The name doesn’t have anything to do with horses. It means hyraxlike beast. It got the name in an unlikely fashion, a long way from Wyoming. In 1838, a brickmaker digging in the clay near an estuary of the Thames River in England uncovered a tiny fossilized jaw. It found its way to the leading paleontologist of the time, a man named Sir Richard Owen, who was known, among other things, for being such a rabid student of anatomy that he arranged to have right of first refusal on any animal that died at the London Zoo. (His wife is said to have complained about the arrangement after coming home and finding a dead rhinoceros in her front hall.) Owen, who had classified hundreds of species of extinct creatures, examined the primitive jaw. It had pointed, low-crowned teeth that looked nothing like those of a modern horse. He first decided it must be the jaw of a prehistoric monkey. Later, on closer inspection, he decided its teeth looked more like those of a pig. Finally, he decided the teeth resembled those of an obscure rock-dwelling critter from the Middle East called a hyrax, which looks like a groundhog but is more closely related to elephants. In 1841, he published a paper describing a new order, using the teeth as evidence. He called his new find Hyracotherium—the hyraxlike beast.
AN EARLY TWENTIETH-CENTURY RENDERING OF HYRACOTHERIUM IN THE LUSH FORESTS.
Owen went on to catalogue uncounted thousands of other fossils, including all of the fossils Charles Darwin brought home from his voyage on the Beagle. He named hundreds of new species, coined the term dinosaur, and founded London’s Natural History Museum, but he never did figure out that Hyracotherium was not a hyrax. That happened decades later and thousands of miles away in the United States.
In 1866, a dour-looking young man named Othniel Marsh, with a very straight, prominent brow often accentuated with a straw boater hat, was named the first professor of paleontology at Yale University. He got the job primarily because his fabulously rich uncle, the industrialist George Peabody, built a paleontology museum for Yale. Marsh was a passionate, and fairly talented, cataloguer of bones. And his timing was excellent. Some of the greatest fossil beds in the world were about to become accessible through railroads opening the American West.
In 1868, Marsh bought a ticket on one of the first trains to the frontier. Passing through what is now western Nebraska, at a whistle stop called Antelope Station, he chatted with some workers who told him they had encountered unusual bones while digging a well. They thought the bones were from ancient humans or maybe even tigers. Hopping off the train, Marsh examined them hungrily and, as he later wrote, “soon found many fragments and a number of entire bones, not of man, but of horses, diminutive indeed, but true equine ancestors.” With the train waiting, Marsh threw the bones in a sack and went on, but later, on closer examination, found fossils of four separate horse species, including a small, odd-looking horse with not one toe, but multiple toes. Describing it, he wrote, “During life, he was scarcely a yard in height, and each of his slender legs was terminated in three toes.”
Marsh went back to the West repeatedly in the next few years to search for bones, often in dangerous and remote regions where native tribes and the US Cavalry clashed. For safety, his researchers carried both rock hammers and Colt revolvers. On one early expedition near Bighorn Basin, he employed a young man named William “Buffalo Bill” Cody as a scout, until Cody had to ride off to respond to some trouble with Pawnee warriors. Within a few years, Marsh had so many crates of fossilized bones stored in the attic of Yale’s Peabody Museum that he had to prop up the ceiling with extra beams.
The collection catalogued more than thirty species of horse ancestors that had lived in North America. At Yale, Marsh began to piece together a horse lineage. He started with horses from the relatively recent past. They had a single hoof that marked them distinctly as relatives of the modern horse. These animals, he found, were clearly related to somewhat older, smaller animals, which had a main hoof with two smaller hooves on either side. Those appeared to be related to even older animals that had three fully developed hoofed toes on each foot. Before this, many scientists had given little thought to how the strange beasts they found in fossil deposits were related to the present. But Marsh had found evidence of a lineage that had changed over time. It became a foundational argument for evolution.
