The Horse

by Wendy Williams

  In free-roaming horses, this slow emergence is a blessing. It allows a horse to ingest sand on Sable Island or to eat food like gorse, both of which wear down teeth. If the teeth of free-roaming horses didn’t constantly emerge, they would quickly wear out and the horses would die without reproducing.

  Dawn horses didn’t have huge teeth. They didn’t need them. Their delicate muzzles were often pointed upward, and their front teeth, their upper and lower incisors, were efficient at nipping buds off branch tips. These noteworthy nipping teeth would stay with them and change as plant life changed for the next 56 million years.

  Neither were the dawn horses’ molars large. They didn’t need to be. Instead, their molars were shaped for smooshing fresh fruits, rather than grinding them up. These teeth would change dramatically over the coming years, to accommodate new foods, such as all kinds of grass. The dawn horses would never have been able to make a living by grazing on grasses, but this didn’t matter anyway because there were no grasslands, as it was still too wet for grasses—“drought specialists,” according to the Saskatchewan author Candace Savage—to proliferate.

  Still, for their day, the teeth of dawn horses and dawn primates were high-tech and full of subtleties. Most field paleontologists can look at a fossil tooth and know not only whether it comes from a mammal—but from which species.

  Some can do this with only a fragment of a tooth—a skill for which I have a great deal of respect, as I lack the required patience. Countless paleontological papers have been written discussing the size, down to the micron level, of the various bumps and lumps on only one tooth.

  This is because often a tooth is all a paleontologist might have to work with. Bones are fragile things, but teeth, hard and dense and already partly mineralized, last forever. Consequently, it’s not unusual for a new species discovery to be based entirely on the identification of only one tooth. Nevertheless, spending a day reading a stack of scientific papers describing the dimensions of various mammal teeth can be a tedious business. One’s eyes (my eyes) tend to glaze over.

  “Teeth in and of themselves are not entirely fascinating,” I said one day to Chris Norris, by way of backing into the issue without (hopefully) seeming rude. I’d made a similar comment to another paleontologist who’d said, with great indignation: “Hey, that’s my life’s work you’re talking about.” I’d tried not to offend this time around.

  Norris, fortunately, agreed with me, admitting that he himself was not initially taken by the study of fossil teeth. But, he explained, there’s a lot more information available in an individual tooth than people might realize.

  This surprised me. I hadn’t thought of teeth as providing any information at all. Another paleontologist, Mike Voorhies, explained it to me this way: “Teeth have a memory.”

  It turns out that teeth can provide all kinds of clues to an animal’s diet and lifestyle. Compared to those of most reptiles, our mammal teeth are super-sophisticated. Most reptiles can only slice and dice. We mammals chew—grind, even—which allows us to eat tough stuff like raw carrots, which in turn allows more opportunities to eat. We gained the ability to get quick sugar fixes from foods like ripened fruit (thank you, Cretaceous Terrestrial Revolution), which in turn helped us grow larger brains. The teeth of Polecat Bench horses and primates show that we developed this fondness for quick energy at least 56 million years ago.

  Thus began what paleontologists call the arms race between mammals and plants.

  “So,” I said to Norris, “teeth are where the rubber meets the road?”

  “It has to do with being a mammal,” Norris answered. Being warm-blooded is expensive, energy-wise. You have to get the most out of every bite you take. “Mammals have to be extremely efficient in their digestion. In mammals, that efficiency starts in the mouth. We’re not like crocodiles, that just gulp things down. Mammals chew, and many mammals chew plants.”

  So it’s a case of “Waste not, want not.”

  Plants, of course, then developed their own survival strategies. This explains why some of them, like gorse, evolved some pretty effective defensive measures.

  “Plants don’t have the option of escape,” Norris said, “so what they have to do is defend themselves. For a plant, it significantly matters if it can stop you eating more than a few leaves of it. For mammals, the mouth is the front line of that battle.”

