Part 3 Guide and Adviser 74

Contents for this section 77

1 Welcome to Agate Fossil Beds

[Illustration: James H. Cook examines a fossil fragment at the

quarries near Agate Springs Ranch about 1918.]

[Illustration: Besides fossils, Cook also collected Indian artifacts

and kept many of them on the walls of his study in the ranch house.]

Worlds of Past and Present

Imagine that you are a healthy young man, raised conservatively in

Michigan several years after the end of the Civil War. You are a skilled

all-around hunter and trapper. The railroad has just spanned the

continent, and stories of the West, its dangers, its people, and its

opportunities come to you frequently. You and a friend decide you must

see this land for yourself, and you save your money carefully against

the day when you will be ready to go west. Around 1869, at age 12, that

day comes.

At Fort Leavenworth, Kansas, you meet several cattlemen who tell you and

your friend where to get work as cattle herders. Before many years have

passed you have been a cowpuncher in Texas, you have fought Comanches,

and you have bossed a ranch crew for a wealthy Englishman. You go on to

fight the famous Apache Chieftain Geronimo as a scout with the U.S.

Cavalry, and you befriend a famous Sioux chief, Red Cloud. You marry,

buy a ranch in western Nebraska, and raise a family. And you become

something of a legend in your own time, your ranch known for its

hospitality to Indian, scientist, traveler—to one and all, rich or poor.

A movie script? Not at all—these are the essentials of the life of James

H. Cook. Known as “Captain,” James Cook became the owner, in 1887, of

the Agate Springs Ranch, founded earlier by his father-in-law. Under

Cook’s watchful eye, the ranch prospered and became a second home both

for the Oglala Sioux and for paleontologists bent on excavating the

fossilized remains of the life of 20 million years ago, found here along

the Niobrara River in western Nebraska.

This land, now encompassing Agate Fossil Beds National Monument, is

punctuated with low bluffs ascending westward toward the Rockies. It is

a land of sharp contrasts, of cool, inviting riverbanks and parched

ridges, the most famous of which are the fossil-bearing Carnegie and

University Hills. The surrounding grassy plains are a tapestry of wild

grasses—prairie sandreed, blue grama, little bluestem, and

needle-and-thread. The wildflowers lupine, spiderwort, western

wallflower, sunflower, and penstemon add touches of blue, purple,

orange, yellow, and red to the tapestry. In summer the dark green spears

of the small soapweed, a yucca, dot the brown grasses of the hillsides.

And just as they did more than 20 million years ago, cottonwoods and

willows provide shade and shelter for birds and other animals along the

river.

[Illustration: Professor Othniel C. Marsh, back row center, of Yale

University and his students look as if they are equipped for a

frontier hunting expedition. But instead of looking for live animals

in the West, they were hunting for fossilized remains of ancient

beasts. Marsh and his crews made many such trips, and it was on one

early trip that Cook and Marsh met, in Sioux country.]

[Illustration: Professor Marsh and the great Sioux chief Red Cloud

greet each other in New Haven in 1880.]

[Illustration: Professor Edward Drinker Cope competed with Marsh for

the best fossils. He once made the mistake of reconstructing a

skeleton hind end foremost; Marsh never let him forget it.]

Looking out over the rippling grasses, you grasp the fact that Nebraska

is larger than all of New England and feel the awesome spaciousness of

the Great Plains. The word “distance” has a different meaning here than

it does in the East. When James Cook came to the upper Niobrara River,

the closest town was Cheyenne, Wyoming—more than 160 kilometers (100

miles) to the southwest.

It was there, in Cheyenne, that Cook met Dr. Elisha B. Graham in 1879,

the year Graham selected this land for a cattle ranch as an investment

and as a summer retreat for his family. Graham named the place the 04

Ranch, apparently because it is near the 104th meridian. Cook visited

the ranch often in the early 1880s and courted Elisha and Mary Graham’s

daughter Kate. They were married in 1886 and lived near Socorro, New

Mexico, for a year before returning to Nebraska with their newborn

child, Harold, and buying the ranch from Dr. Graham, who moved to

California.

