Chapter III: Part 3: Guide and Adviser 74 (2)

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.

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.

The modern plains are dominated by short, curly, sodforming buffalo grass, a plant well adapted to the area’s light rainfall, periodic droughts, low humidity, rapid evaporation, and high winds. The dominant vertebrate animals are burrowers and grazers, and dogs are the primary carnivores. Hoofed animals such as the pronghorn, the ultimate in the running and bounding adaptation; jumpers and hoppers, such as jackrabbits and jumping mice and rats; and burrowing mound builders, such as the prairie dog (a large ground squirrel), the pocket gopher, and harvester ants typify the major occupations of plains animals.

The environmental type seen on the Great Plains of North America is elsewhere best developed in the Pampas of Argentina, the Puztas of Hungary, the Veld of Africa, and the Steppes of Russia. In the climatic classification of the climatologist and geographer, the term _steppe climate_ is applied to all these areas, the Great Plains included.

If the savanna is the halfway station between forests and grasslands, then the fossil fauna of the Early Miocene at Agate was a fauna in the beginning of a serious transition. In the vicinity of Agate, the fauna from the Late Oligocene was dominated by mammals with low-crowned teeth. The crown is that part of the tooth which is above the roots and exposed beyond the gums. Among the herbivores, the browsers can live a long life with low-crowned teeth. But when any appreciable amount of grass, particularly the short, tough grass of the plains and the abrasive dirt and sand that accompanies it, becomes part of an herbivore’s diet, there is a great increase in the rate of tooth wear. Teeth which have evolved for browsing quickly wear down to the gums and the individual dies of starvation.

Accompanying the development of extensive grasslands came the evolution of the high-crowned tooth. This process begins simply with the growth of a taller crown that erupts completely from the gum right after the milk or deciduous teeth fall out. Another step is the development of a longer or higher crown most of which is held in the jaw and then slowly pushed out as the chewing surface is worn down. This is the “mechanical pencil” effect in that the “lead” may be pushed out as needed. Teeth of this type are perhaps best seen in the later horses. From their appearance in the Late Paleocene until the end of the Early Miocene, all horses had low-crowned teeth. With these they could chew the soft leaves and twigs of trees and shrubs, first in the forests and later in the groves and clumps on the developing savanna. By Early Miocene (i.e., Harrison) times, there was only a slight increase in crown height in _Parahippus_, but it had evolved an increasingly complicated crown pattern which served to lengthen the time it took for the tooth surface to wear down flat. With the greater aridity of the changing climate, the teeth of _Parahippus_ became higher and higher crowned, as the individuals with the best teeth lived longest and had greater opportunity to produce offspring than those with lower-crowned teeth. In some species, the tooth material called cement, which ordinarily covers the roots of the teeth, began also to cover the enamel of the crown and give additional wearing strength to the teeth. Soon after the beginning of the Middle Miocene, two species had developed cement-covered teeth whose crowns were high enough to warrant placing them into two new genera of horses, _Merychippus_ and _Protohippus_. These forms, first recognized in the Lower Sheep Creek Beds in the Agate area, were the first horses to use the mechanical-pencil effect, having cheek teeth that continued to rise out of the jaw as the tooth was worn down. _Merychippus_ later gave rise to a line of three-toed horses, which lived on into the Pliocene; _Protohippus_ gave rise to a line which ultimately led to _Equus_, the modern horse.

The ultimate in high-crowned teeth occurs when roots do not ever form at the base of the tooth; additional crown material is constantly added at the bottom of the tooth as it is pushed out of the gum. This type of growth resembles the foundry process of extrusion, where metal or plastic is pushed through a mold to produce a continuous strand. This extreme development is seen in the incisors or gnawing teeth of beavers, gophers, and other Late Oligocene rodents, and in the grinding teeth of only a few forms. The cheek teeth (the grinders) of modern pronghorns (artiodactyls), gophers (rodents), and rabbits (lagomorphs) are typical of this kind of development today. During the Middle Oligocene, only the strange little fox terrier-sized, flat-headed oreodon _Leptauchenia_, the tiny “deer” _Hypisodus_, and the rabbit _Palaeolagus_ had mechanical pencil-type teeth. Some of the rhinos then had fairly tall crowns, but these don’t really qualify as high-crowned teeth. It was not until later on, when the grasslands took over completely, that high-crowned teeth really came into their own.

