The Mark of Death Upon the Land

Palaeocastor

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 whenPalaeocastorskeletons were found in the bottoms of the spirals, almost everyone had to concede that they were truly beaver burrows. Admittedly, the skeleton of aNothocyonwas 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 alldirections. 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. SeveralPalaeolagus(“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.

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.

During the first rumbles of this upheaval there were occasional instabilities in the weather of the Great Plains. From the fossil evidence of Carnegie Hill, University Hill, and theStenomylusquarry, we can see that drought touched the land.

What happens when disaster stalks the land? That question, so pertinent to an understanding of fossil deposition at Agate, can be answered best by looking at the normal scheme of life. Animal populations are cyclic, increasing rapidly to near the highest numbers which can be supported on available food supplies. If times are good, animal populations can be quite high. If the food supply decreases, massive dieoffs result. Successive cycles of plenty and poverty then produce high populations followed by dieoffs.

The fossil evidence suggests that a prolonged drought occurred during the Golden Age at Agate, resulting in death everywhere. The vast numbers of rhino skeletons preserved at Carnegie Hill and University Hill provide paleontological evidence that the drought must have lasted for several years.

Climates change slowly, and there are wet and dry cycles. Every rancher and farmer discovers this when he plows new land during a wet cycle, for sooner or later drier years catch up with him. He expects the optimum to be the standard; but he is badly hurt during average times, and really suffers when thedry years come. It is the same with populations of wild animals. When the times are good and the grass and trees are lush, fat, and green, more of the young survive and the whole population flourishes. The plant-eaters expand their herds and the meat-eaters increase to keep up with the better food supply the plant-eaters provide. In each case the standards for survival are lowered, and the less than perfect can survive and in turn produce young of their own. But when the water fails and plants refuse to grow, the herbivores starve and the carnivore population in turn declines. Nature is indifferent—neither cruel nor kind. When times are bad every species is improved, for the strongest and most tenacious survive to reproduce themselves. There are benefits to hardship.

So it was at Agate only a year or so after the day of our visit. The river died for a while. As with many rivers much of its water flowed beneath the surface, through the sand and gravel of the bed. When the ancient Niobrara died there was still water moving through the sands and filling the low spots in its bed. Some animals could dig down to it and survive, others could stake claims to the diminishing water holes. So the thirsty, suffering herds ofMenoceraswent to the river and found no water. The strong held the water holes. The smart dug into the sand and made their own water holes. The rest died. They died by the hundreds, and thousands. Mixed with the carcasses ofMenoceraswere other victims: occasional chalicotheres, giant pigs, oreodons, cats, dogs, and a variety of equally thirsty smaller animals. Perhaps most of the animals went farther up or down stream, or perhaps they chose not to die at the river. Whatever the pattern of dying might have been, we know thatMenocerasleft untold numbers of skeletons on the broad, flat, and dry bottom of the ancient Niobrara.

Finally the rains fell in the mountains to the west. The river filled with water again and ran in sheets across the plain. At Agate the millions ofMenocerasbones and lesser numbers of the bones of other animals were swept for a few hundred meters downstream and into some sort of backwater or river lake—possibly a great meander, or an oxbow lake. There, like a gigantic mass of jackstraws, they were piled in a tangled mat 30 centimeters (12 inches)thick, covering an unknown number of hectares. All we really know is that they were moved far enough to get thoroughly jumbled, but not far enough to be badly broken or much eroded by the action of the water.

The mass of bones was soon buried by the sands and silts dropped by the reborn river, and by wind-carried debris swept off the parched land. Once buried, the bones were partially petrified by mineral water flowing beneath the surface. The land was built up a few hundred meters by sediments continually brought down from the mountains to the west. Eventually, continued uplifts of the Rockies and the Great Plains combined with erosional cycles to leave the modern Niobrara River. The two erosional remnants known today as Carnegie and University Hills were produced by the cutting of the modern river system. On the sides of these hills were exposed the tangle of bones which marked the site of ancient tragedy.

But this wasn’t the only scene of mass death to be preserved here in the fossil record. A few kilometers away an earlier drought took a toll of many other animals. The little gazelle-like camelStenomylustells the same story in scores of skeletons east of theMenocerasburial ground.

These graceful little camels may have died at the edges of their vanished water hole. The skeletons are mostly undisturbed except for a few pulled apart by meat-eaters. Scores of their dried out, mummified carcasses were buried about the same time as the rhinos on the river’s dry bottom. Like theMenoceras, the camels lay there for millions of years, intact in their death poses, the muscles in the backs of their necks pulling their heads back sharply into an unnatural position. There they lay until men discovered them.

Our imaginary journey into the past has reached its end. We have seen a day at Agate as it might have been 20 million years ago. We have watched the animals going about their daily lives during times of plenty and have seen it as it was later, when death’s heavy hand left a magnificent fossil heritage. This unique place is a window into the past, a window through which we can look back at any time and observe life at Agate millions of years ago.

