Part 3

There’s another herd of small horses moving across the plain toward the river from the south. These are feeding as they move across the savanna, eating both leaves from the scattered trees and grass from the prairie. They seem to be enjoying their mixed diet and thriving on it, so they won’t be too badly hurt in the geologically near future when they have to eat mostly grass. This is Parahippus (“near horse”), a new kind of horse just recently evolved from Miohippus.

Parahippus is a horse of destiny. For a long time some individuals of Miohippus carried a little extra wrinkle of enamel on the crowns of their upper grinding teeth—and now the wrinkle occurs in all individuals of Parahippus. Because of it, Parahippus can eat grass without wearing out its teeth before reaching breeding age, making it possible for most individuals to reproduce before dying. The little wrinkle is passed on. It’s only a small advantage, but such is the stuff that survival and evolution are made of. Parahippus is the forerunner of a vast array of different three-toed, long-limbed prairie horses that will be the most numerous members of their family until nearly the end of the Pliocene. From one of their descendants will come the first one-toed horse—the direct ancestor of our modern horses.

More herds are moving in on the river as the morning grows. There’s a group of something very small moving through the tall grass, but it’s completely hidden. Only the swaying of the 60-centimeter-tall blades shows that a number of animals are hurrying toward the river. Now they’ve moved out into an area of cropped grass, and we can see a herd of the diminutive deerlike Nanotragulus (“dwarf goat”). Not a great deal larger than a house cat, these little “deer” have tall grinding teeth well adapted for grass-eating. Their ancestry goes back for millions of years into the Late Eocene, when some of their ancestors stood less than 15 centimeters (6 inches) high at the shoulder. But their entire family is soon to become extinct. They are part of the grazing community, although they eat leaves and softer vegetation just as readily. We call them “deer” because they look just like miniature deer, but the two families are really only distantly related.

As this group scampers toward the river, we can see that they have a peculiar crouching gait—their forelegs are so much shorter than their hind legs that they seem to be running continuously downhill. They are dainty little animals with small, delicate heads and short, slender limbs. They can bound swiftly away if danger threatens, but they’d rather hide in the thick brush. A few of the females have fawns with them, tiny things less than 10 centimeters (4 inches) tall. Look at them all scatter! The shadow of a hawk has passed over the group, and in their fright they’ve dived for some nearby willows. Young Nanotragulus either learn to duck down at the sight of a passing shadow or they don’t get a chance to learn at all. This time they all got away, and the hawk will have to look elsewhere for a meal.

Buteos or buzzard hawks are common along the Niobrara in the Early Miocene, sailing on the warm updrafts on broad, short wings that let them ride the lightest airs. They swoop down on the mice and pocket gophers, young rabbits and beavers, baby “deer” and sometimes careless birds that live on this savanna. All manner of meat-eaters depend on the small animals for food, and the little Nanotragulus are most vulnerable as they move through the canopy of grass.

There don’t seem to be any other herds moving into view just now; but while we’re on the subject of birds, over there in that patch of short grass is a bird rarely seen in North America anymore. It’s a guan, a ground-living bird related to the grouse and sagehens. It must be far from home this morning; most of its time is spent in the thick brush farther back from the river. A heavy body and long neck and tail make this animal easy to identify.

Let’s look at some of the individuals and small groups that are moving or resting within view. The camels with the very slender legs and long necks are called Oxydactylus (“sharp finger”). They are browsing on the willows where the herd of Nanotragulus ran to hide. Oxydactylus is an important camel, standing about at the midpoint in the evolution of this North American family of mammals. The camels will remain stay-at-homes in the continent of their origin until they spread into Eurasia and South America at the beginning of the Pleistocene Epoch, some 17.5 million years after the rhinos died out at Agate. There are many species of Oxydactylus; the one we are looking at stands about 1.2 meters (4 feet) high at the shoulder. Notice that they don’t have humps on their backs; in this lush land there is no need to store fat against a time of possible starvation.

