Georgia’s Pleistocene Horses

A herd of handsome, reddish-brown horses graze on a small beautiful prairie at the bottom of a lightly forested foothill.  A stallion pauses and raises his head from a clump of grass and white asters, while his harem of six with three foals continue tearing at the tawny and green broomsedge.  He detects the distinct odor of dire wolf nearby.  He whinnies in alarm and begins high-stepping as if signaling for his mates that they must gallop away from this place and now.  The mares get ready to follow his lead, but one seems reluctant to leave.  He nips her on her side to get her going, and they stampede 400 yards to the other side of the prairie where he leads them on a well-used trail through a young stand of oaks and pines.  A flock of grouse explode into the air, startling the horses and sending them in a different direction.  They hurdle bushes, vines, and fallen trees and reach a small creek.  Here they encounter a lone bull mastodon which they perceive as no threat.  They stop and drink, the wolves no longer close enough to pose an immediate threat.

A scene such as I described was probably a common one in North America during the Pleistocene.  Fossils of horses are usually found in most Pleistocene sites.  In Georgia disarticulated horse bones and especially teeth have been recovered from Ladds Mountain and Kingston Saltpeter Cave in Bartow County, and from the Isle of Hope, the Mayfair site, Porter’s Pit, Savannah River Dredgings, and Turtle River Dredgings along the state’s coast.

I perused the scientific literature to determine how many Pleistocene species of horse there were in what’s now Georgia and have concluded there were at least two.  An early paleontologist (Leidy, 18th century) identified one horse fossil from Georgia as belonging to the complex-toothed horse (Equus complicatus), also found in many eastern sites from Kentucky to Mississippi.  Horse fossils from South Carolina have been identified as belonging to the brother horse (Equus fraternus).  And Dr. Clayton Ray wrote that horse teeth from Ladds compared favorably to those of the Mexican horse (Equus conversidens).  However, a genetic study conducted by scientists from the San Diego Zoo compared DNA from the modern horse (Equus caballus) with that from Pleistocene-age fossils of the Yukon horse (Equus lambei), and they determined the two were actually one and the same species.  This calls into question the supposed large number of species of North American horses living here during the Ice Age.  It’s likely the number of species is inflated and many of the supposed different species are also simply no different than the modern horse, so loved by many today.

The confusing classification of Pleistocene horses is no surprise.  Different scientists in different places looked at newly discovered fossil horse teeth and couldn’t find an exact match, so they declared them as belonging to new species when the differences from known teeth were probably interspecific and due to a wide range of natural variance because horses were so abundant and widespread.  Imagine if horses went extinct today, and 10,000 years later a scientist in one region found bones of a large Clydesdale horse, and a scientist in another region found bones of a Shetland pony.  Each would conclude they’d discovered a new species, not realizing the great variability within the species.

True horses, known as caballoids, inhabited Georgia, but wild asses or donkeys, known as hemionids, lived here as well.  In Florida fossils of the pygmy onager (Equus tau) are sometimes found.  The hemionids are also split into many different species but probably can be simplified into one.

Upper left photo is of a wild horse.  Upper right is a photo of a horse tooth identified as Equus complicatus, that was discovered near Natchez, Mississippi.  Bottom photo is of Asian wild asses.  The confusing number of Pleistocene species of horses can probably be simplified into two:  the true horses or caballoids, and the wild asses, or hemionids.  The horse photo is from www.rewilding.org.  The ass photo is from www.birding-southamerica.com. The tooth photo is from an interesting website www.backyardnature.net.  An essay on this latter site discusses the glacial loess found in northern Mississippi.

During the Pliocene the zebra was probably the most common horse species in what’s now Georgia.  The zebra may have been one of the first single-toed horses to  evolve from their 3-toed ancestors.  There’s no way of knowing whether it had stripes like modern zebras.  Photo from www.JamesWarwick.co.uk.

Farther back in time, during the Pliocene, a species of horse (Equus simplicidens) resembling Grevy’s zebra roamed over what’s now Georgia along with the last of the 3-toed gazelle horses which were a dominant herbivore throughout the Miocene.  Horses first evolved in North America, and scientists found evidence of the very oldest known species of horse (the dawn horse) at the Red Hot Fossil Site in Mississippi.  A Wal-Mart has been built next to the site, but the site is still protected.  Fossils from here date all the way back to the Eocene, some 50 million years ago.  The dawn horse is famous for being at the bottom of the horse family tree in biology textbook discussions of horse evolution.

What happened to North America’s horses?

They became extirpated from the continent about 12,000 years ago.  I think human hunting contributed to the destruction of the entire population.  Humans didn’t kill every last individual, but they increased the mortality enough so that combined with normal natural mortality due to disease and predation, it exceeded the horse’s ability to reproduce and maintain a viable population.  Climate change models of extinction for North America’s horses don’t make sense.  When Europeans re-introduced horses, the beasts thrived in feral populations everywhere from Georgia to Nevada.  It’s hard to imagine a climatic phase that occurred 12,000 years ago that for only a narrow window of time rendered the entire continents of both North and South America unfit for horses.  And there is even archaeological evidence of humans hunting North American horses.  Clovis arrowheads with horse blood on them have been discovered associated with horse bones at Wally’s Beach in Alberta, Canada.  The scientists who studied the site think human-hunting combined with climate change caused the extinction of horses on the continent, but I disagree.  It was either one or the other.  I favor human hunting because horses survived millions of years of sometimes drastic climate changes but didn’t become extinct here until man shows up in the archaeological record.

Unlike bison, wild horses don’t migrate long distances.  Once humans exterminated them from a region, they were gone.  The last wild horses and asses lived in only the most remote areas of Asia and north Africa, indicating these regions remained less populated with people than any region in North America.

It pleases me to think of wild horses galloping across the woods and savannahs of Georgia.  Today, a nearby neighbor keeps horses that I sometimes hear whinnying in the evening.  I relax and imagine myself living in a cabin 36,000 years ago.

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