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, 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.