In 1876, the world’s leading proponent of evolution, a largely self-taught English biologist with broad, drooping sideburns, named Thomas Huxley, sailed to the United States to deliver a series of lectures on the controversial new theory, formally proposed by Charles Darwin twenty years earlier. Evolution had a lot of prominent backers, but it did not have much hard evidence to support it. Huxley argued that evidence could be found in the fossil record, and he focused on fossil horses from Europe that seemed to show a lineage of change over time. But his evidence was thin: three species so distantly related that drawing a line between them was at best sketchy, and not very good for the lecture circuit.
Before lecturing in New York, Huxley made his way to Marsh’s museum for what he described in a letter to his wife as a quick “canter over the fossils.” He ended up staying for weeks. The two bone hunters hit it off fabulously, studying at the museum by day and going for carriage rides in the evening. “His collection of fossils is the most wonderful thing I ever saw,” Huxley wrote to his wife. A biography written by his son described how Huxley would ask if Marsh had a specimen to exemplify each transition. “Marsh would simply turn to his assistant and bid him fetch box number so and so, until Huxley turned upon him and said, ‘I believe you are a magician; whatever I want, you just conjure it up.’ ” 2
Marsh had dozens of examples of horse ancestors that showed a gradual evolution from three toes down to one hoof. As the two delved through the specimen boxes, Huxley posited that somewhere in the distant past, there must have been an original member of the horse family with four toes. He called this theoretical beast Eohippus—the Dawn Horse.
THOMAS HUXLEY’S 1876 DRAWING OF THE FIRST HORSE AND FIRST MAN.
He drew a cartoon for Marsh of a human ancestor riding on a multitoed horse. Yale still has the doodle in its special collections. Months later, after Huxley had sailed back to England, Marsh wrote to tell him that he had found the four-toed Eohippus. In fact, it had been at the museum during Huxley’s visit, but Marsh had so many crates of fossils from out West that he had not yet had time to clean the mud off all of them.
Unbeknownst to either, the creature Marsh called Eohippus was the same one Sir Richard Owen had called Hyracotherium thirty-five years earlier. The name Eohippus reigned for decades, until 1932, when an English researcher compared Eohippus and Hyracotherium and concluded they were the same. Since zoological naming rules give priority to the earliest description, Eohippus was scrapped. Hyracotherium is considered the proper name today. In the United States, you still hear a lot of ancient-horse lovers refer to the first horse by its American name, which has a nicer ring. George Gaylord Simpson, who had an even finer knowledge of horse fossils than Marsh, got around the problem by referring to the first horse as eohippus with a lowercase E—a sly way of noting that, while it was not the proper name, it was the right one.
I trailed Rose up through a jumble of boulders and out onto a bare shale flat. He moseyed with his arms loosely tucked behind his back, like a man strolling through an art museum but with his eyes trained down, as if all the paintings were screwed to the floor. In one hand he gripped an ice pick, ready to pry a jaw out of the hard ground. To outside observers, it must have looked maddeningly dull to stand out in the heat and wind, slowly scrutinizing each inch of the middle of nowhere, but each step presented a challenge—thousands of new shapes in the ground for the brain to quickly classify. Tan rock, not a fossil. Broken turtle shell, too common, not worth collecting. Sheep poop, not worth collecting. White-colored rock, oblong, requires further inspection. Nope, not a fossil.
The process continued like a mantra with each step, and took considerable concentration. It insisted that in order to better understand the distant past, we
remain fixated on the present. After an hour, I began to wonder whether we would ever find a jaw. Then Rose suddenly bent down and bobbed up holding three pieces of beige stone. He fit them together loosely in his hands to form the spreading spade of a scapula bone.
“What is it?” I asked, hopefully.
“Not what I’m looking for,” he said. He tossed them back in the dirt. “I’m after the little things. They are much more important. Easier to carry, too.”
As he ambled on, I pressed him about the fossil he had just tossed aside. It was a Coryphodon, the largest mammal of the Eocene, he said. About three feet high, maybe a thousand pounds—unrelated to the hippopotamus but with a similar heft and love of aquatic plants.
“Smallest brain-to-body ratio of any mammal you find here,” he said—an Eocene lummox, and an evolutionary dead end.
We ambled on. I turned my eyes back to the ground, trying to sort through the galaxy of pebbles for something useful.
