At Messel the horses matured quickly. Researchers have found a foal of about three months (they can tell by the maturation of the bones) that’s already two-thirds the size of an adult. Some of the pregnant mares are themselves so young that they still have their own baby teeth. Reproducing at such an early age is unusual in modern horses, but it may well have been normal then, suggests Franzen. Since these horses lived for only a few years at most, this ability to reproduce quickly is probably one reason why they were so prolific. Messel also reveals other biological features that made the horse resilient enough to endure through the ages. One of them, surprisingly, is the horse’s unique digestive system.
When I was taking care of Whisper, I found myself fascinated by his strange eating preferences. In the barns where I rode as a child, horses were fed a small bit of grain and a flake of hay in the morning, and ditto in the evening. That seemed to suffice. But Whisper quickly taught me that he wanted something different: he wanted to head over to a certain corner of the pasture for a bite of one thing, then walk somewhere else for another choice blade. He wasn’t persnickety: he never turned down good food. But left to his own devices, his food choices were extremely varied. This kind of casual on-again, off-again snacking throughout the day is typical of horses on their own. It’s “throughput” eating, different from the way cud-chewing cows, for example, eat.
Messel shows us that the difference between the grazing style of horses and that of cows has its roots in deep time. Even-toed animals like cows are ruminants who digest and redigest their food in a series of stomachs. This is why cows are said to chew their cud and spend roughly half their time in a pasture ruminating rather than grazing. Cows are patient food processors.
Horses are not. They spend much less time digesting each bite they take and a lot more time taking bites wherever they can find them. Their constantly grinding teeth and high-speed digestive system extract whatever energy is there. Our own stomach, and the four stomachs of cows, are of prime importance when it comes to digestion. But for the horse, the stomach is merely one stop—and not necessarily the most important stop—along the digestive tract. A carrot is digested in the mouth, in the stomach, in the intestines, and also in the cecum.
This cecum is an important key to the evolution of the horse and an important reason why his eating habits are so flexible. The cecum is large—quite large—and contains bacteria that break down food like gorse into nutrients that the horse can use. It’s a major reason why horses can survive on a high-fiber, low-protein diet, and it’s an important reason why horses, over 56 million years, were more adept than many other animals at eating newly evolved plants.
Many mammals have a cecum. We do. It’s a small pouch that sits between the small and large intestines. The modern horse cecum, on the other hand, is four feet long. Without this organ, essentially a fermentation vat, horses would not be able to live in so many different ecosystems or eat such a variety of foods. The horse’s digestive system is like a conveyor belt where food moves in at one end, passes through at comparatively high speed, and comes out the other.
The cecum houses combinations of bacteria, protozoa, and fungi that cooperate to break down cellulose. These digestion-facilitating communities are not the same from horse to horse, nor do they stay the same in any one horse throughout the animal’s life. Instead, the composition of the community in a horse’s cecum changes according to the diet the horse is eating. This change, however, must occur gradually, over the course of several days. Horses who change their diet too quickly may suffer greatly or even die. This is why you can’t turn a horse out on spring grass for too long early in the season. The microbes in the cecum become imbalanced by a sudden, rich nutrient influx. Even altering the quality of the hay a horse eats can sometimes cause problems if the shift is too sudden.
Barn managers and horse owners often find this frustrating, but because of Messel, we know that an enlarged cecum as a solution to the problem of digestion is a solution with a deep history: as far back as the middle Eocene, Messel proves, horses could survive on forage that other animals would have to pass up.
While Schaal and I watched his team in action, I saw within the course of only a few hours the discovery of several complete specimens. One man shucked open a rock layer to discover an insect with two long, delicate antennae, each ending in a small sphere. Another peeled open a layer to find a fossil fish. He glanced at it and threw it over his shoulder into the scrap pile.
“Hey,” I said instinctively, forgetting my manners. How is it possible to throw away something so precious?
