CENOZOIC
(continued)

Oligocene

The Oligocene deposits of the Scotts Bluff region consist of extensive sheets of sandstone and clays with local lenses of gravels. These have been referred to as the White River Beds because of their extensive exposures in the White River basin of South Dakota where they have a total thickness of about 675 feet. The White River Beds have been separated into two formations, the Chadron Formation below and the Brule Clays above.

The Chadron Formation, often referred to as the Titanotherium beds, does not outcrop at Scotts Bluff proper, for here the river has not yet cut its channel deep enough to allow it to be exposed. It does outcrop in the valley of the North Platte about four miles above Scotts Bluff in the vicinity of Caldwell and Mitchell, from whence it extends on either side of the valley westward into Wyoming. The formation is widely exposed in the Big Badlands of South Dakota and along the northerly facing escarpment of Pine Ridge, which extends from South Dakota thru northwestern Nebraska and into Wyoming. These beds are known to underline an extensive area of later formations in western Nebraska and northeastern Colorado.

This formation (Desc. by Darton, N. H., U. S. G. S. Prof. Pap. No. 17., pp. 40-41) is composed predominantly of sandy clay of light greenish-gray color, usually with coarser beds at the bottom, including deposits of gravel, often several feet thick. The beds above these gravels are of bright dark-red color, notably in the region about Adelia. Locally they contain thin beds of volcanic ash intercalated with the sediments. The thickness of the formation varies from thirty to sixty feet. Where the bottom of this formation is seen, it is usually lying on the irregular, black or generally rusty surface of the Pierre shale.

This formation is very often referred to by a much older name, Titanotherium beds from the name of the large, extinct animals whose bones occur in it so abundantly. Professor H. F. Osborn made a monographic study of the Titanotheres working out in the greatest detail their occurrence, structure, evolution, environment and habits. His work is our principal source of information on this important group of mammals. In it may be found excellent illustrations of skeletons and restoration drawings, of not only the Oligocene, but also the ??? older Eocene forms. An excellent portrayal of a herd of these animals showing their environment during Chadron time is given in the frontispiece.

A list of illustrations of several of the best known species which lived during Chadron time are given below:

The Titanotheres are the largest animals found in the White River Badlands, their size being comparable to that of the present day elephant. With the exception of turtles and Oreodons, they are also the most abundant forms. In general appearance the Titanothere showed some resemblance to the rhinoceros, particular the head. The limbs are stouter, especially the fore limbs which have some resemblance to those of the elephant but are shorter and more supple. There are four short, thick-hoofed toes on the front foot corresponding to the second, third and fourth digits. On the hind foot only the second, third and fourth are present. The body is short, as in the elephant; the shoulder is conspicuously high, caused by the elongation of the spinous process of the anterior dorsal vertebrae. The neck is thick and extremely muscular, due to the aggressive use of the nasal protuberances. The skull is long, low and saddle-shaped with remarkable nasal prominences at the extreme end, bearing in the later forms, long protuberances. The skull varies greatly in different species and in the sexes. The ears are placed far to the rear while the eyes are surprisingly near the front. The teeth, usually thirty-eight, are large, especially the molars. The incisors are rudimentary or lacking in the later forms, being replaced by the development of a prehensile upper lip for nipping off twigs and grass. The nature of the thick skin is not positively known but is believed to be similar to that of modern forms, showing similar characters and habits.

The Titanotheres had their origin in Early Eocene time and reached the culmination of their development in Lower Oligocene time, suddenly to become completely extinct at the end of that time. The cause of the sudden extinction of such a seemingly well adapted group is puzzling. Changes in the climate at the close of Chadron time may have been indirectly responsible. However, Professor Osborn states that it may have been due to certain diseases which these animals were unable to combat.

The Middle and Upper Oligocene is represented by the Brule Clays. (Darton, N. H., 1903, pp. 37-40) This thick mass of sediment underlies the greater part of the region and consists of massive clays, or a mixture of fine sand and clay, of pale flesh color, containing numerous fossil beds characteristic of the Oreodon series of the White River group. It is widely exposed in the Valley of the North Platte River and Pumpkinseed Creek, extends along the lower portion of the northern slope of Pine Ridge, and is bared in the narrow depression of Lodgepole Creek and Sidney Draw. There are extensive exposures of the Brule Clay in the northern face of Scotts Bluff, where from the base of the overlying Gering beds to the river, there is a vertical interval of 500 feet of continuous outcrop and the formation has a small additional thickness below the level of the river. (Darton, N. H., 1903, pls. 30 & 31)