“Jaw!” Rose yelled. He bent down, then popped up grinning with a fragment of fossil jaw. It was about an inch long, with a back molar still embedded in the bone, glinting in the sun. The small, narrow tooth had four simple peaks for chewing. It did not look like a scale version of modern horse molars, which have a wide maze of ridges used to grind grass with a sideways motion. Hyracotherium’s teeth looked more like plain old teeth—a few crests and valleys for chewing and that’s about it.
“These are very primitive teeth, not much specialization in the direction of modern horses,” Rose said, looking down at the fossil in his palm. “The story of the horse is really the story of its teeth. They have changed so much you wouldn’t be able to recognize the species if you couldn’t connect them over time.”
The sun rose higher and the shadows disappeared. Rose found about twenty more jaws. I found none. Noon is the best time to find jaws because the light glints off the teeth. But it is also the most brutal time to be in the badlands. The searing sun was amplified as it bounced off the light-colored rock. We shambled and sweated. We sweated and shambled. We stopped to drink water. We sweated more. The moisture evaporated instantly in the dry wind, leaving a crust of salt that felt like fine dust. Or maybe it was dust. There was plenty of that around, too. I could feel it infiltrating my shoes and grinding between my toes. My neck and ears were crisping. I was wearing an old baseball cap and envied Rose’s big straw brim—the only shade in miles. It gave me an appreciation for the modern horses that live out here. There isn’t much to eat. There is less to drink, and there is no cover from the sun or wind. I barely made it through a day, and they had been doing it for as long as anyone could remember.
I started to wonder why anyone would want to gather fossils all day, every day, every summer, for a career. I was so hot I wanted to call it quits, speed down to the cool, green Bighorn River and jump in. But it was strangely addicting. Every time I decided to give up and try to find a small rock outcrop that might offer shade, I would see a small shelf or ravine that looked promising. And I would think, Maybe there are jaws there.
Then I saw one. A perfect outline of a jaw, the little black teeth glinting just like a broken beer bottle. I picked it up, cupped it in my palm, and felt my heart beat faster. In my hand was the first horse, which didn’t look like a horse or eat like a horse, but even so, it was key to understanding the wild horse today—why it developed in the West, and how it was able to do so well here.
The evolutionary saga of the horse is a kind of cast-out-of-Eden story. The horse started in a lush paradise that it lost as the global climate changed. Since then, it has had to find its way over dry, barren ground, where it has managed not just to survive but also to thrive. That evolution helps explain why the animal made such a tight bond with humans, and why it was able to spread all over the world in the modern era when so many other animals disappeared.
The story starts with little bands of Hyracotherium. The first of them appear to have evolved on what is now India, when it was still an island in the Indian Ocean. When India slammed into the southern coast of Asia, Hyracotherium spread out and feasted on tender leaves and berries all over greenhouse Earth’s forests, eventually reaching to Wyoming and beyond.
A few tantalizing fossil finds suggest that then, as now, horses were social. In 1952, George Gaylord Simpson, who at the time was a curator at the American Museum of Natural History in New York, found an array of twenty-six Hyracotherium skeletons in southern Colorado. Later analysis showed the group had far more females than males, and the males were bigger, suggesting bands with one dominant male and a harem of females.
That same band structure is still around today wherever horses roam free. Each band numbers anywhere from four to maybe twelve animals. It consists of one dominant stallion, his mares, and their offspring. The stallion acts as security, watching for both competing stallions and for predators. You will often find the stallion standing on a rise overlooking his band. Despite his size, though, he is not in charge. There is always a dominant mare that leads the band. She decides where they go, when they eat, and when they drink. If chased, the mare leads the way, the band follows, and the stallion herds from the rear, keeping his band together while holding a position where he can fight off predators.
There is not enough fossil evidence to know whether Hyracotherium bands worked in the same way, but the record suggests that horses have been living in these stallion-dominated family bands for a very long time.