Cook began at once to make improvements to the ranch. He planted trees

by the hundreds and carried water to them faithfully to get them

started. As settlers failed to “prove up” their land claims over the

years, he added new lands to the ranch and changed the name to Agate

Springs Ranch in recognition of the native moss agates and the many

springs in the valley. He and Kate raised fine race horses as well as

cattle.

The period in which the Cooks took over the ranch was one of transition

from the frontier days of migrations and Indian wars to more settled,

orderly lives. Ranching and farming became the dominant mode of life in

the eastern approaches to the Rockies. Even oil exploration played a

part in the development of the land. The transition was a difficult one

for many, Indian and settler alike.

In some ways Kate Cook represented both the old and the new in Nebraska.

She was a fine horsewoman; one day she rode a bucking horse through the

streets of Cheyenne sidesaddle to win a bet for her husband. She was

refined, too, having taught herself French so she could read French

literature. Her mother, Mary Graham, became the first postmistress for

the small community around Agate.

And James Cook was more than an adventuresome frontiersman. He was

actively interested in community and national affairs and in current

scientific questions. He became a patient, knowledgeable mediator

between the Indians and the settlers, and he was looked upon by the

Oglala Sioux as a friend and host, and sometimes employer.

The Cooks became involved in a great scientific enterprise quite

accidentally around 1885, the year before their marriage. On a ride up

the conical buttes not far from the ranch house, a glitter under a rock

shelf caught Cook’s eye. They found fragments of bones scattered on the

ground. At first they assumed the bones were those of an Indian. But

Cook found instead “a beautifully petrified piece of the shaft of some

creature’s leg bone.” They carried it back to the house but didn’t

report the find until after they bought the ranch. Erwin Barbour of the

University of Nebraska was the first to respond to their reports and in

1892 became the first professional geologist to visit the area and do

some prospecting.

The Cooks’ discovery thrust them and their ranch into a subtle battle in

the American West, a continuing struggle to find the best fossils with

which to reconstruct the ancient past. For centuries it had been thought

that life on our planet was only a few thousand years old, but by the

late 19th century science had evolved beyond that point of view. Now

paleontologists and their excavation teams were scouring the West in

search of fossils that might provide clues to the beginnings of life.

The two most noted antagonists in this feverish search were Professors

Edward D. Cope of Philadelphia and Othniel C. Marsh of Yale University.

Cook knew them both, but the discoveries at Agate would wait for the

next generation of scientists.

[Illustration: University Hill, left, and Carnegie Hill dominate the

Niobrara River Valley. The hills received their names from the

paleontological teams that worked them from the University of

Nebraska and the Carnegie Museum.]

Cook had first met Marsh in 1874. Marsh had just arrived in Oglala Sioux

country to hunt fossils, but the Sioux Chief Red Cloud was suspicious.

Red Cloud was convinced Marsh and his men were just another party of

gold seekers. Cook, an able linguist, was then trailing cattle from

Texas to the northern railroads and reservations. He persuaded Red Cloud

that Marsh wanted only “stone bones” and averted a potentially

disastrous clash. This incident led to a lifelong friendship between

Cook and Red Cloud. And for Marsh this proved to be the last of many

expeditions as he relied on others to send him specimens from likely

fossil sites throughout the West.

[Illustration: An upland sandpiper, one of many birds that can be

seen at Agate Fossil Beds, perches on a fence post.]

Professor Cope and his crews often worked the same localities as Marsh.

Like Marsh, Cope tried to get the best specimens, and each occasionally

outbid the other for them, leaving bewildered farmers and ranchers to

puzzle over these men’s obsession with the past. Conflict also arose

over the naming of animals previously unknown to science.