  It’s a teeth-versus-plants knockdown, and evolution is always raising the stakes. As plants became more and more aggressive in adopting don’t-eat-me strategies, many animals would fail to keep pace. Horses, however, would consistently rise to the occasion and turn out to be supreme combatants.

  Horses, science is showing us, are natural early adopters.

  Scientists suspect that the earliest horses and primates lived everywhere in North America, but there aren’t many sites that directly prove this. They probably exist, but they are hard to find, and when they are found they are sometimes even harder to excavate. As a young man, the paleontologist Chris Beard (another Polecat Bench graduate) learned this firsthand. Beard found an early primate fossil in Yale’s collection that was in a drawer marked “Mississippi.”

  This, Beard thought, was an error.

  “I knew for a fact that there were no Eocene primates in Mississippi,” he told me. All the textbooks said that. But, “lo and behold,” he said, there it was.

  Curious, Beard went hunting. And, lo and behold, he found Eocene primate and horse fossils in Mississippi—in a horrid place, prospecting-wise. Poison ivy. Sticky pines. Snakes. Mud. Kudzu. Hot. Humid.

  “The worst place in the world to find fossils,” Beard said. “Backbreaking labor,” he also said. “We had to remove all of the overlying rock and get down to the productive layers. Then we used small tools to harvest this productive layer of rock.”

  Listening to Beard, I started to think of modern-day Polecat Bench as rather pleasant and resort-like.

  “We used screen washing,” he explained. “To the average person it looks like panning for gold. We take this sediment that has fossils in it. We run it through a fine-mesh window screen. Anything bigger than the mesh is left behind. The screen-washed concentrate or residue goes back to our lab at the museum. Technicians look under a microscope to try to separate the wheat from the chaff.”

  He did this during the spring and the fall for ten years. Finally, he was victorious: He found a fragment of one horse tooth. He also found an early primate fossil. “It turned out to be incredibly interesting from my perspective. The horse fossil is incredibly fragmentary. It’s only part of a lower molar. If I showed you the fossil, you would be unimpressed. But it’s completely diagnostic.”

  Beard likes adverbs.

  I wondered what “completely diagnostic” meant.

  “As a mammal, your teeth have an incredibly complicated topography, full of crests and valleys. For every species of mammal, this topography is distinctive—a fingerprint at a crime scene.”

  So there we were—primates accompanying horses down in Mississippi during the early Eocene—at just about the time we were enjoying life together in wet and wild Wyoming. We think of horses as grassland creatures, but, just like us, they apparently enjoyed a tropical lifestyle tens of millions of years ago. When I thought about it, I decided that the free-roaming modern horses I’d seen in wetlands around the world were just doing what they’d always done. Living on sea islands comes just as naturally to horses as it does to us.

  3

  THE GARDEN OF EDEN APPEARS, THEN VANISHES

  Living horses, with their high-crowned grinding teeth and single hooves, bear little resemblance to the first horses … poodle-sized creatures with four toes and low-crowned, bumpy teeth.

  —MICHAEL NOVACEK, Dinosaurs of the Flaming Cliffs

  The wind is a crude excavator. It has all the finesse of a pickax. So although the winds of the American West have revealed lots of fossils, the finer details of an animal’s life are usually lost.

  If any early horses
had mustaches like the Garranos studied by Laura Lagos, for example, those mustaches won’t be preserved on Polecat Bench. We can find teeth. We can sometimes find bones. But the more delicate tissues are rare indeed. The Wyoming wind has seen to that.

  This lack of detail is frustrating. In Wyoming we can learn almost nothing about the lifestyle of these earliest horses. How did they live? How many foals did the mares have? What did they eat? Hard facts, like soft tissues, are distressingly scarce. We suspect that the Wyoming horses ate fruit, because of the clues provided by the shapes of their teeth. We can deduce—but there’s no direct evidence.

  There is one special place in the world, though, where the secrets of Eocene life are revealed with spellbinding clarity. Even the totally uninitiated, like me, can understand the overwhelming importance of what’s buried here. Standing on a small viewing platform placed on a hillside just south of Frankfurt, Germany, I looked down into a huge hole in the earth. I was at another fossil site that also once hosted a plethora of early horses and early primates. My guide was the paleontologist Stephan Schaal.

