For millions of years during the late Cretaceous period (near the end of the Age of Dinosaurs) what is now Nebraska lay beneath the waters of a great sea. About sixty million years ago, this period ended. The seas retreated and the great Rocky Mountain system was formed to the west. Then, for several million years, these mountains were attacked by erosion. Rock debris worn from their flanks was carried eastward to the lowlands and deposited widely for hundreds of miles by streamsancestors of today's North Platte, Niobrara and White Rivers. These early Tertiary deposits and later ones, now exposed in the Great Plains, make up the formations which are so well known for their richness in mammal fossils.
The Rockies were worn down to a gently rolling land. Then broad warping of the whole region in Oligocene times reelevated the mountains several thousand feet. Rock debris was again carried eastward covering the Great Plains with layer upon layer of gravely, sandy deposits.
Before the Rockies were pushed up this second time, the climate of the Great Plains was fairly warm and moist, and much of the region was covered with deciduous forests. However, the uplifted mountains intercepted moisture-laden winds from the west, causing rain to fall mostly on their western slopes. Precipitation diminished to the east and over a period of many years the forests were gradually replaced by vast grasslands.
To survive, animal life had to adapt to the changing environment. Prairie grasses were an abrasive food which wore down the teeth of grazing animals, shortening their lives. One adaptation which counteracted this was the development of high-crowned teeth that continued growing in length during the lifespan of the creature. As the grinding part of each tooth wore down, new growth from the roots took its place.
The development of grasses was one of the greatest events in the history of life. Grasses played a tremendous part in the evolution of mammals, and it was primarily during the Miocene, sometimes referred to as "The Golden Age of Mammals," that the development of grazing animals reached its zenith both in variety of species and in numbers.
Weathering and erosion of the uplifted Rockies during the late Oligocene and early Miocene resulted in a rapid increase of sedimentary deposits upon the Great Plains. Because of increased gradient, the streams flowed swifter than before, and carried more and coarser sediments. Great floods took place, at which time many animals no doubt were trapped in rapidly rising waters and their carcasses swept downstream to be deposited at a bend of a river or other places where the waters were slackened. Then river sediments covered the remains. This process was repeated, in some cases several times at the same place, until layer upon layer of animal remains were sealed beneath the earth's surface. As alternate layers of sediment covered these deposits and ground waters saturated or percolated through the beds, the sands were compressed and cemented into sandstone.
Over a long period of time, the skeletal remains of the entombed animals were fossilized as the porous bones were filled with mineral matter. Many geologists believe this to be the most likely explanation for the origin of the Agate Springs fossils. Others believe differently, one theory being that the animals became trapped in quicksand when coming to the river to drink and sank into an ancient quagmire. In any event, the conditions that brought about the large concentrations of animal remains within a relatively short period of time must have been unusual indeed.
These deposits remained buried for approximately fifteen million years until exposed by the valley cutting of the present day Niobrara River and by abrasive weathering action of the elements.
Conditions that existed when the little Stenomylus camel perished were somewhat different. Most of the remains found at the Stenomylus Quarry were of complete articulated skeletons entombed in the sandstone just as they lay in death. For some unknown reason practically all of those quarried had their heads and necks arched back grotesquely toward their backs as if death were caused in some agonizing manner. Most of the little creatures were no doubt quickly covered by windblown sands because there is but little evidence of the carcasses being torn apart by carnivora. Later they too were buried under river debris like the other Agate Springs fossil deposits.
In Miocene times large areas of the grassy plains were inhabited by beaver-like animals that lived in colonies similar to the prairie dog towns of today. Casts of the burrows made by these animals are found extensively north of the Niobrara River. These casts are called Daimonelices or "Devil's Corkscrews," so named because of their similarity to immense corkscrews that might have been used by demons. Measuring six to eight feet in height and from three to eight inches in diameter, each of these strange formations has a basal extension approximately four feet in length leading off from the vertical burrow at a slightly rising angle. This extension appears to have been the animal's nesting chamber. These ancient creatures packed the insides of their burrow walls much as prairie dogs do now. Abandoned burrows were penetrated by a mesh of roots which filled them, much as present day roots often clog a tile drain or sewer line. These Daimonelices therefore actually consist of a combination of earth materials and the fossilized remains of vegetable tissue. Different levels of the earth's surface during the Miocene can easily be seen in places where the tops of the "Corkscrews" are roughly level. Remains of these small prehistoric beaver have been found encased in some of the fossil burrows.
All of the Agate Springs fossil deposits are found in what geologists have named the Arikaree group of the Miocene epoch. A more detailed classification of this group shows three divisions: The Gering, Monroe Creek, and Harrison Formations. It is the Harrison formation which contains practically all of the known fossils just described at the Agate site. The sediments of this group are principally sandstone. The quarries are in the lower portion of the Harrison, and the "Devil's Corkscrews" are thought to be in the upper portion, which was laid down perhaps a million or more years after the lower part of the formation.