He explained that the Senckenberg Museum in Frankfurt, which houses many of the fossils, already had so many of those fossilized fish that it wasn’t worth preserving more. In fact, there are so many fossils at this site that the problem is not finding them, but preserving and storing them.
I saw this firsthand when Schaal took me to the Senckenberg and offered to show me one of his prize horses. On an upper floor of the museum, he pulled open one of the many cabinet drawers that contain preserved fossils. Inside this one was a tiny mare carrying an even tinier partially developed foal—with developing teeth. The baby lies “backward” in the uterus, as do today’s foals.
He pulled open another drawer and out came another horse fossil. But this one looked strange. He explained that a chunk of the fossil’s backbone had been removed by the business end of a bucket loader that had been digging up the Messel pit in order to make a garbage dump.
I did a double take.
“How did that come about?” I asked.
In the 1980s, the German government decided to convert Grube Messel—then not yet a World Heritage site—into a national garbage pit. To many government officials, a big empty pit seemed like a great place to stash the modern world’s embarrassing detritus of plastic bags and bottles.
In their defense, Messel did not at that point appear to be a prime research site. It was taken for granted. Local people had for centuries mined the pit for coal. Villagers knew that strange animals, animals no longer present on Earth, could sometimes be found pressed in between the clay layers, but those ancient fossils were not a priority when Germany, like Britain and the United States, found itself in the throes of industrialization and needed all the energy it could get.
Moreover, preserving the fossils was difficult: The same friable wet clay on which I had slipped and which had preserved the skin shadows of ancient animals dried and turned to dust almost as soon as it was dug up. When the clay dried up, the fossils disintegrated. The irony was haunting, sadly reminiscent of the Greek tale of Orpheus and Eurydice: the mere act of looking at the fossils—just bringing the fossils into the light of day—destroyed them.
Without adequate preservation techniques, there seemed to be little point in mining the fossils themselves, but some nineteenth-century researchers did realize that the site contained important fossils and worked hard to get them noticed. In 1875, one Rudolph Ludwig found a fossilized crocodile, putting Messel on the paleontological map. A subsequent paleontological survey affirmed the site’s value, but then World War I and World War II intervened. The site was operated as an energy resource until Allied bombs destroyed its infrastructure. After the war, the mine was not reopened, and German officials were stuck with a useless hole in the ground.
Without the ability to preserve the fossils, which were by then recognized as significant, there seemed to be no future for the site. Finally, researchers found an artificial resin that, applied with the correct technique, solved the problem. Painted quickly after the fossils were removed from the pit, the resin held the bones in place.
Messel could now be legitimately thought of as a fossil hunter’s “El Dorado,” in the words of Schaal. Nevertheless, the garbage dump plan moved ahead. Bucket loaders began gouging away the layers of clay. Scientists ran ahead trying desperately to preserve what they could. Schaal, then a young researcher, was in front of one of these machines when it scooped up some clay and expos
ed a skeleton, which Schaal managed to save. The fossil turned out to be a complete horse (or would have been complete, save for that chunk of backbone removed by the bucket loader). This very fossil was what Schaal had shown me in the museum drawer.
This dramatic event drew attention to the problem. As the public learned more about the importance of the site, protests grew, and in 1990 the government abandoned the garbage-dump plan. Five years later, Messel became a World Heritage site. One of the horse fossils even appeared on a German postage stamp.
Messel today is one of Germany’s most important Lagerstätten. Lagerstätten is the plural. Lagerstätte, the singular, is one of those long German words that require a whole lot of English words to explain. Basically, it means “a place where fossils are so outrageously well-preserved that scientists will never know all there is to know about the fossils there.”