The first fossils were collected in volume in 1904 by Olaf Peterson of the Carnegie Museum in Pittsburgh. Excavations have continued, off and on, to the present. As early as 1892, Erwin Barbour’s student F. C. Kenyon had retrieved a few bones from the site but their significance was overlooked. Rancher James Cook first picked some up in the 1880s and may have first noticed such deposits, without particularly recognizing them, in the 1870s.Other institutions soon joined Carnegie in extracting slabs of the greatMenocerasbone-bed, and occasionalMoropusandDinohyusspecimens. The University of Nebraska opened a new quarry in 1905. Henry Fairfield Osborn, president of the American Museum of Natural History and one of the greatest popularizers and exponents of evolutionary science, and his chief preparator Albert Thomson began work in 1907. F. B. Loomis of Amherst College discovered the nearbyStenomylusquarry the same year. Yale University’s R. S. Lull soon followed.From 1911 to 1923 the American Museum became the main excavator at Agate, but increasingly their attention was drawn elsewhere, including the later Miocene Snake Creek Beds 20 miles to the south. There, for awhile, great excitement centered around a worn tooth thought to be from an early human ancestor until the tooth was proven to be from an ancient peccary.Until 1981, only occasional excavations for bonebed slabs andStenomylusmarked the next 50 years. Then, Robert M. Hunt Jr. of the University of Nebraska reopened the main quarries and a little-known side area, and found evidence of an extensive carnivore den of the beardogDaphoenodon.In some cases, individual fossil bones were removed one by one, a very slow and painstaking process but when possible large blocks of fossil-bearing sediments were removed and shipped to laboratories for cleaning and analysis. The tools, chemicals, and special conditions necessary to extract the best specimens and most complete information are available only in a laboratory such as the one which is shown on pages40and 41 at the Carnegie Museum in Pittsburgh, Pennsylvania, in 1905. Slabs from Agate Fossil Beds were taken there so paleontologists could examine the evidence and figure out the past.See pages86-87 for a listing of museums with specimens from Agate Fossil Beds.Extracting a slabMembers of Peterson’s crew built a box around a slab in theStenomylusquarry around 1908 in preparation for shipping to the Carnegie Museum.With a team of horses, O. A. Peterson’s field crew moves dirt out of theStenomylusquarry around 1908. The boxes in the foreground are resting on the quarry’s lower bone layer. Several specimens to the left have been strengthened with plaster for shipment to Pittsburgh.Crates of prepared specimens had to be taken to Harrison, 37 kilometers (23 miles) north of Agate for the rail trip to the East. Note that the wagon is just a flat platform and that the driver is using the largest crate as a seat.

Other institutions soon joined Carnegie in extracting slabs of the greatMenocerasbone-bed, and occasionalMoropusandDinohyusspecimens. The University of Nebraska opened a new quarry in 1905. Henry Fairfield Osborn, president of the American Museum of Natural History and one of the greatest popularizers and exponents of evolutionary science, and his chief preparator Albert Thomson began work in 1907. F. B. Loomis of Amherst College discovered the nearbyStenomylusquarry the same year. Yale University’s R. S. Lull soon followed.

From 1911 to 1923 the American Museum became the main excavator at Agate, but increasingly their attention was drawn elsewhere, including the later Miocene Snake Creek Beds 20 miles to the south. There, for awhile, great excitement centered around a worn tooth thought to be from an early human ancestor until the tooth was proven to be from an ancient peccary.

Until 1981, only occasional excavations for bonebed slabs andStenomylusmarked the next 50 years. Then, Robert M. Hunt Jr. of the University of Nebraska reopened the main quarries and a little-known side area, and found evidence of an extensive carnivore den of the beardogDaphoenodon.

In some cases, individual fossil bones were removed one by one, a very slow and painstaking process but when possible large blocks of fossil-bearing sediments were removed and shipped to laboratories for cleaning and analysis. The tools, chemicals, and special conditions necessary to extract the best specimens and most complete information are available only in a laboratory such as the one which is shown on pages40and 41 at the Carnegie Museum in Pittsburgh, Pennsylvania, in 1905. Slabs from Agate Fossil Beds were taken there so paleontologists could examine the evidence and figure out the past.

See pages86-87 for a listing of museums with specimens from Agate Fossil Beds.

Extracting a slab

Members of Peterson’s crew built a box around a slab in theStenomylusquarry around 1908 in preparation for shipping to the Carnegie Museum.

With a team of horses, O. A. Peterson’s field crew moves dirt out of theStenomylusquarry around 1908. The boxes in the foreground are resting on the quarry’s lower bone layer. Several specimens to the left have been strengthened with plaster for shipment to Pittsburgh.

Crates of prepared specimens had to be taken to Harrison, 37 kilometers (23 miles) north of Agate for the rail trip to the East. Note that the wagon is just a flat platform and that the driver is using the largest crate as a seat.

Paleological laboratory.

