley paleontologist Daniel Adams thought differently. “The points of similarity [between the North American cats and the African cheetah] are so extensive and of such a complex nature,” Adams wrote in 1979, “that a hypothesis attributing their origin to other than common genetic descent would require pushing the concept of parallel evolution to an unprecedented extreme.” He grouped the North American fossils together under a subgenus – Miracinonyx, a name coined decades before by E.D. Cope – within the genus of the African cheetah Acinonyx. More than that, Adams argued, cheetahs might have evolved in the New World and then spread to the Old.

Better fossils resolved the debate. In 1990, fossil carnivore expert Blaire Van Valkenburgh and colleagues described a nearly-complete cheetah-like cat found in a West Virginia cave. This specimen, compared to others, showed that the leggy North American cats were two species of a distinct genus that was closer to cougars than cheetahs. (Adams had been misled by functional adaptations of the cat skull and legs which had evolved independently.) The earlier Miracinonyx inexpectatus and the later Miracinonyx trumani were false cheetahs – specialized cats whose genus evolved in North America around three million years ago. A molecular analysis of recovered Miracinonyx DNA published in 2005 by Ross Barnett and colleagues confirmed this relationship. America’s svelte Pleistocene cats were agile cougar cousins, not true cheetahs.

Regardless of their ancestry, though, the sleek form of Miracinonyx has inspired paleontologists to envision the carnivore as a cheetah copycat. The long legs and enlarged nasal openings – for better oxygen intake while running – appear to indicate that Miracinonyx sprinted to chase down prey. Pronghorn expert John Byers took this assumption to propose that pronghorn co-evolved with the false cheetahs and other fast carnivores, making the speed of the herbivores a trace of an evolutionary arms race that ended 10,000 years ago.

Byers’ hypothesis became the de facto explanation for pronghorn speed. Advocates of Pleistocene Rewilding – the controversial notion that Old World species should be introduced to New World parks to kickstart evolutionary interactions that have gone dormant since the loss of American megafauana – have even suggested that African cheetah be brought to North America to reinvigorate the evolutionary competition that gave pronghorn reason to run. (I can only wonder what pronghorn would say to this misguided idea.) The logic is simple – fast predator, faster prey. The problem is that visions of false cheetahs running down pronghorn are based on the appearance of speed rather than hard evidence.

We don’t know very much about the natural history of either Miracinonyx species. Their skeletons are cheetah-ish, but that’s not nearly enough to pin these carnivores as the inspiration for artiodactyl agility. In fact, the ecological context of Miracinonyx bones hints that these cats were not simply speedy specialists who prowled open grasslands.

In their 1990 study, Van Valkenburgh and collaborators noted that later Miracinonyx bones have been found from Nebraska to Pennsylvania and Florida in deposits which accumulated under varying conditions. These cats were apparently just as at home among coastal savannahs as mountain stream valleys. More recently, at the 2010 Society of Vertebrate Paleontology meeting, John-Paul Hodnett and coauthors presented a poster about Miracinonyx that frequented caves in prehistoric Grand Canyon, Arizona. There was a distinct lack of fast-running, open-savannah prey animals during the same time period – the researchers noted that the extinct mountain goat Oreamos harringtoni was the most common possibly prey animal in the area. Rather than speeding over the grasslands, Hodnett and colleagues reported, the Grand Canyon Miracinonyx may have lived like snow leopards, bounding down sheer rock faces in pursuit of mountain goats.

This isn’t to say that Miracinonyx never bolted after equally-swift prey. It’s only to point out that we don’t know much about the cat’s ecology, feeding habits, or hunting strategy. There are a few ways we could find out a bit more, though.

Coprolites attributable to Miracinonyx might contain identifiable bone fragments of the cat’s prey. And while such a find is a longshot, perhaps a trackway made by a Miracinonyx running or launching itself into pursuit could tell us about how these cats actually moved. Both lines of evidence suffer from the complexities of accurately attributing a particular trace fossil to a trace-maker, though. Another route may be to compare the isotopic clues in the teeth of Miracinonyx to those of their potential prey, as was recently done for two sabercats and a bear dog found in Spain. By ascertaining where herbivores were feeding, and how geochemical signatures of prey became locked in carnivore teeth, paleontologists could narrow down the preferred habitats and prey of Miracinonyx. Furthermore, a poster presented by Natalia Kennedy and coauthors at the 2012 SVP meeting outlined a new attempt to compare the spine of the modern cheetah to that of Miracinonyx and other extinct cats to see how skeletal anatomy influenced flexibility and lifestyle.

Miracinonyx might have been the reason for the swiftness of pronghorn. False cheetahs and archaic pronghorn overlapped in time, if not habitat, for as much as three million years. But saying Miracinonyx was certainly a speed demon that gave pronghorn a reason to run is only supported by the barest amount of evidence. If we’re going to understand the evolution and natural history of these animals, we must first untangle their histories and the specific details of their ecology. The Just-So story of how the pronghorn got its speed has yet to be tested by the evidence which resides in the fossil record.

References:

Barnett, R., Barnes, I., Phillips, M., Martin, L., Harington, C., Leonard, J., Cooper, A. 2005. Evolution of the extinct sabretooths and the American cheetah-like cat. Current Biology. 15, 15: R589-90

Hodnett, J., Mead, J., White, R., Carpenter, M. 2010. Miracinonyx trumani (Carnivora: Felidae) from the Rancholabrean of Grand Canyon, Arizona and its implications for he ecology of the “American cheetah”, in Program and Abstracts, Journal of Vertebrate Paleontology, 30:sup2, 1A-198A