There was no dramatic change in the fauna at the beginning of the Miocene, and many Oligocene genera carried over into the new epoch. Most of the Eocene hold-overs, primitive animals that had survived in the extensive forests, became extinct when the forests began to retreat; but for the most part the record continued undisturbed. This is to be expected where the deposition of sediments continues without interruption. (Remember that the epochs, periods, and eras were originally based on breaks in the European sedimentary record reflecting local events which would not necessarily show up in North America’s sediments.)

By the time the Harrison Formation was deposited, the development of the halfway world of the savanna was beginning to affect the fauna. Although the Oligocene and the very earliest Miocene mammal faunas were highly varied and rich in types of animals, much of this was due to the continued presence of primitive and archaic forms, and to the explosive development of rhinos and oreodons. With the savanna becoming the dominant landscape, the shift to grazing and away from browsing became evident. Or, at least, the presence of animals that both browsed and grazed was indicative of changing times. As was mentioned earlier, grazing and burrowing are characteristics of plains herbivores. In such a transitional period we would expect to find an increase in burrowers and grazers as grasslands became more common.

Evolutionary Change

Animal species respond to environmental changes in a variety of ways.
Simply put, some species die off, some adapt physically, and some move
to a different habitat. On the next few pages are examples showing how
three species responded to long-term environmental changes in the area
around Agate Fossil Beds. The _Stenomylus_ line died off; _Miohippus’_
evolutionary line remained a grazing animal but changed physically
over the years, eventually becoming the modern horse; and the
_Palaeocastor_ line moved from land to water, gradually evolving into
the beaver.

Each of these three animals is portrayed here with partial skeleton,
musculature, and outer skin to help you see its general composition
and to emphasize certain physical features that developed in the
species over time. Paleontologists, of course, work this way. From
fragments and bones they reconstruct full skeletons, and from surmises
about muscular structure, often based on present-day animals, they
project the appearance of the animal. The artist, in this case Jay
Matternes, then brings together these bits of evidence to give us a
picture of life long ago.

Stenomylus

A small, gazelle-like camel similar to the present-day gerenuk of
Africa. _Stenomylus_ is the second most common animal found in the
fossil beds at Agate. _Stenomylus_ had hard hooves like modern
antelopes and deer, unlike modern camels which have flesh-padded feet
adapted to desert terrain. The three-hued coat is inferred from the
coat of the modern gazelle, a similar form in adaptation.
_Stenomylus’_ evolutionary line eventually died out in North America
at the end of the Pleistocene. No one knows why both camels and horses
died out on this continent.

The distance between grid lines represents five centimeters.

2 _Stenomylus’_ musculature was adapted for high-speed running,
similar to the present-day pronghorn.

3 The back structure suggests that _Stenomylus_ would have made
short, choppy leaps, not the graceful, arcing leaps of a modern
impala.

4 Limbs were long in proportion to the body, allowing the animal to
run with great speed.

5 _Stenomylus_ had a hard, chitinous hoof, an adaptation for greater
running speed, and for sure footing on rough terrain.]

Miohippus

Over the last 60 million years the horses have evolved from small, terrier-sized animals to the diversity of size we know today, from the huge Clydesdales to the diminutive Shetland ponies. The three-toed early horse known as _Miohippus_ was about the size of a sheep. The descendants of _Miohippus_ apparently went in two directions in their evolution: One group continued to be forest-grazing, three-toed horses that eventually reached the size of modern horses but died out later. The other group, through such intermediate forms as _Parahippus_, became grassland forms that led eventually to the modern one-toed horses. Horses became extinct in North America at the end of the Pleistocene, but no one knows why. They continued to evolve on other continents and were re-introduced in historic times.

The distance between grid lines represents five centimeters.

2 Limbs were long in proportion to the body, an evolutionary trend
in the horses for speed in open-plains running, rather than darting
about in forests.

3 Most of the weight of the animal was on the middle toe, which has
become a single toe in modern horses. This is an adaptation for
endurance and stability in open grasslands.

4 The upright mane is a primitive horse characteristic; wild horses
today have reverted to this trait.

5 The coat is shown as striped, a probable holdover from earlier
horses that dwelled in forests, where a striped coat would provide
camouflage.