By late Harrison time, valleys here had been well filled with rock debris from the west, and the landscape was probably one of numerous shallow marshes and lakes, drained by sluggish rivers. Within the bottoms of these shallow lakes, beds of limy mud accumulated. Such beds are widespread in the Nebraska panhandle, and many contain agate deposits. The agates were produced by slowly percolating ground waters which carried a high concentration of silica. Each nodulein some cases these are well banded agatesrepresent points where silica in solution in ground water was deposited. First accumulating around small nuclei, each center of deposition gradually enlarged as more ground water slowly seeped through the enclosing earths, depositing silica in thin layer after thin layer. Some of the agates contain dendritic deposits of manganese oxide which resemble minute fossil ferns. These are the "moss agates" of the rock collector. Today agates can be found lying on top of the ground along the Niobrara River bluffs, particularly in the vicinity of the "Devil's Corkscrews."
The Miocene epoch which endured for some twelve million years, has been called the "Golden Age of Mammals" because of the tremendous increase both as to species and numbers of mammal creatures that roamed the earth. The presence of a new environment of grass-covered plains resulted in great changes by the animals, especially grazers, to adapt themselves to these new conditions. The Agate site is an outstanding example of this important chapter of evolution and many evidences of interesting mammal life have been found here.
By far the most common mammal was a two-horned rhinoceros named Diceratherium. This small, swift creature was about the size of a Shetland pony and roamed the plains in great numbers. It possibly was as numerous then as the buffalo was in the 1700's.
The most unusual looking animal was the Moropus. It appears to have had some characteristics of several present day mammals and some which cannot be associated with anything living today. The Moropus was large and heavily built, about seven feet tall at the shoulders. Its head was horselike, the neck suggested faintly the giraffe, its torso the tapir, the front legs a rhinoceros, and the hind legs a bear. Most unusual were the feet which were armed with large claws, used no doubt for defense and for digging up underground succulents such as roots and bulbs. Not at all suited for a grasslands environment, the Moropus probably became extinct later in the Miocene.
Perhaps the most ferocious and meanest dweller of the Nebraska scene in Miocene times was the Dinohyus or "Terrible Pig," a monstrous beast over seven feet tall at the shoulders and about ten feet long. It had a massive head with large tusks, a small brain and was no doubt very stupid. For a pig, its legs were quite long and slender. Many specimens with broken bones have been found, thus indicating the Dinohyus must have been a most aggressive creature which was wounded in battle frequently.
Remains of these animals, the Diceratherium, Moropus, and Dinohyus have been excavated at the Carnegie and University Hill Quarries. Here parts of approximately 800 individuals of the first, 25 of the second and 2 of the third have been quarried.
Large herds of gazelle-like camel roamed the Miocene plains. This delicate and graceful little animalthe Stenomyluswas slightly over two feet tall had long, slender legs and deer-like-hoofsnot like the present day camel at all, but more like the guanaco of South America. Fossil skeletons of over 100 of these creatures have been removed from the Stenomylus Quarry.
Colonies of little beaver named Palaeocastor, somewhat similar to the prairie dog of today, dug spiral burrows down into the ancient plains. Casts of these burrows called Daimonelices have been described previously.
Fragments of the fossil remains of other animals have been found in the Agate area from the Harrison or later formations. Some of these are described briefly as follows:
The Syndyoceras was an antelope-like mammal about the size of a very small deer. It had two pairs of horns; one grew out of the middle of its face and curved outwardly, and one pair grew out of the top of its skull and curved inward like that of a cow. Fossil remains of this animal have been found in the Marsland formation which is immediately above the Harrison.
A large dogDaphaenodonabout the size of today's wolf, no doubt preyed upon many species of Miocene animals. Its head resembled that of a wolf but the rest of the body was cat-like. The tail was long and heavy, the claws were sharp and somewhat retractable. As this animal depended upon surprise to capture its prey, it was not built for long pursuits.
A little three-toed horseMerychippusabout the size of a domestic sheep inhabited the region, but probably long after the Agate Springs Fossil deposits of the lower part of the Harrison were laid down.
Herds of Desmathyus, a small pig-like creature similar in size and appearance to today's javalina were plentiful. Many animals probably respectfully kept their distance when these mean-tempered, fleet-footed mammals appeared on the scene.
The Promerycochoerus was the largest known genus of the Oredonts, about the size of a large pig, and in appearance like a small hippopotamus. Although this was one of the most abundant creatures of Miocene times, it was not common at Agate.
Last Updated: 12-Nov-2010