Paleontology is the study of ancient life through the fossil remains of that life. Today, there are thousands of museums, societies, professional groups, and academic institutions around the world devoted to this study. Fossil remains are still being dug out of the ground in a number of localities, such as Dinosaur National Monument in Utah, but by far the great bulk of fossils now being studied were excavated during the last 100 years.There are now about 250,000 known separate species of fossil plants and animals. Biologists are still working to explore, find, and classify all living species; they estimate that 4,500,000 species of plants and animals are now living at our own brief moment in the nearly five billion years of our planet’s history. As you can see, the fossils now known represent only a tiny fraction of all the plants and animals that have ever lived. Yet a great deal is now known about even the simple forms of life more than three billion years ago.How has this come about? What has happened since the days of our great-grandfathers to cause this vast increase in knowledge? Men must have picked up and discussed fossils for tens or perhaps hundreds of thousands of years. We have no way of knowing what the earliest men thought about them. Their significance has been revealed slowly in the way we tend to look at time, but perhaps not so slowly when we consider how short a period man himself has been on Earth.Lucretius, a Roman writer of the first century B.C., thought that the Earth was very young. He interpreted the fossils known to him as the remains of monsters that had grown out of the Earth just after it came into existence. Evidently he had seen partial fossils and believed them to be whole, because he postulated that the Earth had brought forth creatures that lacked one or more limbs or other body parts. Lucretius assumed, as have many others, that the varieties of animals he knew of were fixed for all time and did not change. But he did recognize the principle of evolution, that things change as time goes on, in his description of human history.Lucretius described four ages of human life, progressing from early hunters up to the highly civilized life he knew under the Roman Republic. His work was rediscovered during the European Renaissance, when scholars once again began to inquire into the nature of seemingly inexplicable things like fossils.Toward the end of the 18th century the confusion over the importance of fossils and their relative antiquity forced a scientific showdown. For hundreds of years, fossil bones of extinct animals unlike any ever seen had been turning up, often with tools nearby that appeared to have been shaped by human hands. A growing feeling that the Earth and therefore the fossils were very old indeed was a topic of frequent discussion in Europe and in the New World, despite the assertion by Archbishop Ussher a century earlier that the Earth was not quite 6,000 years old.Explorers and scientists had found fossils in deep layers of rock widely separated by other layers of rock, leading many of them to conclude that now-extinct forms of life had existed before the Biblical flood. A pioneer French paleontologist, Georges Cuvier, tried to solve this dilemma in the late 1700s by postulating that there must have been several worldwide floods before the one described in the Christian Bible. Finally, this solution collapsed under the weight of new evidence as more and more studies proceeded.In the 1830s an English geologist, Sir Charles Lyell, popularized the principle of uniformitarianism—the idea that processes we observe now, such as the steady erosion of mountains, the gradual buildup of silt as sediments in rivers, lakes, and oceans, have always occurred since the origin of the Earth. This, he then reasoned, meant that the Earth must be many millions of years old at least, instead of merely a few thousand years old.A wave of interest in fossils and their antiquity swept communities around the world in the 1840s and 1850s. Americans interested in science from Thomas Jefferson on had advocated the collection and study of fossils, and a feverish race to build up study collections got underway that lasted into the 20th century. Today, scientists believe the Earth is more than 4.5 billion years old, its life more than 3 billion years old.Karl Von Linné, 1707-1778, is known as Linnaeus after the Latin form of his name. A Swedish botanist, he established a hierarchical system for classifying plants and animals that is still in use in a modified form. His organizing principle was the degree of complexity of the organisms he studied. This resulted in a system with seven levels: Kingdom, Phylum, Class, Order, Family, Genus, and Species, in descending order from the broadest category to the most specific. Students remember the system by the sentence “King Philip Crossed the Ocean For Good Soup.” Without realizing it, Linnaeus prepared the ground for the evolutionists, who later were able to demonstrate the gradual ascent of life forms from simple to complex by using his scheme of classification.Jean-Baptiste de Lamarck, 1744-1829, a French physician and ex-military man, founded the modern study of animals without backbones and coined the term invertebrates to describe them as a group. When his battle wounds forced him to take up a new career, he studied botany and published a study of French plants. He later turned to invertebrates, and between 1815 and 1822 published the classicHistoire naturelle des animaux sans vertèbres. He applied his vast knowledge of living invertebrates to paleontological work, greatly enhancing the knowledge of fossil invertebrates. Lamarck was also an evolutionary theorist, and he believed that a single characteristic acquired by an animal during its lifetime could be passed on to its descendants by heredity (modern genetic theory was unknown at that time). He saw that evolution must have taken a long time to occur, and he supported the principle which has since become known as uniformitarianism.Georges Cuvier, 1769-1832, was a French anatomist and paleontologist who specialized in the study of animals with backbones, the vertebrates. He had a long and brilliant career as a professor, eventually becoming France’s minister of the interior in 1832. His skill as a comparative anatomist enabled him to understand how vertebrate fossils should be reconstructed to form a complete skeleton, and he was one of the first to use the small muscle scars on fossil bones to reconstruct the extinct animal’s musculature. His classic workRécherches sur les Ossemens Fossiles de Quadrupèdswas published in 1812. He is known for his theory of a series of natural catastrophes, each supposedly obliterating all extant life, to account for the great variety of ancient fossils. This theory was later supplanted by the theory of continuous evolution supported by Darwin, Lyell, and others.Charles Darwin, 1809-1882, is today a household name that is still invoked in controversy as it was more than a hundred years ago. An extraordinarily patient and insightful biologist, Darwin contributed the idea of natural selection, the “weeding out” of unfit individuals and species, and described it as the guiding principle of the evolution of life on this planet. His bookOn the Origin of Species by Means of Natural Selection, published in 1859, is the most important landmark in evolutionary studies. This was the culmination of decades of work, leading to conclusions startlingly similar to those of his fellow Englishman, Alfred Wallace. Darwin knew nothing of the genetic principle of biological heredity and variation, which has now assumed equal importance with natural selection in the study of the evolution of life. For paleontologists, Darwin’s work meant they must look for transitional forms of life and not content themselves with Cuvier’s assumptions that past life forms had been static and unchanging. During his travels in South America, Darwin contracted a disease, now known as Chagas’ disease, and suffered intense pain and discomfort the rest of his life. He died of a heart attack on April 19, 1882, and was buried in Westminster Abbey in London a few days later.Charles Lyell, 1797-1875, revolutionized the study of geology partly by publicizing the earlier work of James Hutton, who died the year Lyell was born in Scotland, and partly by infusing the science with his own highly disciplined point of view. His greatest contribution was the firm establishment of Hutton’s principle of uniformitarianism, or uniformism, which became the foundation for all modern geological work. Put simply, this is the principle that the processes we see operating to form and shape the Earth today have always operated in the past. Once this is admitted, it becomes clear that past geological time is vast, not short, a truly stunning notion for Lyell’s time but a commonplace fact today. The first volume of hisPrinciples of Geologywas published in 1830; in his later works he championed Darwin’s own revolutionary point of view, adding his own powerful arguments in support of the idea of natural selection.Alfred Wallace, 1823-1913, was the co-originator, with Darwin, of the principle of natural selection, or “survival of the fittest.” The main difference between the two was that Wallace did not believe that natural selection explained things as well as Darwin thought it did, which has been borne out to a large extent by modern studies of genetic variation. Wallace worked in South America, along the Amazon and Rio Negro rivers, and in East Asia. He showed that the animals on either side of a line between Borneo and the Celebes Islands are radically different in their makeup and origin. Now known as “Wallace’s Line,” his work has been vindicated by additional modern studies. Although Wallace did not become as well known as Darwin, his brilliant, independent studies lent a great deal of weight to the Darwinian view of evolution.