The hills of Agate Springs Ranch proved to be a rich archive of ancient

life. Dr. Barbour and his students confined their efforts to what soon

became known as University Hill. Thus began a quiet rivalry with Olaf

Peterson and his crew from the Carnegie Museum in Pittsburgh,

Pennsylvania, who worked what became known as Carnegie Hill. The most

numerous fossils these teams found at Agate are the remains of the

pony-sized rhinoceros _Menoceras_, but the site also is known for

fossils of the gazelle-like camel _Stenomylus_, the early small horse

_Miohippus_, and the corkscrew burrows of an ancient beaver,

_Palaeocastor_.

Other distinguished scientists visited Agate over several decades. Among

them were Henry Fairfield Osborn and Albert Thompson of the American

Museum of Natural History in New York. The collections these men made at

Agate are still being studied and exhibited.

James and Kate Cook’s older son Harold caught the fever, too. He became

a trained geologist and in 1910 married another geologist, Eleanor

Barbour, daughter of the distinguished Nebraska geologist. The new

generation of Cooks continued the tradition of hospitality and

scientific interest, encouraging further excavations of the fossil

treasures of Agate. Perhaps Harold Cook’s greatest moment of scientific

glory came in 1926, when he and other scientists participated in the

finds at Folsom, New Mexico, which proved to be a turning point in the

study of the human prehistory of North America. George McJunkin, a black

cowboy, had spotted the ribs of an animal protruding from the banks of

an arroyo. The Folsom spearpoint and the bones of an extinct form of

bison found there indicated that humans had lived on this continent for

more than 10,000 years, a startling revelation at that time though today

scientists put the figure at more than 40,000 years.

[Illustration: Next to the fence stands a windmill, which once

provided water for excavation teams.]

[Illustration: The narrow Niobrara River winds through the

surrounding tableland, carving out bluffs and exposing occasional

fossils.]

In time the Cooks’ house became a repository for a substantial number of

Indian artifacts and natural history specimens. On summer weekends and

holidays tourists ventured out to the ranch to see the Cook Museum of

Natural History. James Cook personally guided many through the

collection, but usually the whole family participated, leading the

curious through three rooms and a small hallway.

Harold Cook wanted Agate Springs Ranch to provide an enduring memorial

to the ancient past. Soon after his death in 1962 his second wife,

Margaret Crozier Cook, and friends began a campaign to add the fossil

beds to the National Park System. Their efforts succeeded in 1965, when

Congress authorized the establishment of Agate Fossil Beds National

Monument.

Today you can walk about Carnegie and University Hills where the great

digs took place. You can see a few exposed fossil specimens, and you can

try to recreate in your mind the life and landscape of this part of

Nebraska 20 million years ago. To help you do that, we have asked

paleontologists James R. and Laurie J. Macdonald, in Part 2 of this

handbook, to take you on a journey to the past and then examine the

evidence.

Welcome to the worlds of past and present at Agate Fossil Beds National

Monument.

2 A Landscape Rich With Life

_Text: James R. and Laurie J. Macdonald_

_Illustrations: Jay H. Matternes_

[Illustration: This mural depicts life in the Agate Fossil Beds area

in the early Miocene Epoch, about 20 million years ago when the

story on the following pages takes place. The painting is a

composite of life at that time; if you had been there then, you

would not have been able to see all of these forms of life together

at any given moment! The original mural hangs in the fossil halls at

the Smithsonian Institution’s National Museum of Natural History in

Washington, D.C.]

[Illustration: Key to mural]

1/Moropus

2/Promerycochoerus

3/Menoceras

4/Oxydactylus

5/Daphoenodon

6/Stenomylus

7/Dinohyus

8/Merychyus

9/Palaeocastor

10/Parahippus

11/Syndyoceras

A Visit to the Past

Come enter into our imaginations and return to a day along the Niobrara

River in western Nebraska 20 million years ago. Let’s go back and have

an imaginary look. To set the mood, think about the wild animal movies

made in modern Africa during the last 40 years. Think of a land swarming

with life, of extensive grasslands dotted with trees through which great

herds of grass- and leaf-eating animals are wandering. Look sharply into

the shadows under the trees and amid the high grass where the

meat-eaters are resting or stalking their prey. When you have this

picture of wildlife in mind we’re ready for our journey into the past.