Speaking of camels, here comes a herd of Stenomylus (“narrow tooth”) bounding through the tall grass on the south bank of the river. This is a strange little long-neck camel that strayed off the main line of the family’s evolution. Less than 60 centimeters (24 inches) high at the shoulder, it looks very much like the living African antelope called the gerenuk. Stenomylus, with its long and delicate legs and tall cheek-teeth, is perfectly adapted for living in and eating the abundant grass which billows on this tree-dotted plain. Yet many of the little Stenomylus are going to share a tragic time with hundreds of Menoceras only a year or so from this day we are visiting. Later, we’ll move ahead to that time so you can see that natural disaster, now preserved in rock, as it happened.

When you travel back 20 million years in time you would expect to find unbelievably bizarre animals. So far, we’ve seen some offbeat specimens, but there has been nothing really out of this world. Now, if you look to the north by the lone oak tree, you will see a real prize. Do you see that hulk stepping out of the shade? No, it isn’t the Dragon of the Ishtar Gate, though it might pass for a mythical beast. What a wonderful animal! A head like a large horse’s, a neck somewhat slimmer, long front legs, sloping back, short hind legs, and a little switch tail. Watch with your field glasses when it moves out onto the bare ground. See the feet? They don’t have hooves; each toe ends in a great curved claw! This is one of the fabulous chalicotheres, a relative of the horses and the rhinos. There were never very many of them living at one time, but the family lived in Eurasia from the Eocene, some 55 million years ago, through the Pleistocene; and here in North America from the Late Eocene to the Middle Miocene.

This chalicothere is named Moropus (“sloth foot”), and it is little wonder that when paleontologists first discovered his foot bones (without an associated skull) they thought they had found the feet of a ground sloth. Let’s watch Moropus as it ambles slowly across the plain, its strange stilted walk a little like that of the modern giraffe. Other animals move aside as Moropus strides through the grass. He’s a browser, an occasional grass-eater, and even a digger of easily accessible roots and tubers. Like his cousins the rhinos, he isn’t at all bright, and he has a very short temper. When he’s annoyed, he kicks out with those claws and every animal with good sense leaves him alone. He’s respected by meat-eaters and plant-eaters alike. He walks by himself and everything else detours around him.

Look down toward the river, and we may see an exception. That two-meter (six-foot) high “pig” walking away from the river, covered with mud, is heading right toward the Moropus. His name is Dinohyus (“terrible pig”), and he’s just as short-tempered and stupid as Moropus. He looks like a giant peccary, but his size and over-large head give him away as an entelodont (“complete tooth”). These are pig-like animals, usually of large size, that aren’t related to the domestic pigs at all. Dinohyus’ skull is nearly one meter (three feet) long, and those tusks are as thick as a man’s wrist. Though we missed seeing him earlier, he must have been wallowing in the mud under the overhanging willows. Now he’s heading away from the river in search of lunch. He’s not very choosy about what he eats; it might be succulent leaves or fruits, or even the carcass of a dead animal. Dinohyus is an omnivore, eating almost anything that has nourishment.

Right now it looks as though he’s on a collision course with the Moropus. He’s seen the larger animal, has stopped in his tracks, and is pawing the ground with his front feet. Up goes his head—and listen to that roar! He’s getting a good temper worked up. Off he goes at a full gallop, right toward the Moropus. It’s hard to believe that an animal as big as that pig could charge so fast. And look at the Moropus! He’s finally realized in his dim way that he’s about to be attacked. Up he goes on his hind legs, holding his front legs out ready for a downward blow with all eight claws. But suddenly Dinohyus shifts his course just slightly, lets out another loud bellow as he avoids the Moropus, and thunders off toward the open prairie.

Dinohyus has a smaller relative around here somewhere, a little fellow just over one meter (three feet) high, called Entelodon. His head is long and low and has flaring cheekbones and bumps along the underside of the jaw like his larger cousin’s. Another pig that lives along the Niobrara is Desmathyus (“bond [filling a gap] pig”), a true North American pig or peccary. Its appearance probably wouldn’t surprise anyone; it looks very much like the peccaries that live in the American Southwest today. Its distant cousin, the domestic pig, was domesticated in the Old World from a European species of wild hog, and it was spread throughout the world by European colonists. In America, peccary evolution has run a long and conservative path. This group has changed relatively little in the 35 million years since it first appeared in the Late Eocene.