There are now about 250,000 known separate species of fossil plants and animals. Biologists are still working to explore, find, and classify all living species; they estimate that 4,500,000 species of plants and animals are now living at our own brief moment in the nearly five billion years of our planet’s history. As you can see, the fossils now known represent only a tiny fraction of all the plants and animals that have ever lived. Yet a great deal is now known about even the simple forms of life more than three billion years ago.

How has this come about? What has happened since the days of our great-grandfathers to cause this vast increase in knowledge? Men must have picked up and discussed fossils for tens or perhaps hundreds of thousands of years. We have no way of knowing what the earliest men thought about them. Their significance has been revealed slowly in the way we tend to look at time, but perhaps not so slowly when we consider how short a period man himself has been on Earth.

Lucretius, a Roman writer of the first century B.C., thought that the Earth was very young. He interpreted the fossils known to him as the remains of monsters that had grown out of the Earth just after it came into existence. Evidently he had seen partial fossils and believed them to be whole, because he postulated that the Earth had brought forth creatures that lacked one or more limbs or other body parts. Lucretius assumed, as have many others, that the varieties of animals he knew of were fixed for all time and did not change. But he did recognize the principle of evolution, that things change as time goes on, in his description of human history.

Lucretius described four ages of human life, progressing from early hunters up to the highly civilized life he knew under the Roman Republic. His work was rediscovered during the European Renaissance, when scholars once again began to inquire into the nature of seemingly inexplicable things like fossils.

Toward the end of the 18th century the confusion over the importance of fossils and their relative antiquity forced a scientific showdown. For hundreds of years, fossil bones of extinct animals unlike any ever seen had been turning up, often with tools nearby that appeared to have been shaped by human hands. A growing feeling that the Earth and therefore the fossils were very old indeed was a topic of frequent discussion in Europe and in the New World, despite the assertion by Archbishop Ussher a century earlier that the Earth was not quite 6,000 years old.

Explorers and scientists had found fossils in deep layers of rock widely separated by other layers of rock, leading many of them to conclude that now-extinct forms of life had existed before the Biblical flood. A pioneer French paleontologist, Georges Cuvier, tried to solve this dilemma in the late 1700s by postulating that there must have been several worldwide floods before the one described in the Christian Bible. Finally, this solution collapsed under the weight of new evidence as more and more studies proceeded.