Projecting ourselves back those 20 millions of years, we find ourselves

in a landscape filled with animals. Some of the animals are not much

different from those living today, but others are so bizarre that you

may have a hard time believing they really existed.

Dawn is building a new day, and those animals which hunt and feed at

night are disappearing into their lairs. There are so many kinds of

livings to be made that the day isn’t long enough for all animal

varieties to be about and active only in daylight. As the light spreads

over the land we see that it is an open, sunny place of mixed grasses

and trees—mostly grassland, but here and there single trees or small

clumps not big enough to be called groves. We would call it a savanna.

A broad river runs through the land, and for lack of a better name we

can call it the Niobrara or “Running Water,” the name given by Indians

to the much smaller modern stream. This ancient Niobrara was a bold,

wide stream with deep pools and sandbars. It was not cutting a valley as

its modern counterpart is doing, but was carrying sand and silt down

from the then-young Rocky Mountains. It sometimes flooded, and as it

spread out over the wide, flat plain it deposited layers of sand and

silt that geologists today call the Harrison Formation. The ancient

Niobrara and other similar rivers spread the same sediments over nearby

areas in eastern Wyoming and southwestern South Dakota. In the

sediments, paleontologists would one day find millions of bones.

Our ancient river was the center of life for untold numbers of animals.

They lived in it, along its banks, in the willow thickets that grew on

its more permanent sandbars, and on the broad, tree-dotted plains that

stretched to the horizon beyond the river’s normal course. Great herds

of small horses, rhinoceroses, camels, and other dwellers on the savanna

came to water holes to drink and perhaps to wallow in the cool and

refreshing pools. Although all else might be anticlimactic, let’s look

at the rhinoceroses first.

We know that in the modern world rhinos belong in Africa and

southeastern Asia, not North America, yet this continent was the major

home of these strange beasts for millions of years. Rhinos did not

become extinct in North America until about 5 million years ago, during

the Pliocene (see geologic time chart on page 46). Along the ancient

Niobrara the rhino herds are not made up of the giant mammals we see in

zoos today. The ones we see moving slowly toward the river on this early

Miocene day are no larger than big domestic pigs. They are the first

among the rhinos to have horns—not one behind the other, but a pair near

the end of the nose, side by side. To scientists today these rhinos are

known as _Menoceras_. The name _Diceratherium_, once used both for these

small rhinos and a larger type of ancient rhino, now refers accurately

to just the large rhino.

[Illustration: _Menoceras_]

Look off to the south. There’s a herd moving slowly but purposefully

down to its favorite watering hole. You can see that the males have the

paired nose horns, and that the females, which are about the same size,

do not. Trotting amid the herd are fat little colts whose seriousness of

purpose belies their youth. The herd seems to be made up of about 50

individuals moving through the tall grass like a flattened dark gray

cloud. Suddenly they are startled by a large cat or dog stalking through

the grass, and all the males on the side nearest the enemy bunch

together to face the hungry hunter. Most of the herd continues to flow

across the plain, and when the danger is past the guardian males catch

up in a lumbering gallop.

Finally the herd reaches the river and spreads out through the shallows.

It is hot today, and the flies are biting even through the tough rhino

hides. Many of the adults go into deeper pools to roll and soak while

the young drop their serious attitudes and frolic in the shallows.

As the day wears on, the herd leaves the river and feeds on the leaves

and stems of scattered trees and willow thickets along the river. When

twilight comes, the herd draws together, colts and females toward the

center and bulls around the edges. After a period of milling and

pushing, the herd finally beds down for the night, with only the

perimeter guards moving around on the edges.