6 A large, deep mandible supported teeth adapted to grazing and the
grinding of grasses and other wild plants. The teeth were
deep-rooted and continuously erupted as the surface was worn down by
the grit and dirt that came with the large quantities of plant food
consumed daily.]

Palaeocastor

_Palaeocastor_ was an ancient beaver whose mode of life was like that of a modern prairie dog—land-oriented instead of water-oriented. _Palaeocastor_ was small, about 12 centimeters (5 inches) high, and about 30 centimeters (12 inches) long. Its fossilized spiral burrows, called _Daemonelix_, survive to tell us what its habitation was like, a feature unique to _Palaeocastor_ among all the fossil beavers. The _Daemonelix_ shown here dwarfs a member of Olaf A. Peterson’s field crew from the Carnegie Museum. The bones of a _Palaeocastor_ and one of its predators were found at the bottom of one such burrow, helping to prove that _Palaeocastor_ was responsible for making these corkscrew holes in the ground.

The distance between grid lines represents five centimeters.

2 The complex musculature supported the use of the forelimbs in
burrowing. _Palaeocastor_ had a collarbone or clavicle, like us, for
greater agility in using the forelimbs.

3 The forelimbs were adapted to burrowing in the ground.

4 The tail is like that of a modern burrowing rodent, such as a
muskrat, whereas the modern beaver has a different, very specialized
tail.]

A grazing animal is fairly easy to recognize, but how can we recognize a burrower? Some have radically adapted limbs and claws. Obvious cases are the common garden mole and the armadillo. The mole has powerful attachments for the muscles of the upper arm on the humerus, a bone so flattened that its width has come to match its length. Moles also have long, broad digging claws. The armadillo, which is also a digger but not a burrower in the same way a mole or a gopher is, has large curved claws for digging. Moles did start to become quite common in the Late Oligocene, so we can assume that a good burrowing environment was present.

Another group which became extraordinarily common in the Late Oligocene of western North America was that of the ancestral pocket gopher. Direct proof that this group actually burrowed does not exist, but the abundance of fossil gophers suggests that they might have lived underground in colonies.

A real surprise at Agate is the number of beaver burrows. The famous _Daemonelix_ or “devil’s corkscrew” attests to the dense population of _Palaeocastor_. By that relatively advanced stage of beaver evolution, the animals might be expected to behave like the modern-day muskrat, perhaps digging dens along stream borders and spending some of their time in the water. The presence of skeletons in the spiral burrows, however, indicates that _Palaeocastor_ was primarily a burrower, one which perhaps lived very much like our present-day prairie dog. Despite that, there is no apparent structural modification to indicate burrowing abilities.

Changing environmental conditions were pushing _Palaeocastor_ toward extinction in the Early Miocene. The disappearance of that ancient beaver, while not unusual, presents a problem for the careless observer who might assume that ancient animals behaved like their modern counterparts. The burrowing beavers of Miocene Agate certainly have no modern counterparts.

While we can delineate in a general way the prehistoric life of Agate, we can’t describe the past in any detail. Plants most directly reflect the effects of climate—and plant fossils are absent at Agate. As the base of the food chain, plants carry the influences of climate on to the plant-eating animals. From the numerous animal fossils found at Agate we have learned most of what is known about the environment of that time. Sediments tell a good part of the story, and floras from other localities help, but much of Agate’s ancient ecology must be inferred from the bones.

Today, standing on the porch of the visitor center or walking along the path to University and Carnegie Hills, visitors find themselves in the midst of the shortgrass prairie. Five distinctive plant communities share this prairie, coexisting in a dynamic relationship which depends upon local climate variations.

Even to the untrained eye, it is evident that the basic short-grass pattern has been modified by the shape of the land and by the Niobrara River. In the stream valley, along the tributaries, and on shaded north-facing slopes, the shortgrass community is mixed with taller grasses. If a dry cycle began, the short grasses would take over the whole area by migrating downslope from the exposed prairies. Of interest is the fact that over-grazing by either domesticated or wild animals will have the same effect as a dry period in that taller grasses will be replaced by short ones.

Let’s examine the five communities present today so we can appreciate the complexity of relationships between living things and the earth upon which they depend.

First, we can begin in the Niobrara River itself. The river’s water-dwelling plant inhabitants include algae, which grow underwater.