In the 1830s an English geologist, Sir Charles Lyell, popularized the principle of uniformitarianism—the idea that processes we observe now, such as the steady erosion of mountains, the gradual buildup of silt as sediments in rivers, lakes, and oceans, have always occurred since the origin of the Earth. This, he then reasoned, meant that the Earth must be many millions of years old at least, instead of merely a few thousand years old.

A wave of interest in fossils and their antiquity swept communities around the world in the 1840s and 1850s. Americans interested in science from Thomas Jefferson on had advocated the collection and study of fossils, and a feverish race to build up study collections got underway that lasted into the 20th century. Today, scientists believe the Earth is more than 4.5 billion years old, its life more than 3 billion years old.

Karl Von Linné, 1707-1778, is known as Linnaeus after the Latin form of his name. A Swedish botanist, he established a hierarchical system for classifying plants and animals that is still in use in a modified form. His organizing principle was the degree of complexity of the organisms he studied. This resulted in a system with seven levels: Kingdom, Phylum, Class, Order, Family, Genus, and Species, in descending order from the broadest category to the most specific. Students remember the system by the sentence “King Philip Crossed the Ocean For Good Soup.” Without realizing it, Linnaeus prepared the ground for the evolutionists, who later were able to demonstrate the gradual ascent of life forms from simple to complex by using his scheme of classification.

Jean-Baptiste de Lamarck, 1744-1829, a French physician and ex-military man, founded the modern study of animals without backbones and coined the term invertebrates to describe them as a group. When his battle wounds forced him to take up a new career, he studied botany and published a study of French plants. He later turned to invertebrates, and between 1815 and 1822 published the classicHistoire naturelle des animaux sans vertèbres. He applied his vast knowledge of living invertebrates to paleontological work, greatly enhancing the knowledge of fossil invertebrates. Lamarck was also an evolutionary theorist, and he believed that a single characteristic acquired by an animal during its lifetime could be passed on to its descendants by heredity (modern genetic theory was unknown at that time). He saw that evolution must have taken a long time to occur, and he supported the principle which has since become known as uniformitarianism.

Georges Cuvier, 1769-1832, was a French anatomist and paleontologist who specialized in the study of animals with backbones, the vertebrates. He had a long and brilliant career as a professor, eventually becoming France’s minister of the interior in 1832. His skill as a comparative anatomist enabled him to understand how vertebrate fossils should be reconstructed to form a complete skeleton, and he was one of the first to use the small muscle scars on fossil bones to reconstruct the extinct animal’s musculature. His classic workRécherches sur les Ossemens Fossiles de Quadrupèdswas published in 1812. He is known for his theory of a series of natural catastrophes, each supposedly obliterating all extant life, to account for the great variety of ancient fossils. This theory was later supplanted by the theory of continuous evolution supported by Darwin, Lyell, and others.

Charles Darwin, 1809-1882, is today a household name that is still invoked in controversy as it was more than a hundred years ago. An extraordinarily patient and insightful biologist, Darwin contributed the idea of natural selection, the “weeding out” of unfit individuals and species, and described it as the guiding principle of the evolution of life on this planet. His bookOn the Origin of Species by Means of Natural Selection, published in 1859, is the most important landmark in evolutionary studies. This was the culmination of decades of work, leading to conclusions startlingly similar to those of his fellow Englishman, Alfred Wallace. Darwin knew nothing of the genetic principle of biological heredity and variation, which has now assumed equal importance with natural selection in the study of the evolution of life. For paleontologists, Darwin’s work meant they must look for transitional forms of life and not content themselves with Cuvier’s assumptions that past life forms had been static and unchanging. During his travels in South America, Darwin contracted a disease, now known as Chagas’ disease, and suffered intense pain and discomfort the rest of his life. He died of a heart attack on April 19, 1882, and was buried in Westminster Abbey in London a few days later.

Charles Lyell, 1797-1875, revolutionized the study of geology partly by publicizing the earlier work of James Hutton, who died the year Lyell was born in Scotland, and partly by infusing the science with his own highly disciplined point of view. His greatest contribution was the firm establishment of Hutton’s principle of uniformitarianism, or uniformism, which became the foundation for all modern geological work. Put simply, this is the principle that the processes we see operating to form and shape the Earth today have always operated in the past. Once this is admitted, it becomes clear that past geological time is vast, not short, a truly stunning notion for Lyell’s time but a commonplace fact today. The first volume of hisPrinciples of Geologywas published in 1830; in his later works he championed Darwin’s own revolutionary point of view, adding his own powerful arguments in support of the idea of natural selection.

Alfred Wallace, 1823-1913, was the co-originator, with Darwin, of the principle of natural selection, or “survival of the fittest.” The main difference between the two was that Wallace did not believe that natural selection explained things as well as Darwin thought it did, which has been borne out to a large extent by modern studies of genetic variation. Wallace worked in South America, along the Amazon and Rio Negro rivers, and in East Asia. He showed that the animals on either side of a line between Borneo and the Celebes Islands are radically different in their makeup and origin. Now known as “Wallace’s Line,” his work has been vindicated by additional modern studies. Although Wallace did not become as well known as Darwin, his brilliant, independent studies lent a great deal of weight to the Darwinian view of evolution.