* * *
The Senckenberg Research Institute and Natural History Museum is the 1815 inspiration of the poet Johann Wolfgang von Goethe, an early proponent of science for the public. Prior to that time, collectors were wealthy men who kept their natural history treasures in their own homes, where only a few invited guests could see them. These collections, expensive to obtain, were symbols of status. Early collectors displayed stuffed animals from all over the world, had personal greenhouses in which unusual or even bizarre plants grew, and kept rock collections, coral collections, and just about anything else from the world of nature that the wealthy could think of to amass. If you were invited to dinner, you were expected to view (and to be impressed by) the proprietor’s hoard of natural history objects, and, by implication, his powerful global reach. Goethe believed in the democratization of science—that these collections should be available to everyone. Many citizens of Frankfurt agreed with him.
Today the Senckenberg Museum remains much as it was during Goethe’s era, filled with rows of cases containing shells and birds and corals for visitors to contemplate. There’s very little explanation. But in the museum’s basement is a modern exhibit, full of information that focuses on Messel and its horses.
While there, I watched a short animation that presented one scientist’s view of how a dawn horse might have moved. In the animation, fossilized bones rise out of their 47-million-year-old bed of clay and assemble themselves into a complete dawn horse. The poor little fellow ambles forward in an ungainly gait that’s anything but horselike in the modern sense. It’s not quite a walk and not quite a trot. It’s something else entirely, something you’ll never see a modern horse do.
Intrigued, I called the scientist behind the animation, Martin Fischer of Friedrich Schiller University in the former East Germany. An evolutionary biologist and specialist in animal locomotion, Fischer had created the animation more than a decade earlier.
“If we did this today, I would actually change some things. I would lower the body in between the limbs,” he said. “This horse should move more like a fox.”
To me, I said, the animal looked like it was crawling.
“It should be even more crawling, because this poor animal was not a horse at all and had no idea that it would become a horse,” he said. “Small mammals of this size all have a curved spine.”
I thought of my border collie and his emphatically flexible backbone that can easily undulate not just up and down, but from side to side. Were the dawn horses like that?
Somewhat, he answered.
He continued: “The straight spine which you know for the horse is a very late acquisition from evolution. If you look at a galloping horse, you will see that the galloping comes out of the lower spine, the last seven vertebrae. It’s very late in evolution that this occurs and you have to be very high up so that you can gallop from your legs.”
Small animals, he said, would not gallop like this.
This was interesting. It’s certainly true that larger horses gallop more smoothly than smaller horses, but I hadn’t thought of the gallop as an artifact of height. It helped explain why even now, when I think about some of the ponies I rode as a child, my spine aches.
“Horses are actually the only dorsal-stable animal we have. That’s why we can ride them,” Fischer continued.
This sounded very convenient for us humans.
“Think of a cow,” he suggested. “The spine movement is so tremendous that you cannot sit on the cow. That’s why your American cowboys can only stay on a cow for five seconds.”
I mulled that over. It is possible to sit on a cow or a camel or even an elephant, I thought, but Fischer’s right that they’re not very comfortable. I’ve never ridden a cow, but I have ridden both camels and elephants, and each time I realized how much I missed my horse.
I asked what he thought the little dawn horses would have looked like when they ran.
“Think of a fox or dog of the same size and you’re very close to how they moved. They could gallop, but not like a modern horse. They had to gallop in their back.”
I explained that I had always felt sorry for these first horses, with what I imagined to be a cumbersome gait.
“You have this horse-centered view,” he answered. “You have to get out of that to see what these early horses moved like. In today’s horses we have a limb-dominated gait. In the early horse, we have limb-dominated gaits in the walk and trot. But to go faster, they had to bring in the backbone.”
This explained why Margery Coombs had told me that the dawn horses scampered. The gallop as we know it lay well in the future.
Modern horses, Fischer explained, have “this dorsal-stable locomotion.” By this he meant that relative to other living mammals, modern horses have a firm but flexible back shaped in a way that allows a rider to find a firm, well-balanced, easy-to-maintain seat.
“This is a very peculiar thing. Particular to horses,” said Fischer. “And it’s the prerequisite to riding.”
