08 July 2016

Fossil Vertebrate of the Month: Miracinonyx

Natural Trap Cave
Photo by the author
This year being the centennial of the US National Park Service, the ecosystems and animals of Yellowstone have been on lots of peoples' minds (and itineraries).  Certainly, the Greater Yellowstone Ecosystem remains the best place in the continental United States to see what remains of North America's megafauna, but a site roughly a hundred miles to the east attests to how much more diverse that fauna used to be.  On the hills above Bighorn Canyon on the Wyoming-Montana border sits Natural Trap Cave, one of the most important Pleistocene vertebrate localities in the world.  Then as now, the cave is a huge hole in the ground, and while a grate keeps anything too big from falling in anymore, during the Pleistocene it was a death trap for tens of thousands of animals.  I was fortunate enough to be able to spend some time working at the site this month, and while my contributions consisted mainly of rodent bones and teeth, the cave has yielded fossils of a wide range of animals.  Some of these - bison, wolves, wolverines, to name a few - would be familiar to Yellowstone visitors today.  Some - mammoths, horses, and camels, for example - are now extinct but are nevertheless fairly familiar.  Others are more obscure, and perhaps the weirdest of these often overlooked animals is the American cheetah Miracinonyx.  Really, I should have quotes around the word "cheetah," as the evolutionary history of Miracinonyx has long been a bone of contention.  It was first described on the basis of fossils from Pennsylvania by the great paleontologist E.D. Cope in the 1890s, who placed it in Uncia, the same genus as modern snow leopards.  Other finds from the east led 20th Century researchers to conclude that it was closely related to modern Puma, the genus that includes modern jaguarundis and cougars.  It was specimens from Natural Trap Cave described in the 1970s that first demonstrated that Miracinonyx had extremely elongated legs similar to those of cheetahs, indicating that it too hunted its prey by chasing it down.  This was something of a bombshell, as most cats are ambush predators specialized for sneaking up on prey and pouncing; modern cheetahs are the only modern big cat that is a really effective pursuit predator.  It was widely suggested that this similarity in morphology meant that North American and African cheetahs not only hunted in a comparable and unusual way, but that they were very closely related.  Consequently, during the 1980s most authors referred Miracinonyx specimens to the Acinonyx, the genus of modern cheetahs.  All this taxonomy may seem very dry and dull, but it has an important implication: that despite their geographic distance, cheetahs in North America and Africa were similar because they were close relatives descended from a recent common ancestor, an evolutionary phenomenon known as parallelism.  However, since 1990 our understanding of the evolutionary history of Miracinonyx has changed substantially.  That was the year that Blaire Van Valkenburgh and colleagues performed an analysis of the morphology of a specimen from a West Virginia cave and showed that, in fact, the animal that paleontologists had been referring to for years as a cheetah was actually a very close relative of cougars.  This finding was later backed up by an analysis of ancient felid DNA, which likewise showed a close relationship between Miracinonyx and Puma.  The North American cheetah's evolution switched from being an example of parallelism to one of convergence, in which two unrelated species evolve similar traits due to similar environmental pressures.  It has been further argued, though with considerably less direct evidence, that the evolution of pursuit predation in Miracinonyx may have had a major impact that can still be observed today.  As "cheetahs" got faster, it would likely have put pressure on their prey to evolve traits that would allow them to run faster as well, which would in turn lead to further pressure towards speed-related traits in Miracinonyx in a process known as coevolution (or, more colorfully, an evolutionary arms race).  This could explain the blazing speed of the still-extant pronghorn Antilocapra, which has been clocked at over 60 miles per hour.  This is far faster than any living mammalian predator in North America, but well below the top speed of modern cheetahs.  So if you do make it to one of the western national parks this centennial year and you're fortunate enough to see a herd of pronghorn running at full tilt, spare a thought for the bizarre cat that may have made such velocity necessary.
Miracinonyx and Antilocapra
Reconstruction by Mauricio Antón

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