Between the river and the dry ground is a second community—the marsh—which is often more wet than dry. The marsh has its own characteristic plant association. Most familiar are the cattails, mints, and willows, but just as important ecologically are arrowleaf, rush sedge, marshweed and blue verbena. These are moisture-loving plants that thrive on being thoroughly soaked during the wet part of the year.

Beyond the marsh on the valley floor is a third community. Here the water table (the top of the saturated soil and rock zone) is close enough to land surface that the plants can easily send their roots down into the saturated zone. Here, in what the plant ecologists call the “sub-irrigated floor plain” we find a mid-grass community. Eighty-five percent of the vegetation is slender wheatgrass. Its wheat-like heads may, under favorable conditions, grow to a height of one meter (three feet). At Agate it is seldom over knee high. Kentucky bluegrass takes care of another 10 percent of the plant population. Imported from Europe as a pasture grass in the 1600’s, it spread so rapidly that it often beat the settlers onto new land as they moved westward. The remaining five percent includes imported redtop and such native grasses as switchgrass, foxtail barley, little bluestem, prairie cordgrass, and inland saltgrass. Wildflowers such as Flodmon thistle, yarrow, heath aster, salsify, and blue-eyed grass complete the community.

Moving farther away from the stream, we rise up onto terraces within the valley. These terraces represent levels where the stream paused in its downcutting and cut sideways for awhile. At a drier level, on deep, well-drained sandy soils, they support the fourth or mixed-grass community.

No exotics have yet appeared in this plant community. The grasses include prairie sandreed, sand bluestem, blue grama, needle-and-thread grass, and Indian ricegrass. Wildflowers include the prominent phlox, penstemon, and lupine. Unwelcome (to man and his grazing animals) is _Astragalus_, the selenium-concentrating plant better known as loco weed. The brittle prickly pear and spiderwort cactus are found here too.

At higher levels in the terrace community, slightly steeper slopes and shallower soils cause some change in this mixed-grass assemblage. Here the dominant grasses are little bluestem, threadleaf sedge, needle-and-thread grass, and blue grama. Lupine disappears, and common pricklypear becomes the only cactus. In this community is found the yucca, its flowers a beautiful soft yellow in season and its spiny leaves painful at any time of the year. Avoid this plant; yucca spines break off under the skin and soon cause irritating festers. The yucca moth, often seen flying around the yucca seed pods, lays eggs in the plant’s lemon-sized fruits. Inside the fruit are long rows of flattened, wedge-shaped seeds. When the yucca moth eggs hatch into caterpillars, they eat their way through the seeds, killing them. On the other hand it is the yucca moth with its long tongue that is solely responsible for pollinating the yucca flower! If you find a yucca fruit in early summer, you can (elsewhere than in the park) slice through it and see the caterpillars at work.

On the high bluffs and overgrazed terraces is the fifth community, the short grass. This community too can be divided into two slightly different parts. The bluffs support blue grama grass, needle-and-thread grass, and Sandberg blue grass. Flowers and shrubs include _Eriogonum_, brittle pricklypear cactus, pepperweed, penstemon, broom snakeweed, fringed sagewort, and yucca. The other part of this community, the overgrazed terraces, have threadleaf sedge, needle-and-thread grass, and blue grama. Except for the familiar penstemon, all the flowers are restricted to this community. Gronwell, menzania, and bee plant are indicators of overgrazing.

Certain cyclical variations are characteristic of these plant communities. First, the shortgrass and mixed-grass areas ebb and flow with changing moisture conditions from year to year. Second, grass populations change with the seasons. Cool-season grasses (foxtail barley, Indian rice grass, Kentucky bluegrass, needle-and-thread grass, Sandberg blue grass, and slender wheatgrass) flourish during spring and fall. During the warm summer the blue grama, inland saltgrass, little bluestem, prairie cordgrass, prairie sandreed, and switchgrass predominate. This natural adaptation to seasonal conditions uses the greatest potential of the growing season and at the same time provides species that will flourish in both wet and dry cycles.

After reading this last section, you might look back at the section on Early Miocene ecology. Comparison reveals that a great deal of information can be obtained by examining living plants. In contrast, the lack of fossil flora from the Early Miocene at Agate has resulted in a scarcity of ecological information from that early epoch. Scientists begin their reasoning by such comparisons; you can begin your own exploration of the past in the same way.

