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.
[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.
[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.
[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.
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.