Now for another weird and wonderful beast! Trotting daintily out of a thicket on our left is a herd of something you might think were deer or pronghorn. But if you look closely you’ll see that they have two pairs of curving, unbranched horns on their heads, not the single pair of prongs you’d expect on a pronghorn. These are Syndyoceras (“together horn”), members of a family of mammals found only in North America and now extinct. Even on the Early Miocene day we’re visiting, they are scarce, moving only in small herds. The rear pair of horns is not remarkable, but the front ones, which rise from a large bump near the nose, curl up and away from each other, ending in blunt tips.

The first member of this family was Protoceras, which lived in the hills and mountains of western South Dakota during the Late Oligocene, just a few million years before the day we are visiting at Agate. Paleontologists have found battered scraps of its skeletons in the White River Badlands where perhaps they were washed by heavy spring rains running off the hills to the west. Protoceras had six bony bumps on its head that presumably bore short horns; one pair was over the nose, another was over the eyes, and a third was near the back of the skull. Probably it was the direct ancestor of Syndyoceras.

If Syndyoceras fails somehow to qualify as grotesque, let’s jump a few million years into the “future” and look at his Late Miocene descendant, Synthetoceras (“combined horn”). Here was an animal on a par with unicorns and cyclopses. Like Syndyoceras he had two tall horns at the back of his head; but something had happened to the curved ones on his nose. They had, during several million years of evolution, grown together into a single shaft and then spread out again to the sides and up. What a pity there were no little boys then, for here we have the world’s first and only self-propelled slingshot! Tie a rubber band to the tips of his nose horns, fill your pocket with pebbles, and saddle up.

You may be wondering by now about the smaller animals—the rodents and small carnivores. We have not seen any of them so far. Most carnivores work at night, but there should be a few about. Down by the river is a pair of Oligobunis (“little cusp”) hunting near the water’s edge. They look something like modern badgers but are really more closely related to the weasels. If you look just to the right of the herd of Stenomylus we were following earlier you might be able to catch a glimpse of a stalking cat about the size of a mountain lion. It’s probably either an advanced Nimravus (“ancestral hunter”) or an early Pseudaelurus (“false cat”), but we’ll have to get a closer look before we can be sure. Whichever it is, it’s on the main line of cat evolution and will eventually end up in our familiar Felis and the other living cats. There should also be some sabretooth cats lurking about; they are found in nearby deposits of the same age, though not at Agate itself.

If you look very closely at the thicket just south of us on the hillside you can see several fox-like dogs hunting rodents. This Nothocyon (“false dog”) seems to have filled approximately the fox niche during the Early Miocene. Some coyote and wolf-sized dogs are in the area too. Daphoenodon (“blood-reeking tooth”) is about coyote size. Temnocyon (“cutting [tooth] dog”), is a little larger, probably substituted for the wolf in the local fauna, and is characterized by its heavy head and long, strong jaws. If we could get a close look at its teeth, we would see that they are like those of the Cape Hunting Dog living today in South Africa.

Let’s move away from the river a half-kilometer (0.3 mile) or so and see if we can find something different. That thicket ahead might produce a couple of rabbits. Look, there at the edge of the thicket: a Nothocyon has caught something. It’s a Meniscomys (“crescent mouse”), an early relative of the living mountain beaver Aplodontia. Today, a single species of Aplodontia, the last of the line, is found only in the mountains of the West Coast. It’s the most primitive living rodent, not related to the Canadian beaver, and sole survivor of a suborder which was the earliest rodent group to evolve. Meniscomys was one of the most prominent members of the group during the Miocene. It had a round furry body, a round head with protruding incisors a bit like a true beaver’s, and no visible tail.