The daily routines of the other herds of grass- and leaf-eating animals

generally follow somewhat different patterns from that of the rhinos. Of

them, only the piglike oreodons wallow in the river. The others spend

nearly all their time out on the savanna, coming to the river only at

dawn or dusk to drink.

Oreodons were among the most abundant medium-sized animals of the Middle

Tertiary. A strictly North American group, they have been described as

looking like a cross between a sheep and a pig. As small as a house cat

or as big as a domestic pig, these mammals reached a peak in abundance

and variety between the Middle and Late Oligocene (though they are known

from the Late Eocene through the Late Miocene). This peak probably has

never been equalled by any other group of mammals in such a size range.

As we look out among the herds of animals dotting the plain, we can see

only a few small bands of oreodons. There’s a group coming toward us

now. These are some big, ugly ones with large triangular-shaped heads.

The backs of their cheek bones flare out far to the sides, so that with

their narrow snouts they are most peculiar looking. Their bodies are

long and rather nondescript, and their legs are short but slender. This

particular kind is known as _Promerycochoerus_ (“before ruminant hog”)

and is just about the largest of the oreodons. They are really a rare

sight here at Agate. Perhaps the large herds of _Menoceras_ fill their

ecologic niche locally, and the oreodons have found they cannot

successfully compete with the rhinos for food, water, and living space.

After all, not everything can fit into Paradise.

[Illustration: _Promerycochoerus_]

Look to the northeast: there’s a herd of _Miohippus_ (“Miocene horse”)

wading into the river to drink and browse in the willows along its

banks. Let’s walk toward the herd slowly and quietly. We should take an

especially good look at this herd—they are part of a doomed race! The

genus _Miohippus_ is making its last stand at this time. When conditions

change, well adapted species may restrict their ranges to what is left

of the old environment; they may adapt, if they are able, to the new

conditions; or they may not survive if they cannot adapt.

_Miohippus_ did all these things. Some species of the genus became

extinct. Some evolved into something else. But the end result was the

complete termination of the genus _Miohippus_ as paleontologists

recognize it. Much of the environmental pressure coming to bear on the

genus _Miohippus_ was a result of mountain building to the west. As the

young Rockies rose, rain-bearing winds from the oceans far to the west

were wrung of their moisture. This same circumstance makes the high

plains a land of little rain today.

The scattered trees and groves we see from our vantage point of long ago

will disappear and be replaced with a sea of drought-tolerant grasses.

In effect, the savannas will give way to prairies. _Miohippus_ will soon

be yielding its place to descendants which can eat grass as a steady

diet. Grass is much harsher on the teeth than the foliage that

_Miohippus_ eats. In eating grass, grazing animals pick up sand and silt

enough to quickly wear away teeth designed for leaf-eating. The

descendants of _Miohippus_ will become better runners, too, with longer

and more powerful legs. As the trees disappear there will no longer be

friendly clumps of greenery to hide behind when hungry meat-eaters are

on the prowl. From now on, fleetness of foot will be a most important

factor in horse survival.

_Miohippus_ will also give rise to somewhat larger forest horses that

will survive on into the Pliocene in patches of woodland. They will be

little changed except in size (some came to be nearly as large as the

modern horse), and some of them will even cross the Bering Land Bridge

into Eurasia. This is the last time, however, that we’ll see these

primitive horses in large numbers here in North America.

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_.

[Illustration: _Parahippus_]

_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.

[Illustration: _Oxydactylus_]

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.

[Illustration: _Stenomylus_]

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.

[Illustration: _Moropus_]

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.

[Illustration: _Dinohyus_]

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.

[Illustration: _Syndyoceras_]

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.

[Illustration: _Daphoenodon_]

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.

[Illustration: _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 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.