The lower portion of the Brule Clays is often referred to as the Oreodon beds because of the abundant remains of Oreodons found in them. These animals are the commonest fossil mammals of the White River Beds. They originated in the Eocene, ranged through the Oligocene and Miocene, and became extinct in the lower Pliocene. They are distinguished by many primitive characters and according to Cope they constitute one of the best marked types of mammalia the world has seen. They occupy a position somewhat intermediate between the ruminants (cud-chewing animals) and the suilline pachyderms (pig-like, thick-skinned animals). Leidy, in his description of the Oreodon, suggested that it might very appropriately be called a "ruminating" hog. The most common form found in the Middle Oligocene is Oreodon culbertsoni, a restoration of which may be seen in Scott. (1913, p. 259, fig. 136). A complete articulated specimen may be found in the Museum of Paleontology at the University of California. This species is a little larger than the red fox. These animals evidently reamed the woods and plains in great herds, probably feeding upon the more tender grass of the forested areas.

The remains of two primitive types of rhinoceroses are found in the Middle Oligocene deposits, one an aquatic form, and the other an agile, fast running type, well adapted to the plains. To the former type belongs Metamynodon, a heavily built form with short toes in front, eyes and nostrils much elevated for convenience in deep wading, canine teeth enlarged into re-curved tusks, and a prehensile upper lip apparently tending toward proboscoid development. This animal much resembled the present day hippopotamus both in build and habit. The skeleton measures nine and one-half feet in length and stands four and one-half feet high at the shoulders. An excellent restoration painting has been made by Knight for the American Museum of Natural History, illustrations of which are available in the Matthew Library, University of California; or may be obtained from the American Museum of Natural History. Reproductions may be found in O'Harra (1920, pl. 29). Of the cursorial rhinoceroses of this time Hyracodon is well known. It was a small, light-chested, swift-footed, hoofed, harmless creature much resembling the Miocene horses and evidently well fitted for living on the grass-covered higher lands. Restorations may be found in Scott. (1913, p. 341, fig. 130).

Of the Middle Oligocene horses one of the commonest and most typical was Mesohippus bairdi. The adult animal averaged about eighteen inches in height, was slender limbed and well adapted for speed; the hind limbs were much longer than the fore. In the fore foot were three digits, the median one enlarged and supporting most of the weight; however, the lateral digits touched the ground and were not entirely functionless; in addition there was a small splint, the rudiment of the fifth digit. The hind foot had also three toes.

Many specimens of Canidae (dogs, wolves, etc.), have been found in the White River Badlands but most of the material is fragmentary. Recently, however, a few complete skeletons have been obtained. Of the several species, Cynodictis gregarius and Daphaenus felinus are the best known. Cynodictis gregarius was most abundant and, as the name implies, seems to have roved the country in packs. It was smaller than the red fox. Daphaenus felinus reached approximately the size of a coyote. (O'Harra, 1920, pp. 78-82, and Scott, 1913, pp. 520-530).

The cat family is well represented in the fossil fauna of the Middle and Upper Oligocene. Two genera are of particular importance namely Haplophoneus and Dinictis. (Illus., O'Harra, 1920, figs. 31, 32; pl. 27; Scott, 1913, figs. 264, 265). These are early forms of what are commonly known as saber-tooth cats, or tigers, a name given to them by reason of two great sword or saber-like canine teeth of the upper jaw. They were not as large as some of the later forms of this group but nevertheless were vicious creatures. Hoplophoneus was the largest of the two, doubtless fully as large as the present day leopard and apparently much more powerful. In Hoplophoneus the fangs were very long and slender and the projecting flange on the lower jaw correspondingly deep. Dinictis had shorter canines and less prominent flange. The cause of the development of these abnormally powerful upper canines and the uses to which they were put have been the cause of much speculation. It may have been that this was an adaptation of the canines to a particular method of attack. (Matthew, W. D.). The lower jaw is so shaped as to allow the mouth to be opened to an unusual extent so as to give greatest freedom to the saber-tooth in stabbing the prey. Hoplophoneus. in addition, had a strong body, stout neck and legs, and highly developed strong retractile claws. His food must have been in large measure the thick-skinned rhinoceroses, clotheres, oredonts, and other similar animals. The somewhat smaller Dinictis probably preyed more successfully on the smaller swift-footed animal.

In the Upper Oligocene, Eusmilus dakotensis, a large saber-tooth cat approaching the size of the African lion was the largest known carnivore of its time. (O'Harra, 1920, pp. 83-87.)

Rodents are known from the Middle and Upper Oligocene and include ancestral squirrels, rabbits, beavers, and rats. Many other interesting forms are found in the Upper Oligocene beds, but for lack of time and space not all can be described. (See check lists).