  This site, called Grube Messel, dates to the Eocene epoch, just as does Polecat Bench. But where Polecat Bench is now windblown and desiccated, the German site is serene and lush, much as it was 47 million years ago. Its treasures are safely ensconced between layers of wet claylike material.

  While Schaal and I stood and talked, gentle breezes rustled the leaves on the plentiful trees and shrubs. The early fall temperatures were cool but not cold. The air was crisp and invigorating. From my lookout point I could see below me a marshy area rich with plant life—about as different from the present-day world of Polecat Bench as I could imagine.

  Down there, Schaal said, pointing to one of the world’s most spectacular fossil sites, lay the graves of hundreds, probably even thousands of ancient mammals, including dawn horses and early primates. Also entombed at the site are hundreds of thousands (maybe millions) of Eocene insects along with countless species of vegetation.

  At this World Heritage site Eocene life is floridly, gloriously, sumptuously present. Everything is there. From soup to nuts. Almost nowhere else in the world is such a complete ancient ecosystem so well-preserved.

  When animals and insects died at Grube Messel 47 million years ago, almost 10 million years after the Polecat Bench horses and primates, they sank down into an anaerobic deep-water lake bed. In that oxygen-free environment, without bacteria to break down the animals’ tissues, their bodies did not decompose. Instead, the flesh, feathers, ligaments, tendons, and skeletons lay intact, season after season, year after year, on through the ages. Layers of detritus drifted down through the pond water to blanket them until, finally, they were entombed in a mille-feuille pastry of ultrathin alternating layers of silt, then algae, then silt and algae again.

  To peel back these layers is to turn, quite literally, the pages of an Eocene Book of Life. Opening that book by separating the layers, we can easily see that the world of 47 million years ago was similar in many ways to the world of Polecat Bench. Despite the passing of almost 10 million years, the world was still wet and swelteringly tropical. The heat hadn’t stayed high, though. The extreme temperature spike that began the Eocene and that, graphed out, reminded me of the Eiffel Tower, lasted only a few hundred thousand years. World temperatures, which had risen so markedly at the beginning of the Eocene, plunged almost as abruptly, finishing off the PETM. Then, temperatures again began to rise, only this time much more gradually. The graph of world temperatures across time now looks like a gentle hill rather than a spike.

  Again, the cause of the gradual increase is uncertain. Evidence from deep ocean cores shows that the planet’s atmosphere was then filled with greenhouse gases, which resulted in the slow warming, so scientists understand that greenhouse gases brought about the temperature increase—but no one knows where those greenhouse gases came from.

  In any case, by the time the carcasses of horses and primates and the other plants and animals were folded so gently into the clays of Messel, the heat was once again back at early Eocene heights. There was no ice in the world, neither at the poles nor on the mountain peaks. Sea levels were very—very—high, so much so that much of what we know today as Europe was then under salt water. What land was above sea level consisted of islands. From Grube Messel, it was a long swim to Asia.

  Nevertheless, the European life isolated on those islands was rich and plentiful. Inside the layers of Messel, like keepsake flowers pressed in between a book’s pages, horses and primates are accompanied by a secret garden of flora and fauna, some of which we still have with us in the world today.

  Schaal and I walked down the steep hillside to the pond and wetland where a paleontological crew was carefully peeling apart the layers of silt and algae. Mining the fossils here is easy and fun: lots of immediate gratification, no backbreaking labor, no risk of sunstroke, no tramping through kudzu and poison ivy, and, perhaps most important, no working for ten years to find one tooth.

  Invited to try, I found that I could sometimes open the layers with just my thumbnail. No need for hammer, chisel, or shovel. The task simply requires some delicacy. You have to be careful, because the pages of this manuscript are exceptionally frail. I didn’t have to don white gloves, as in research libraries, but the thrill was just as intense.