3 Guide and Adviser

Visiting the Park

Contents of This Section

Visiting the Park 77
Location
Area
Climate
When to Visit
Visitor Center
Activities
Camping
Nearby Accommodations
Transportation
Establishment Date
Address
Access
Protection 80
Park Regulations
Safety Tips
Birding Along the Niobrara 81
Taking the Annual Count
Collections of Agate Springs Fossils 86
NPS Areas With Fossil Exhibits 88
Badlands
Dinosaur
Florissant
Fossil Butte
Petrified Forest
John Day
Hagerman
Nearby National Parks 90
Badlands
Devils Tower
Fort Laramie
Jewel Cave
Mount Rushmore
Scotts Bluff
Wind Cave
Not So Nearby National Parks 92
Bighorn Canyon
Little Bighorn
Rocky Mountain
Theodore Roosevelt
Armchair Explorations 93

Location

Northwestern Nebraska 69 kilometers (43 miles) north of Scottsbluff along the Niobrara River.

Area

1,116 hectares (2,762 acres).

Climate

Temperatures range from winter lows of -38° C (-36° F) to summer highs of 39° C (101° F). Winter temperatures average 1° C (33° F), and winter snow averages 60 centimeters (2 feet) for the whole winter. However, snowdrifts can be much higher. Summer nights are cool, with temperatures averaging 10° C (50° F). Average annual precipitation is 41 centimeters (16 inches), with most precipitation in April and May.

When to Visit

Most people go to the park some time between June and August, but you can avoid the high summer temperatures by visiting in the spring, fall or—if you don’t mind the cold and snow—in the winter. Spring can be blustery, but the fall is usually dry and the days are cool. Check ahead on local weather conditions if you plan a winter visit. Museums and tourist attractions in nearby Fort Robinson are open Memorial Day to Labor Day.

Visitor Center

A ranger is on duty to help you and answer your questions. Fossil exhibits and part of James H. Cook’s personal collection of Indian items are on display in the visitor center, and publications about the park, paleontology, and history are on sale.

Activities

A trail from the visitor center takes you on a tour to both University and Carnegie Hills, with an interpretive display at each. The roundtrip distance is three kilometers (two miles) and takes about one hour. You may fish for German brown and rainbow trout in the Niobrara River if you have a Nebraska fishing license. The park has several tables for picnickers.

Camping

The park has no camping facilities, but there are state campgrounds near Harrison and near Fort Robinson, Nebraska, and a commercial campground on Nebr. 26 between Mitchell and Scottsbluff, Nebraska.

Nearby Accommodations

Hotels, motels, food stores, outdoor supply stores, and restaurants are available in Scottsbluff, Nebraska. A motel, restaurant, gas station, and grocery store are in Mitchell, Nebraska, 55 kilometers (34 miles) south of the park. There are a motel, food store, drugstore, and restaurant in Harrison, Nebraska, 37 kilometers (23 miles) north of the park, and there are motels and restaurants at Fort Robinson, Nebraska, 37 kilometers (23 miles) east of Harrison, or 74 kilometers (46 miles) northeast of the park.

Transportation

Buses—The nearest bus connections are in Scottsbluff, Nebraska. Airport—Scottsbluff, Nebraska, has an airport served by a scheduled commercial airline. Rentals—Cars may be rented at the airport or at car rental agencies in Scottsbluff.

Establishment of the park

June 5, 1965.

Mailing Address

Agate Fossil Beds
National Monument, 301 River Road,
Harrison, NE 69346.

Access

Protection

Park Regulations

To ensure your safety and to protect the park’s natural and historical resources, several regulations have been established by the National Park Service. Collecting of fossils, rocks, plants, or other objects is not permitted. Please be sure to leave everything as you find it along the trails and throughout the park for others to enjoy. If you have any questions about park regulations and policies, please ask the staff. The rangers are here to help you and to enforce the regulations.

Safety Tips

Though snakes are not prevalent, be sure to watch for rattlesnakes as you walk about through the park, along the trails, and near the exhibits at Carnegie and University Hills. Avoid them if you see them, but do not harm them. As a general rule it is best to keep a good distance from any wildlife you see, not only to protect yourself and your children, but to avoid frightening or hurting the animal. It is best to observe wildlife at a safe distance with field glasses. While walking about the park, do not take chances by climbing on loose rock, or going into unauthorized areas, and do not let your children go beyond your control. Park your vehicle in authorized places and observe the normal rules of road safety and courtesy while you are in the park, and when entering and leaving it.