Because of the structure of the modern horse, he added, the best point of balance on a horse is at the withers, where the neck meets the back. I mentioned that when I visited Mongolia, I saw that riders stand in their saddles at the gallop, hovering over the withers. One of the reasons for this, Mongolians had told me, was that it freed up the horse’s legs and tired him out less. It took a while for modern flat-racing jockeys to figure this out. For the longest time, jump jockeys in steeplechase races also used to sit down firmly on the horse’s loins, but Mongolians have understood the best way to ride a fast horse over lots of ground for centuries.
“People in Mongolia ride on the withers of the horse because they know it is even more stable. When you ride up on the shoulder, you ride on the center of gravity. This is the point where it’s absolutely stable and you have the least amount of any kind of motion in any direction,” Fischer explained.
I thought of Huxley’s cartoon of Eohomo riding Eohippus: Riding horses during the Eocene would have been rather difficult. Huxley’s little primate rider would most likely have gone right over the horse’s neck. The equine skeleton that would permit elegant postures and modern dressage wouldn’t come for another 40 million years or so.
And long before that happened, yet another worldwide shift in temperature and plant life would help bring the little horses dangerously close to worldwide extinction. Throughout the early and middle Eocene, the different horse species were quite similar to each other. Horses evolved, but in a rather cautious and conservative manner. While the horses of Messel enjoyed their island-living lifestyle, the horses in North America were changing.
Several hundred miles south of Polecat Bench at a place called Grizzly Buttes, a horse with slightly longer legs showed up only a few million years after the Polecat Bench horses first appeared. This horse, Orohippus, still had four toes on his front feet and he still had tiny teeth that could smoosh grapes, but Orohippus would have had difficulty with coarser foods. He certainly wouldn’t have been able to eat anything like gorse.
Marsh, with his Victorian preoccupation with improvement, thought of this slightly younger and
slightly larger horse as “better” than the first horses—a “new and improved” version, if you will. But now we know that Orohippus wasn’t necessarily “better.” He was just suited to a slightly different natural setting. In fact, for most of the Eocene, horses stuck with the same basic body plan.
They seem to have been one of evolution’s great ideas.
Then, unexpectedly, they weren’t.
Worldwide, horses nearly became extinct.
* * *
Several months after visiting Messel and the Senckenberg, I watched the paleontologist Matthew Mihlbachler unlock and pull open a small drawer sequestered in a distant corner of the monumental horse bone room of New York City’s American Museum of Natural History. During the early twentieth century, the horse was so iconic that the Museum of Natural History’s logo, in true partnership style, consisted of a skeleton of a horse rearing beside a skeleton of a man. In the logo, the man’s arms reach to the sky, just as do the horse’s front legs, making the parallel between the modern horse skeleton and the modern human skeleton crystal clear.
The inspiration for the former logo of the American Museum of Natural History
“Look at this place. There is cabinet upon cabinet of stuff,” said Mihlbachler, waving his hand to indicate horse bones covering the whole of horse evolution, from the days of Polecat Bench well into the Ice Age. The bones extended from one end of the cavernous room to the other.
“There’s all this,” he said, motioning toward endless rows of cabinets upon cabinets containing fossils of dawn horses. “And then there’s this.” He pointed to a few pitiful tiny bones in the corner of the drawer he had just opened: “Here’s the middle Eocene of horse evolution in this one … very … empty … cabinet,” he said. The drawer was labeled “Epihippus.” “There’s this huge bottleneck in horse evolution,” explained Mihlbachler. “You’re looking at it right here.”
There was almost nothing in the drawer, save for a few tiny jaws and various fragments of bone and teeth. Museum researchers have been collecting horse bones for roughly 150 years and have cataloged tens of thousands of horse fossils. Plenty more are still in their plaster jackets, waiting for someone with the time and energy to open up the packages and find the treasures inside. So many fossils, so little time …
The Horse Page 9