  Quarrying for fossils at Messel is, in the initial stage, a bit like quarrying stone: someone cuts a large chunk, a “book” of fossil-containing material several feet square and several feet deep, out of the layers of solidified mud, then carries it up from the pit to where people are working. Then comes the easier, more exciting task of separating the layers.

  I walked from the work site down into the quarry itself, to feel the material in situ, with my own fingertips, where the fossils had lain for all those ages.

  “Be careful,” Schaal cautioned. It had begun to rain heavily. The footing going down into the pit was slippery. A lot of people fall.

  I nodded. But his warning didn’t register. I was too excited.

  My feet slid out from under me. I felt with my fingers a “rock” that was not at all firm, but slightly pliable. This was not rock at all, although researchers sometimes use that term. It felt smooth and soft, like unbaked pottery clay.

  Fossil preservation here is so magnificent that details like the hairs on a horse’s tail or the teeth of a foal still inside the mare are easy to see. Even the colors of an insect’s wings—formed not by pigment but by wing-surface striations that reflect light like the ridges on a CD—are still visible. A jeweled beetle fossil still shows a shiny metallic blue, just as it did when the insect flew through Eocene forests. Bird feather fossils show not only the feather’s shaft and branching, but even the tiny individual hooks at the end of a feather’s individual barb. The details of the scales of a huge crocodile remain clear. Small moths have been found half digested in bat stomachs. Fish bones have been found in the gut of an ancestral relative of the modern hedgehog. Pollen grains are still visible in the pollen sacs of flowers. The flower of an ancient water lily looks very much like a water lily looks today. The delicate bones of the inner ear of various bat species show that, even then, bats had perfected their ability to echolocate. For some plants, we can see not just the roots, but the individual rootlets.

  And just as primates accompanied horses at Polecat Bench, primates are also present at Messel. One, nicknamed Ida by scientists after she was discovered at the end of the twentieth century, is fully articulated and is said to have a Hautschatten—German for “skin shadow.” We can see the complete outline of her body, showing how the skin and flesh covered her bones. We know her long tail was covered by fur because we can still see the individual hairs. Ida is small, not even a foot long. She still has her baby teeth. We can also see her adult teeth that were forming in her jaw when she died. She enjoyed a final meal of leaves and fruits, which we can see partially digested in her stomach. Her hands are gnarled up in deathly rigor mortis. Like us, she has
five fingers. Like ours, the small bones at the tips of her fingers are clearly shaped to be protected by nails rather than by claws or hoofs. Ida is so well-preserved that researchers can see that she had broken her wrist, but that the break had healed. Ida, aka Darwinius, was for a time at the center of a minor controversy, as some paleontologists suggested she was directly related to the primate line that led to Homo sapiens. The current consensus is that she had already separated from that particular group of primates, but only just. The differences between Ida’s line and our own, at that point in time, are minor.

  Among Ida’s companions are several horse species, including a horse with four toes on the front feet and three on the back, closely (but not precisely) resembling the dawn horses of Polecat Bench. So the little dawn horses were, at least for a time during the Eocene, spread far and wide throughout the Northern Hemisphere.

  In Wyoming, researchers find few fully articulated remains, but at Messel, questions about how the dawn horses spent their days are answered: we can see the animals in eerie detail, almost as if they had been X-rayed. The detail provided by some of the horse fossils is as complete as the detail provided by Ida. Just as we can see evidence of Ida’s last meal, gut contents in one of the horse fossils show that the animal died with grape seeds still in the digestive system. In 1975, the German paleontologist Jens Franzen unearthed a complete Messel fossil of a young male Eurohippus, with a skin shadow showing a detailed outline of the animal’s fleshy parts and with a gut showing partially digested plant matter. Later, other well-preserved Eurohippus were found. One fossil preserved the skin shadow of the little horse’s deerlike outer ear, not yet evolved into the pointed ear of the modern horse. Another showed the horse’s stubby tail, complete with the outlines of coarse tail hair. The tail would have accomplished nothing in the way of dealing with horseflies.