Birding Along the Niobrara

Taking the Annual Count

One of the joys of visiting the national parks, author Freeman Tilden once said, is having an unexpected, provocative experience. You go to a park to see or do one thing, and you come across something else that strikes your fancy as well. Tilden called it serendipity. At Agate Fossil Beds National Monument, one such experience might be birdwatching. In this piece, Doris B. Gates writes of her annual bird surveys in this area.

In western Nebraska the northern part of the Great Plains ends at the Pine Ridge, an escarpment circling from Wyoming across Nebraska’s north edge and winding into South Dakota. A major grass of this mixed prairie is little bluestem, Nebraska’s state grass, whose rusty-red hue in fall and winter gives much of the state its characteristic color.

These plains are rarely broken by cultivation and only a few houses with their few trees break the landscape. The land’s major change comes where the Niobrara River, here little more than a narrow creek, cuts a valley whose rock outcroppings provide homes for rock wrens, chipmunks, and bushy-tailed wood rats better known as pack or trade rats.

Here, since 1967, near Agate Fossil Beds National Monument, my partner and I have taken part in the annual Breeding Bird Survey for the U.S. Fish and Wildlife Service. Part of one of our survey routes, Highway 29, crosses the monument’s west end. We know the area—in June at least—quite intimately, when there is nothing quite so beautiful as a sunrise over these flower-dotted, green-grassed rolling hills along the Niobrara.

We go many kilometers and make many bird counting stops, then we drop into the little valley where the Niobrara flows and suddenly we hear and see birds in such rapid succession that we have difficulty getting them all named in the three minutes allowed us under the survey rules. Actually, three stops are influenced by the river: on the south edge we have found a common nighthawk, a lark sparrow, and a Say’s phoebe; on the north end the rock wren sings its un-wrenlike song. Near the bridge, where a narrow belt of shrubs and trees—mostly willows—hugs the river, we have logged the following: common flicker, a red-headed woodpecker, eastern and western kingbirds, western wood peewees, a blue jay, black-capped chickadees, house wrens, a brown thrasher, robins, yellow warblers, black-billed magpies, common grackles, black-headed grosbeaks, American goldfinches, and the non-native house sparrow and starling. Only once did we see or hear a black-billed cuckoo.

_continues on page 85_

If we stop and peer into a large culvert under the highway we may scare out a cloud of cliff swallows whose mud nests are stuck on culvert walls. Barn and rough-winged swallows are more rarely seen—usually near the Agate buildings.

Near scattered farmhouses we may see logger-head shrikes; by one water tank we usually find a few killdeer. These and such birds as the long-billed curlew, upland sandpipers, and sharp-tailed grouse break the near monotony of such prairie birds as western meadowlarks (Nebraska’s state bird), lark buntings, horned larks, and chestnut-collared longspurs. Lark buntings line the utility wires, taking off to sing their territorial songs, and descending with butterfly-like motions.

Hawks are here—red-tails, Swainson’s, ferruginous, marsh, and the little American kestrel—but in small numbers. We search long rows of fence posts for a burrowing owl and occasionally see one. Great-horned owls frequent tall cottonwood trees around the Agate ranch buildings. This is also the country of turkey vultures, golden eagles, and prairie falcons, but we have not been lucky enough to see them yet.

Mammals are more elusive. Cattle pasture conspicuously on land formerly claimed by the buffalo (bison). We see pronghorns each year. A lone coyote is the only other relatively large mammal we have logged. Check a good mammal book and you will appreciate what lives here largely invisible to the untrained eye: shrews, moles, bats, cottontails and two kinds of jackrabbits, pocket gophers, prairie dogs, kangaroo rats, voles, several kinds of mice, two kinds of ground squirrel, muskrats, beaver, raccoons, minks, badgers, longtailed weasels, two kinds of skunks, occasional porcupines and bobcats, white-tailed deer, and mule deer. Consider yourself lucky if you see the swift fox, mountain lion, and the rare black-footed ferret.

Life abounds here in other forms less noticeable to eyes trained on the Breeding Bird Survey: various species of amphibians, reptiles, fish, and the numerous insects associated with grasslands. We hear perhaps too much about rattlesnakes—western Nebraska has only the prairie rattler, whose numbers are now much reduced. Other snakes include western hognosed, blue racer, bullsnake, and the plains, wandering, and red-sided garter snakes.