The largest divisions of geologic time are eras, shown above in chronological order from the oldest on the bottom to the most recent on top. The scale at left shows the relative duration of each era. As the chart shows, geologists further divide time into periods and, in the Cenozoic Era, into epochs. The fossilization of animals in the Agate Springs area of Nebraska took place in the Miocene Epoch. Adjustments to this time chart are made as new data becomes available, so it should not be thought of as an unchanging reference. This diagram is adapted from one in The Emergence of Man series published by Time-Life Books.

Although the whole story of Agate Fossil Beds dates from the formation of the Earth four and one half billion years ago, only the last 600 million years is known in detail. It was about 600 million years ago that many plants and animals began to have hard parts—parts likely to be preserved as fossils. The few fossils contained in older rocks are often folded, twisted, squeezed, and distorted so that their original character is all but erased. That isn’t always the case, of course. Some of these old rocks, the Belt Series in Montana, look as though they were deposited only a few million years ago; they contain traces of algal colonies indicative of the generally simple life forms on the primitive Earth. The old rocks, deposited during the first four billion years of Earth’s history, record the Precambrian Eons, spanning eight-ninths of geologic time.

The evolutionary development of skeletal remains has aided in the study of geologic history. The last 600 million years have been divided into units for ease of discussion and comparison. The three largest divisions are the eras—Paleozoic (ancient life), Mesozoic (middle life), and Cenozoic (recent life). We are viewing all this from our vantage point at (what is now) the most recent episode of the Cenozoic.

To begin at the known beginning, we only need go back to the start of the Paleozoic Era some 600 million years ago. No Precambrian, Paleozoic, or Mesozoic rocks can be seen around Agate, but we know from rocks found in comparable areas about what to expect under the surface at Agate: thousands of meters of sedimentary rocks, most of them laid down in an ocean. During the Paleozoic and most of the Mesozoic eras, up until about 70 million years ago, the west-central United States was covered by seas. The area now occupied by the Rocky Mountains was then a long north-south trough in which thick sediments collected. To the east, the present Great Plains area was the floor of a shallower sea. Sediments collected in the trough and on the sea bottom. Gradually, over a period of 530 million years, the sediments accumulated to a thickness of over 2,100 meters (6,900 feet). A dynamic “give-and-take”process, this sedimentation was the result of periods when the sea rose and fell several times.

If the Paleozoic sounds a little dull, it’s because we haven’t told the whole story. During the Paleozoic Era, all the major groups of organisms evolved. The seas swarmed with trilobites and shellfish of all kinds, some weird and fantastic and some very like those alive today. With them coexisted fish and sea-lilies, seaweeds and giant swimming “scorpions.” For the first time, plants and then animals came out of the sea to live on the land. These events and creatures are preserved in Paleozoic rocks. The Mesozoic Era saw the development of mammals from reptiles, the rule of giant dinosaurs, and the beginnings of flight. Strange reptiles evolved and returned to the sea, or glided through the air on motionless wings. The Sundance Formation, deposited in the northern Rocky Mountains about the middle of the Mesozoic Era, is noted for the masses of bullet-shaped squid shells found in it. Water, land, and air were full of life.

In the Middle Mesozoic, the trough drained and much of it became an area of swamp and tropical forest extending from Montana to southern Utah. This was the domain of our largest dinosaurs, giant reptiles whose bones were preserved in impressive numbers. Como Bluff and Bone Cabin, Wyoming; Morrison, Colorado; and Dinosaur National Monument and Cleveland, Utah, are the sites of quarries where many fine specimens ofApatosaurus,Diplodocus,Stegosaurus, andAllosaurushave been collected. These caches of bones have made our large museums showplaces known throughout the world.

During the last 65 million years of the Mesozoic, the trough fluctuated up and down. During much of that time a broad, shallow sea covered central North America from the Gulf of Mexico to the Arctic Ocean. The sea was filled with fish like the giantPortheus(3.5 meters/11.5 feet long), with squid-like animals floating about in elaborate chambered shells, and with reptiles which had gone back to the water. Where there was a broad coastal plain, it swarmed with dinosaurs, the ones that make Lance Creek, Wyoming and Hell Creek, Montana famous collecting grounds for museum field parties. Yet at the end of the era the trough was a seaway again, and in the Agate area a final blanket of black mud, the PierreShale, was deposited in the sea.

At the end of the Mesozoic Era a profound change came over the land. The trough, with hundreds of meters of sediments accumulated on its bottom, was drained and folded by pressures built up in the Earth’s crust. To the south, the Colorado Plateau rose slowly and smoothly; to the north folding and faulting built complex mountain ranges. This uplift is called the Laramide Revolution because of the magnitude of the change it made on the face of the continent.