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 the _Stenomylus_ quarry,

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 the dry 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 of _Menoceras_ went 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 of _Menoceras_ were

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

that _Menoceras_ left 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 of _Menoceras_ bones 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 camel _Stenomylus_ tells the

same story in scores of skeletons east of the _Menoceras_ burial 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 the _Menoceras_, 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.

Excavations at Agate Springs

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

great _Menoceras_ bone-bed, and occasional _Moropus_ and _Dinohyus_

specimens. 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 nearby

_Stenomylus_ quarry 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 and

_Stenomylus_ marked 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

beardog _Daphoenodon_.

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

pages 40 and 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 pages 86-87 for a listing of museums with specimens from Agate

Fossil Beds.

[Illustration: Extracting a slab]

[Illustration: Members of Peterson’s crew built a box around a slab

in the _Stenomylus_ quarry around 1908 in preparation for shipping

to the Carnegie Museum.]

[Illustration: With a team of horses, O. A. Peterson’s field crew

moves dirt out of the _Stenomylus_ quarry 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.]

[Illustration: 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.]

[Illustration: Paleological laboratory.]

The Beginnings of Paleontology

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.

[Illustration: 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.]

[Illustration: 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 classic _Histoire 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.]

[Illustration: 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 work _Récherches sur les Ossemens Fossiles de Quadrupèds_

was 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.]

[Illustration: 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 book _On 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.]

[Illustration: 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 his _Principles of Geology_ was 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.]

[Illustration: 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.]

[Illustration: _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._]

Geologic Time Chart

Period Epoch Time span (years before present)

Cenozoic Quaternary Pleistocene 10,000 to 2 million

Tertiary Pliocene 2 to 5 million

Miocene 5 to 23 million

Oligocene 23 to 34 million

Eocene 34 to 55 million

Paleocene 55 to 65 million

Mesozoic Cretaceous 65 to 138 million

Jurassic 138 to 205 million

Triassic 205 to 240 million

Paleozoic Permian 240 to 290 million

Carboniferous 290 to 365 million

Devonian 365 to 410 million

Silurian 410 to 435 million

Ordovician 435 to 500 million

Cambrian 500 to 570 million

Precambrian 570 to 4,500+ million

The Geologic History of Agate

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 of

_Apatosaurus_, _Diplodocus_, _Stegosaurus_, and _Allosaurus_ have 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 giant _Portheus_ (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

Pierre Shale, 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 Nebraska the 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 the _Stenomylus_ quarry. 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) east of 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

horse _Merychippus_ in 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.

Ecology: Change and Adaptation

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.

[Illustration: 35 million years ago]

[Illustration: 25 million years ago]

[Illustration: 15 million years ago]

[Illustration: 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-like

_Aepinacodons_ (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.]

[Illustration: 25 million years ago, a savanna dominates the Agate

landscape. Copses of oak and pine are interspersed with open

grassland. In the foreground are several _Parahippus_ (1), an

ancestor of today’s horse, while _Oxydactylus_ camelids (2) move

away into the distance and, overhead, a hawk (3) searches for

rodents.]

[Illustration: 15 million years ago, the Agate landscape has changed

to an open prairie. A small herd of _Merychippus_ horses (1) races

toward the arroyo in the distance, narrowly escaping ambush by a

large, leopard-like cat known as _Pseudaelurus_ (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.]

In western Nebraska the savanna environment lasted for only a very short

time, in a geologic sense, before it gave way to a wave of advancing

grasslands, the third phase of Tertiary environment in the area.

Tallgrass prairie such as that still found 325 kilometers (200 miles)

east of Agate a century ago must have been first among the grassland

types. Trees, when present at all, were restricted to the borders of

streams. Then as the climate became even more arid the prairie or tall

grass retreated eastward, while the forest moved before it even farther

to the east and south, and the modern shortgrass of the plains took its

place. Today Agate lies in one of the valleys whose rivers are slowly

dissecting the High Plains.