Back in the Eocene, bands of tiny horses nibbled at lush forest growth. One well-preserved fossil from Germany showed a digestive system filled with the preserved leaves and pits from wild grapes. Just as Hyracotherium was getting established, about fifty million years ago, Earth began to steadily dry and cool. One of the leading theories is that the cooling was driven by an explosion in aquatic ferns that sucked so much carbon dioxide out of the atmosphere that it turned down the whole planet’s thermostat. Whatever the cause, the forests began to thin. An obscure plant family called grass, which was once relegated to marshes and riverbanks, started to spread. It now covers almost a third of our planet.
If the spread of grass doesn’t seem like a big deal, that’s because you’ve likely never tried to eat it. And for good reason. It’s harsh stuff. As a defense mechanism, grass draws abrasive silica from the ground and stores it in its cells. This sand built into the plant wears down herbivores’ teeth to nubs. An herbivore with no more teeth is a dead herbivore.
Fossils suggest that, as grass spread, the horse at first stayed in the retreating forests. But then, in the Oligocene Epoch, about thirty-two million years ago, Hyracotherium split into two distinct lines in North America: Miohippus and Mesohippus.
Mesohippus (middle horse) basically carried on the Hyracotherium way of life, though there were some notable changes. The fourth toe on its front feet had withered to a bony nodule along the ankle, leaving just three toes on each foot. It was slightly larger, about two feet tall at the shoulder, with longer legs and a longer face. Its back had lost some of Hyracotherium’s rabbitlike hunch. It had developed an extra pair of molars for grinding. But its low-crowned teeth showed it continued to browse on lush forest foliage. Fossil beds formed from swampy areas, like the Big Badlands of South Dakota, are full of Mesohippus fossils, suggesting it lived in moist forests along rivers, going on as if global cooling had never happened.
Its cousin, Miohippus (small horse), struck out on a different path. Like Mesohippus, it had three toes, it was bigger than Hyracotherium and more horselike, but its upper molars were bigger and wider. Paleontologists see this as evidence it had started eating grass. The difference may seem minor, but it’s not. Mesohippus is gone. It was an evolutionary dead end. Miohippus passed on its genes to modern horses that now cover the earth.
Miohippus’s evolution can be explained in part by what it was trying to eat, and in part by what was trying to eat it. By thirty-two million years ago, predators had proliferated across North America. Thousand-pound Hyaenodons, doglike predators
with muscle-piled shoulders and back legs like springs, prowled the continent. Nimravidae, early ancestors of the big cats, waited in the trees to pounce. The forest became dangerous. On the grasslands, at least you could see predators coming.
With the constant pressure of predators, Miohippus started to show characteristics now inseparable with horses. Its eyes moved farther back on its head, putting it in a better position to spot predators while lowering its head to graze or drink. Its legs became longer, stronger, and faster. Its brain became rapidly larger, perhaps because it needed to navigate new social behavior and make quick decisions about potential threats on the open plains.
The global trend in cooling and drying continued to spread. Forests continued to retreat. Grasslands and tundra continued to expand. Changes occurred gradually until about twenty-three million years ago, when something really big happened—George Gaylord Simpson called it “The Great Transformation.” To watch it happen, the “Great Transformation” wouldn’t have seemed all that great. But a crucial change allowed horses to increase around the planet and dominate grasslands and savannas. The transformation was this: Miohippus had gradually evolved through a series of limbs and branches into a distinct descendant called Parahippus (near horse). The big revolution in Parahippus was its molars. They were broad and complex—a series of ridges and valleys that could cut grass like a threshing machine. These back teeth also became much, much longer than typical teeth. Most of their length remained below the gums, housed in the jaw. The new teeth acted a bit like a mechanical pencil. As a steady diet of grass wore down the crown of the tooth, more rose out of the socket to replace it. Biologists call this trait hypsodonty—long-toothedness.
The new teeth could grind grass for decades before wearing out, allowing horses to eat a harsh diet of grass and still thrive and reproduce. Modern horse teeth can be up to five inches long and take thirty years to wear down. This, by the way, is where we get the sayings “He is long in the tooth” and “Don’t look a gift horse in the mouth.” A person could gauge a horse’s useful life expectancy by how much of its back teeth it had left.
Wild Horse Country Page 5