At the beginning of the Cenozoic Era, about 65 million years ago, the Rockies and the Great Plains were slowly rising. Rains fell and rivers carried the water, wearing away the old sediments. Some sediments were carried west into the Pacific Ocean, or deposited on the land and covered over by new sediments. Some sediments remained within the Rockies, settling into basins during the early Cenozoic. Other sediments were carried east by the rivers, beyond the Great Plains and into the Mississippi Embayment. Mountain building and erosion tended to cancel out one another in the Rockies, preventing the mountains from reaching great heights. The mountain bases were perhaps no more than 300 meters (1,000 feet) above sea level, and the crests perhaps 600 meters (2,000 feet) above that.

Before long, the basins within the Rockies filled with sediments. The basins overflowed, and at last sediments began to cover the Great Plains. This was near the end of the Eocene Epoch, about 35 million years ago. Subtropical rivers, flowing sluggishly, rolled out over their banks and left their loads of silt and clay on the featureless plain.

The oldest part of the blanket of sediments, the White River Beds, extended from Saskatchewan to Texas. Nearly 200 meters (650 feet) of muds, clays, silts, and river gravels were laid down during the 11 million years of the Late Eocene and Oligocene Epochs. Magnificent exposures of these beds can be seen at Scotts Bluff National Monument in the valley of the North Platte, in Toadstool Park north of Crawford, Nebraska, and particularly in the Big Badlands of southwestern South Dakota.

The kind of floodplain deposition characteristic of the Oligocene on the Great Plains ended about the beginning of the next epoch, the Miocene. In Nebraskathe process continued, but eventually erosion began wearing away the accumulated sediments. The land to the west was uplifted a little, and the streams flowed off the high ground fast enough to cut down into what they had just deposited. On this eroded surface, the layers of tan silts, fine sands, and clays known as the Gering Formation were deposited.

On top of the Gering Formation, in the Late Oligocene, is the Monroe Creek Formation, the oldest formation actually exposed in the Agate area. This formation is named for exposures in Monroe Creek Canyon north of Harrison, Nebraska, and may be up to 75 meters (245 feet) thick. You can see a little of the Monroe Creek Formation’s pinkish silts and volcanic glass shards exposed in the valley of the Niobrara River. Where it is more exposed by erosion than at Agate, the Monroe Creek Formation forms magnificent cliffs along the Pine Ridge and similar areas of high ground. The best local examples are at Fort Robinson State Park between Harrison and Crawford, Nebraska. A close look can be obtained in Smiley Canyon just west of the fort, where old U.S. Highway 20 is maintained as a scenic drive.

After the Monroe Creek interval, near the end of the Oligocene, deposition of the famous Agate Fossil Beds began. Geologists have named this sequence of grayish silts and sands the Harrison Formation for its occurrence near Harrison, Nebraska. A short interval of erosion separates it in some areas from the Monroe Creek Formation below, and its coarser sands indicate increasing uplift of lands to the west. Wind played a smaller role in deposition than in Monroe Creek times, though that is certainly not true at theStenomylusquarry. The Harrison Formation was the last of the truly widespread deposits seen in the Miocene of the Great Plains. Many rivers flowing eastward from the Rockies contributed sands to Wyoming, South Dakota, and Nebraska.

Generally the Harrison Formation and the overlying Marsland Formation are only moderately fossiliferous, but along the Niobrara River here at Agate, Nebraska, the accumulation of rhinoceros and camel skeletons is one of the wonders of the fossil world. Here, thousands of animals perished in two droughts which coincided with conditions perfect for preservation. About two kilometers (1.2 miles) eastof the Monument headquarters the dried-out, mummified bodies of perhaps a hundred or more little camels,Stenomylus hitchcocki, were buried under windblown sand during the first drought.

Shortly after the camels were buried there was a brief period of erosion and then the Marsland Formation began to be deposited. Named for a little village east of Agate, the Marsland Formation consists of basal river channel deposits followed by about 45 meters (150 feet) of wind-blown tan-and-gray sands. It is in one of these river channel deposits that the Agate rhinoceros quarries are located.

The second drought occurred early in Marsland times and literally hundreds of the little rhino,Menoceras, were preserved when their carcasses were broken up by a reborn river and buried like a mat of jackstraws in a river lake.

After Marsland times there was more erosion, in some places by rushing streams that cut down 91 meters (300 feet) through soft sediments, to the top of the Monroe Creek Formation. In these channels the Runningwater Formation was deposited because it filled in the stream valleys and wound around the high spots. This channel deposit is not found everywhere, but it does have an equivalent in southwestern South Dakota. Other deposits of similar age are found in many parts of the Great Plains, and they contain fossil animals like those found in the Runningwater Formation.

The turbulent streams which deposited the Runningwater Formation were flowing off newly uplifted land to the west. This was the beginning of the most recent major uplift of the Rocky Mountains, and it signalled a great change in the pattern of deposition on the Great Plains. No longer would broad blankets of sediments be deposited by sluggish streams originating in the low, broad warp of the Rockies.

This latest uplift is called the Rocky Mountain Revolution. It brought on a period of alternating cycles of deep channel cutting and stream deposition. Floodplain deposits were restricted to narrow ribbons in river-cut valleys. Even more important than the changes in deposition was the effect of this uplift on the climate. As the Rockies began to rise to their present height, the climate became increasingly arid and the tree-dotted savanna of the old Great Plains gave way to grasslands.