Watch your step. There is the mound of a pocket gopher. It is neither our familiar western gopher Thomomys (“heap mouse”) or the “eastern” pocket gopher of the Great Plains, Geomys (“earth mouse”), but an ancient relative, Gregorymys (“Gregory’s mouse”). It must be pretty successful as a burrowing animal, because we find it all over the western United States in the Early Miocene.

A hundred meters (300 feet) more and we’ll show you the surprise of the day. Here we are in what looks like a prairie dog town. But those aren’t prairie dogs. They’re a little larger, and quite unfamiliar by modern standards. Can’t guess what they are? These are beavers—Palaeocastor (“ancient beaver”) to be exact. Here in the Early Miocene of North America, beavers don’t build dams. In fact they live neither at the water’s edge nor, like muskrats, in the water. They dig deep, spiral burrows in well drained ground. Some of their burrows are 2.5 meters (8 feet) deep, but 2 meters (6.5 feet) is about average. Down and around and around the burrows go, like giant corkscrews, always ending in straight shafts slanting slightly upward so that living chambers will not be flooded by rainwater running down the burrows.

Paleontologists have called the preserved burrows “devil’s corkscrews”—Daemonelix—since the time they were first found. At first, scientists thought they might be holes left by the giant tap roots of some unknown plant. But when Palaeocastor skeletons were found in the bottoms of the spirals, almost everyone had to concede that they were truly beaver burrows. Admittedly, the skeleton of a Nothocyon was found in one burrow; but this predator probably followed a beaver home for supper and just stayed. Three other kinds of beavers lived around Agate in the Early Miocene, but their bones have never been found in the burrows. No one knows what they did for homes: perhaps their burrows were much shallower or were in the river banks where running water soon destroyed them.

Near the river bank in some soft sand is a nest of tortoise eggs. The hot sun has brought the babies out of their shells and they’re stumbling off in all directions. Right now the biggest is only about twice the size of a silver dollar; but when they’re grown they’ll be about 60 centimeters (24 inches) across the shell, or perhaps even larger. They’re strict vegetarians, grazing and browsing on soft plants and leaves. There are probably some pond turtles around too, but we’ve never seen any.

A little farther up the bank, under the roots of that big walnut tree, is a rabbit’s burrow. Several Palaeolagus (“ancient rabbit”) live there with their many offspring. Although they look very much like cottontails, their ears are smaller and they haven’t the same leaping and running ability. They’d much rather hide than flee their enemies.

These dwellers of the savanna, common during the Miocene Epoch, comprise the major species found at the Agate Fossil Beds. Their discovery in the late 1800’s and early 1900’s was highly important to the young science of paleontology. In those decades of major discoveries, large gaps remained in the story of evolution. Quarries like those at Agate helped provide the missing pieces of the puzzle. In their time, the discoveries at Agate were an important contribution toward understanding the world far beyond the dawn of mankind.

Today, advances in paleontology still depend primarily upon major field discoveries, but paleontologists also make use of highly refined analytical and measurement techniques. Closely connected with paleontology are several other sciences, among them geology, zoology, and botany. The paleontologist, for example, must depend on geology to provide important answers about the age of fossil specimens. Fossil botanical specimens, in turn, can provide answers about animal diets and climate. Though paleontology may center on the study of fossil remains, it is an interdisciplinary science. This fact will become increasingly apparent in the following chapters, which reveal the strands of evidence used in constructing the picture of Miocene Agate.

The Mark of Death Upon the Land

Even in Paradise an occasional calamity can occur. Agate’s misfortune appeared in the form of a drought. To the west of the plain built by the ancient Niobrara River, the Rocky Mountains began to rise again. This renewed uplift, after millions of years of relative quiet, eventually led to an even drier climate and a replacement of the savanna with a landscape of unbroken grasslands from the mountains to the Mississippi River and beyond. Trees then could survive only on canyon slopes along the courses of the few large remaining rivers that crossed the plains. Those rivers flowed toward the central lowlands of North America, once an embayment of the Gulf of Mexico. This Mississippi Embayment, as it is called by geologists, extended as far north as the present location of Cairo, Illinois.