Collections of Agate Springs Fossils

Museums You Can Visit

Many museums throughout the world have displays of fossils from the Agate Fossil Beds. Very few of them actually collected their own material. Museum curators are dedicated “horse traders” and fossil-swapping is part of the business. When museums such as the Carnegie Museum of Pittsburgh or the American Museum of Natural History in New York make collections like the ones made at Agate earlier in this century, they usually have some trading stock left over after completing their study collections and exhibits. They then can trade an extra _Menoceras_ slab, for example, for a dinosaur skeleton from some faraway corner of the Earth.

At several museums in this country you can see mounted skeletons of several animals found at Agate, along with _Menoceras_ slabs (sections of rock with the bones still imbedded) or models and dioramas of Agate specimens. To the right are listed, in order of proximity to the park, some of the museums and their specimens from Agate.

The United States Museum of Natural History, Smithsonian Institution, has many fossils that depict the life of the most recent 65 million years and several murals by artist Jay H. Matternes showing the life of each of the epochs. The Miocene mural, reproduced on pages 20-21 of this handbook, is among these reconstructions. It depicts ancient life around what is today known as Agate Fossil Beds National Monument.

The Trailside Museum
Fort Robinson, Nebraska 69339.
_Menoceras_ slab, skeleton, and restoration
_Stenomylus_ skeleton on a slab, and a prepared limb
_Palaeocastor_ in a _Daemonelix_
_Palaeocastor_ in a plaster cast
Museum of Geology, South Dakota School of Mines and Technology
Rapid City, South Dakota 57701.
_Menoceras_ slab, beautifully prepared
The Geological Museum
University of Wyoming, Laramie, Wyoming 82070.
_Menoceras_, mounted skeleton
_Stenomylus_ slab containing most of a skeleton
University of Nebraska State Museum
101 Morrill Hall, 14th and U Streets, Lincoln, Nebraska 68508.
_Moropus_, mounted skeleton
_Palaeocastor_ skeleton in a _Daemonelix_; also, two other
_Daemonelix_
_Menoceras_ slab
_Dinohyus_ skeleton
_Stenomylus_, a group of skeletons
Field Museum of Natural History
Roosevelt Road at Lake Shore Drive, Chicago, Illinois 60605.
_Menoceras_ slab
_Moropus_ skeleton
The University of Michigan Exhibit Museum
1109 Geddes Rd., Ann Arbor, Michigan 48104.
_Menoceras_ slab and mounted skeleton
_Dinohyus_ eating dead Menoceras, a diorama
_Stenomylus_ skeleton and model
Carnegie Museum
4400 Forbes Ave., Pittsburgh, Pennsylvania 15213.
_Promerycochoerus_ slab
_Menoceras_ slab and mounted skeleton
_Moropus_, mounted skeleton
_Dinohyus_, mounted skeleton
_Stenomylus_, three skeletons mounted in a group
The American Museum of Natural History
Central Park West and 79th St., New York, New York 10024.
_Moropus_ skeleton
_Menoceras_ slab and skulls, one used in a sequence showing
collecting and preparation techniques
_Dinohyus_ skull
_Stenomylus_, nine skeletons and a reconstruction of the group
in life
Museum of Comparative Zoology
Harvard University, Oxford Street, Cambridge, Massachusetts 02138.
_Menoceras_ slab
_Dinohyus_ skeleton
_Stenomylus_ skeleton

NPS Areas With Fossil Exhibits

Several fossil sites in the United States are under the protection of the National Park Service. Besides Agate, the major ones are:

Badlands National Park, South Dakota

Prominent deposits from the Oligocene Epoch, predecessor to the Miocene, combine with a rugged, eroded landscape and abundant wildlife to make Badlands a park where the natural processes of the past combine with those of today. The National Park Service maintains a Fossil Exhibit Trail at Badlands and presents fossil cleaning demonstrations. Prominent fossils are those of ancient camels, giant pigs, sabertooth cats, _Protoceras_, and _Brontotheres_. Mailing address: P.O. Box 6, Interior, SD 57750.