The conditions under which the Oligocene deposits were laid down have been described by Hatcher as follows: "The distribution, state of preservation, nature, and character of the animal and plant remains found in the clays and sandstones, as well as the distribution of the latter, absolutely precludes the possibility of their having been deposited in a vast lake, and favors the presence of streams meandering through low, broad, level, open, or wooded valleys, subjected in part at least to frequent inundations, conditions very similar to those at present prevailing in the interior of South America, about the headwaters of the Orinoco, the Amazon, the Paraguay, and Panama Rivers. . . . Molluscs from the Titanotherium and Oreodon clays include such characteristically shallow water forms as Chira, Limnaea, Physa and Planorbis . . . which according to Drs. Dall, Pilsbry, and Stanton are forms inhabiting swamps and small ponds. Dr. Knowlton, who has examined the plants, finds in great abundance the stems and seeds of Chira, which as all know is distinctly an inhabitant of small springs, shallow ponds and brooks. . . . Moreover, remains of forests were found at several places and at different horizons throughout these beds. At various localities in the Hat Creek basin in Sioux County, Nebraska, I discovered remains of the silicified trunks of trees and seeds belonging especially to Hicoria and Celtis. These were found at various horizons from the middle of the Titanotherium beds to the very top of the Loup Fork; and in South Dakota, some 12 miles north of White River, opposite the mouth of Corn Creek, I discovered the remains of a not inconsiderable forest."

Wanless (1923, p. 244), after a careful study of the stratigraphy of the White River beds, states that at the beginning of Oligocene time the streams were slightly entrenching themselves in the old floor of the plain in rather steep-sided valleys. Gradually these were widened and soon a more gentle relief caused the streams to wander somewhat upon the surface of the valley floor depositing sands in their channels and steeply cross-bedded silts as lateral delta terraces on the sides of the valleys. With perhaps a slightly more arid climate the streams gradually aggraded the valleys. When the streams of the Titanotherium age had aggraded to the level of the old plain they were no longer bound by valley walls and were free to wander at will across the plain. Thus when torrential downpours brought sheets of water from the mountains, it spread far and wide leaving layers of silt. The climate was evidently mild and fairly humid.

At the close of Titanotherium time, and with the initiation of the Middle Oligocene, climatic conditions swept to an arid extreme which may have caused the disappearance of a large part of the cover of vegetation. This climatic change may have been responsible for the sudden disappearance of Titanotherium. In the sediments this change is represented by a marked increase in the amount of calcareous cement. Again shallow streams meandered across the plain and spread sheets of silt many miles away from their channels. A large and varied fauna roamed over the greasy plains, and an aquatic fauna typified by the massive rhinoceros, Metamynodon, frequented the river channels. Several times after a period of flood, the surface of the plain remained dry, perhaps for many years, and the surface silts were cemented to a hard caliche or nodular layer.

After a long period of caliche development, there was a change to a somewhat more humid climate. Stream channels became more numerous, and again local ponds with fresh-water faunas characteristic of the Middle Oligocene were developed over the surface of the plain. It was still, however, apparently more arid than in Titanotherium time.

Gradually another factor was imposing itself on these previous rhythms of climate and deposition, namely, increasing contributions of volcanic dust from outside this region. This was in the form of wind-blown material and is believed to have come from the Rocky Mountains and Black Hills where it is known that lacolithic intrusion was occurring on a large scale during Cenozoic times. It becomes an increasingly more important constituent in the Oligocene sediments.

At the beginning of Upper Oligocene time there seems to have been a slight return to more humid conditions as stream channels became mere abundant and extensive, but there soon ensued another period of caliche formation resulting in another series of levels of nodules, probably each representing old land surfaces. The only difference between the Middle and Upper Oligocene caliche nodules is that in the latter volcanic ash generally forms fifty per cent or more of the elastic material of the sediments.

From the occurrence of hackberry fruits said to occur in millions in the Oreondon beds (Middle and Upper Oligocene) Chaney (Chaney, R. W., pp. 54-56. one plate) has postulated the nature of the climate of these times as not unlike the drier parts of the West at present time. A variety of common hackberry is found today along the streams in Western South Dakota. The more common associates of the hackberries in these regions are several species of Populus and Salix, but there is little liklihood under present climatic conditions of any of the leaves becoming a part of the fossil record, since in most cases they become dried and decayed before they have left their prints in the sediments accumulating along the streams. The hackberry and cherry, both having hard shelled seeds might be expected to occur as fossils. The presence of a large number of hackberry fruits indicates beyond question the abundance of those trees along the sites of deposition during the Oligocene, and also the Miocene in the Great Plains. At the same time the absence of hackberry leaves and of leaves of other species which may be supposed to have lived in the same situation, is a strong suggestion of a climate so arid as to prevent their entrance into the sedimentary record.