Several kilometers south of Agate, the Sheep Creek Formation was laid down during the Middle and Late Miocene. The appearance of the grazing horseMerychippusin these channel and floodplain deposits marked the establishment of the grasslands as the newly dominant ecosystem of the Great Plains. At that time the “modern” fauna began to replace the old, and new patterns of life were established.

Again rejuvenation of the stream system, probably reflecting further uplift in the west, started another erosional interval that began to wash away the beds just deposited. When deposition followed in the Late Miocene, a new series of channel and floodplain deposits, the Lower Snake Creek Beds, was laid down. On them was deposited the Upper Snake Creek Beds, and together they span most of the Late Miocene and the Early and Middle Pliocene epochs. Harold Cook collaborated with W. D. Matthew of the American Museum of Natural History, publishing important papers on the numerous finds from these fossiliferous deposits. Animals new to science are still being discovered in the Snake Creek Beds.

After Snake Creek times, the area immediately around Agate was left out of the mainstream of events on the Great Plains. The continuing uplifts of the Rocky Mountains were no longer recorded here in cycles of downcutting and channel deposition. If the cycles continued here, all traces have now been washed away—an unlikely possibility. The view from the high plains above the valley of the Niobrara River reveals only the rolling surface of the pre-Runningwater deposits.

A more complete record is found in the river terraces of major streams, the North Platte to the south and the White and Cheyenne Rivers to the north. These terraces tell the story of continuing uplifts. To the south, east, and northeast of the Platte the record is also written in fossil bones, but these are outside the scope of our story.

Northwestern Nebraska, northeastern Colorado, southeastern Wyoming, and southwestern South Dakota today remain a promised land for paleontologists studying mammal life in North America during the middle and later Cenozoic Era. The fossil deposits in the Agate area are surpassed in importance only by the Late Eocene and Oligocene deposits of the Big Badlands and Pine Ridge in South Dakota.

During the Age of Mammals (the Tertiary Period), three major environments dominated western Nebraska. The first of these occurred during the Paleocene, Eocene, and Oligocene epochs. This was a forest system where trees were the major component of the flora. Meadows were found only in scattered areas and can be considered a minor element. There is no geologic or paleontologic record of the Paleocene and Eocene in the Agate area, but when our present knowledge of the early Tertiary Rocky Mountain floras is projected eastward a bit, a predominantly forested landscape is indicated.

It is in some ways ironic that while the Oligocene land-laid sediments of southwestern South Dakota, western Nebraska, southeastern Wyoming, and northeastern Colorado contain one of the best vertebrate fossil records in the world, the plant record is almost non-existent. Unfortunately the groundwater chemistry that was so right for the preservation of bones was hostile to the preservation of plants. Hackberries (Celtis) and walnuts (Juglans) are the only recorded plant species from the Oligocene in this very large area. Because these are such widespread and climatically tolerant types, they tell us almost nothing about the environment. Indications of the flora at Agate may be obtained, however, from the extraordinary Late Eocene flora found at Florissant, Colorado, south of Denver. Although this deposit does contain some upland species, it generally indicates a warm temperate forest including such things as horsetail rushes, ferns, cattails, grasses and sedges, poplar, willow, birch, oak, elm, serviceberry, sycamore, maple, sumac, and—of course—hackberries and walnuts.

During the Early Miocene, slightly changed climatic conditions brought about by minor uplifts in the Rocky Mountain area transformed the immediate area of western Nebraska into a savanna of mixed trees and grasslands. This second system probably reached its climax just about the time the Harrison Formation was being laid down during the Early Miocene. This was a savanna with scattered clumps of trees, gallery forests, and grasslands. The modern world’s richest and most diverse fauna of hoofed mammals can be found on the savannas of east Africa. On the savannas, grazing and browsing (grass eating and leaf eating) adaptations of the larger plant eaters are represented.

35 million years ago

25 million years ago

15 million years ago

35 million years ago, life along the Niobrara River near Agate would have appeared something like this. Two oreodons (1) have startled an alligator (2) and two hippopotamus-likeAepinacodons(3) along the river bank. Climbing a tree is an opossum (4), one of the oldest forms of life in the world today. Note the many familiar trees and plants, particularly the cottonwood, willow, beech, dogwood, and cattail.

25 million years ago, a savanna dominates the Agate landscape. Copses of oak and pine are interspersed with open grassland. In the foreground are severalParahippus(1), an ancestor of today’s horse, whileOxydactyluscamelids (2) move away into the distance and, overhead, a hawk (3) searches for rodents.

15 million years ago, the Agate landscape has changed to an open prairie. A small herd ofMerychippushorses (1) races toward the arroyo in the distance, narrowly escaping ambush by a large, leopard-like cat known asPseudaelurus(2). A few cottonwoods, elms, sycamores, and willows grow along the river, but cedars predominate in the arroyo in the middle ground, where they are protected from winds that sweep across the plains. Though the animals have changed, the landscape is essentially like this today.

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