Dinosaur National Monument, Colorado and Utah

The late Jurassic muds and sands of the Morrison Formation have been a major source of dinosaur bones for more than a century. Steeply tilted strata near Vernal, Utah, were the source of tons of bones for the Carnegie Museum of Pittsburgh. This quarry site became the nucleus of Dinosaur National Monument. The bone-bearing stratum has been exposed by careful excavation, so that bones and partial skeletons of numerous dinosaurs are exposed in high relief. The entire quarry face is covered by a glass-walled structure that forms a large gallery. Mailing address: 4545 E. Hwy. 40, Dinosaur, CO 81610.

Florissant Fossil Beds National Monument, Colorado

This site has long been famous for its fossils of insects and plants preserved in fine-grained sediments. Specimens of _Brontothere_ indicate an Eocene age for the deposits. Mailing address: P.O. Box 185, Florissant, CO 80816.

Fossil Butte National Monument, Wyoming

Within the strata of this rock remnant of an ancient lake is one of the most extensive concentrations of fossilized freshwater fish known to science. The site is about 18 kilometers (11 miles) west of Kemmerer, Wyoming. Mailing address: P.O. Box 592, Kemmerer, WY 83101.

Petrified Forest National Park, Arizona

Here in the Late Triassic Chinle Formation are widespread deposits of petrified logs. Some are nearly 2 meters in diameter and 60 meters long (6.5 by 197 feet). Preserved in bright colors of opal and other minerals, the most common trees are relatives of the living monkey puzzle or Hawaiian star pine. Paleontologists believe many of the logs floated to the area in Triassic rivers and became stranded. In the museum are displays of various fossil plant species and animal fossils from the same deposits. Mailing address: P.O. Box 2217, Petrified Forest National Park, AZ 86028.

John Day Fossil Beds National Monument, Oregon

With a total of about 5,700 hectares (14,100 acres) in several noncontiguous units in north-central Oregon, this park provides an extensive record of Earth history dating back at least 37 million years. Plant and animal fossils are present in great variety. Mailing address: HCR 82, Box 126, Kimberly, OR 97848.

Hagerman Fossil Beds National Monument, Idaho

Within the banks of the Snake River are preserved the last vestiges of late Pliocene life before the Ice Age and modern flora and fauna appeared. Mailing address: P.O. Box 570, Hagerman, ID 83332.

Nearby National Parks

While you’re in the Agate Fossil Beds area, why not see some other sites in the National Park System? These parks offer a variety of experiences from frontier history presentations to caving.

Badlands National Park is 97 kilometers (60 miles) southeast of Rapid City, South Dakota. This wonderland of bizarre, colorful spires and pinnacles, massive buttes, and deep gorges is open all year, though blizzards may temporarily block roads in the winter. Campfire programs and guided nature walks are presented. Backpackers will enjoy the park’s wilderness area. The park has a herd of about 300 bison and some prairie dog towns. Mailing address: P.O. Box 6, Interior, SD 57750.

Devils Tower National Monument is 47 kilometers (29 miles) northwest of Sundance, Wyoming. Known as Mato Tipila (Bear Lodge) to the Lakota, this towering landmark looms over the Belle Fourche River in the northeast corner of Wyoming. Here the Black Hills meet the plains grasslands, and you will likely see prairie dogs, as well as other mammals and a variety of birds. The park is open all year. Mailing address: P.O. Box 10, Devils Tower, WY 82714.

Fort Laramie National Historic Site is 5 kilometers (3 miles) southwest of Fort Laramie, Wyoming. The first fort on the site was built in 1834 and soon became a lucrative center of the fur trade. The U.S. Army took over in 1849, using the fort to protect the Oregon Trail. The fort was abandoned by the Army in 1890. Several buildings are furnished as they would have been during the Army years of the 1870s and 1880s. The park is open all year. Mailing address: HC 72, Box 389, WY 82212.

Jewel Cave National Monument is located on U.S. 16, 24 kilometers (15 miles) west of Custer, South Dakota. The cave’s name comes from the myriads of jewel-like calcite crystals that adorn its walls. Tours are conducted daily from mid-May through September. Tours, if any, the rest of the year are irregular. Mailing address: RR 1, Box 60 AA, Custer, SD 57730.

Mount Rushmore National Memorial is 40 kilometers (25 miles) southwest of Rapid City, South Dakota. The mountain sculpture of Washington, Jefferson, Theodore Roosevelt, and Lincoln is best viewed under morning light. From June 1 to Labor Day the faces are illuminated at night. The park is open all year. Mailing address: P.O. Box 268, Keystone, SD 57751.