PREHISTORIC HORSES
The past history of horses is well known from an abundance of fossil material, ranging in age from the Eocene down to the present. Modern horses have only one toe in each foot, but there are remnants of two additional toes which may be seen only in the bony structure underlying the skin. Most of their ancestral relatives were three-toed as far back as the Oligocene period. During Eocene time, however, there was a stage which may be regarded as four-toed although it was evidently a temporary condition, linking known horses with more remote forms having five toes.
Oligocene Mammals From Weld County, Colorado
The giant pigs (Archaeotherium mortoni) at the left of the group, and the rhinoceros (Trigonias osborni) were common animals of the western plains region at one time.
Eohippus, the “dawn horse” as it has been called, is one of the oldest and best known of the American horses. Its relation to existing members of the family can be traced by means of changes in tooth structure as well as in the gradual reduction in the number of toes that is seen among intermediate forms. Its ancestors some day may be positively identified in that group of generalized, primitive, five-toed, hoofed mammals which are known to have lived at the beginning of the mammalian era, but such identification has not yet been established. Even Eohippus bore little resemblance to the familiar horse of today. Its height was only eleven inches, and in body form it had much of the appearance of a modern dog. There were four toes on the front foot, one of them decidedly shorter than the others but complete in all its parts, and evidently capable of service in carrying a portion of the animal’s weight. The hind foot had three complete toes and a tiny remnant of a fourth which could not have been apparent externally.
As changes in the structure of the feet progressed, the central toe of the original five continued to increase in size while the adjacent digits became relatively shorter and eventually so reduced in length that they could touch the ground no longer. The smaller bones at the extremities, corresponding to the joints of our fingers and toes, eventually disappeared from the side toes. Then the longer bones of the outer digits lost the broadened supporting surface, where the missing toes had been attached, and became reduced to pointed remnants known as splints. Extreme shortening of the splint bones eventually leaves only a small knob which is often referred to as a rudimentary toe. In the skeleton of a large horse the splints are readily seen, but in some of the earlier species they are so small that they may easily be destroyed or overlooked by the collector who removes the fossilized material from the surrounding rocks. Even then, the bones of the wrist and ankle may indicate in an unmistakable manner that an additional toe once was present, for each bone is supported by another, and at the point of attachment there is a characteristic surface whose purpose is usually obvious.
Throughout the Cenozoic era the changes continued. Among the horses of the North American Oligocene were Mesohippus, approximately the size of a collie dog, and Miohippus which was slightly larger. Both were three-toed, but the rudimentary splint of a fourth toe was still present in the front foot. Parahippus and Merychippus carried on during the Miocene period, the latter being characteristic of the time, and showing, in addition to other progress, a decided trend toward the modern structure of molar teeth. There was some increase in size but the largest horse of that period was hardly more than a small pony.
A Pleistocene Horse of the Texas Plains (Equus scotti)
Hipparion and Protohippus, living during Upper Miocene and Pliocene time, represent later stages of the three-toed condition. The side toes were completely formed but greatly shortened, only the central toe touching the ground. In some of the species the outer toes had also become very slender, approaching the splint condition. By this time the molar teeth were longer and better adapted for feeding on grasses which were becoming sufficiently abundant to attract some of the forest dwellers into the open country.
During the Pliocene period, in the genus Pliohippus and also in Hipparion, the feet were far advanced in structure, with most of the species single-toed, the side digits having reached the splint stage. Pleistocene horses of the genus Equus, like living species of that genus, were strictly one-toed animals, ranging over grassy areas and highly specialized for a life in that kind of environment.
Specialization is to be noted partly in the foot and leg structures where the modifications have contributed to greater speed and travelling ability. This is of great service to an animal of the plains where food and water are often scarce, and great distances frequently have to be covered in order to obtain sustenance. The horse, as we know it, is built for speed, its limbs and feet being elongated to permit a greater stride, and also modified to decrease the weight without loss of strength. The ordinary ball-and-socket joint is replaced by a pulley-like construction which limits the direction of movement but provides an excellent mechanism for locomotion, especially over flat, open ground. Flexibility in other directions is sacrificed for greater strength, and the foot incidentally becomes less suited for other purposes.
This is what is meant by “specialization”—a departure from “generalization.” The study of fossils provides numerous illustrations of specialized development which contributes greatly to an interest in prehistoric life. Any specialized structure or habit which increases fitness for a particular way of living is also known as an “adaptation.” Quite in line with the idea of specialization and adaptation is the change which occurred in the construction of the horses’ teeth, for the dental equipment of the modern grazing animals differs widely from that of the browsing creatures which lived on the soft leaves and other plant substances of the forests.
The Structure of Molar Teeth
The large lower molar of a long-jawed mastodon shows worn and unworn cusps, with the enamel layer forming a heavy border around the central dentine where the surface covering has been worn through. In the grinding teeth of rhinoceroses (illustrated at the right) the crown pattern is quite different, but both types are adapted for softer foods and are similar in having the protective enamel on the outside only. The central tooth shows the condition after the shallow surface depressions have been removed by wear.
The cheek teeth or grinding equipment of the horses underwent as complete a change as the feet. Modification resulted in a new type of tooth which enabled herbivorous animals to take advantage of a kind of vegetation which was late in arriving and has since become the principal diet of the ungulates. The grasses are coarse and harsh as compared with the leaves of forest shrubbery, requiring more thorough grinding to make them digestible. In addition they contain minute particles of silica, which is a highly abrasive mineral that quickly wears down the tooth substance, especially the softer materials found in tooth construction. An increase in the length of the tooth would offset the excessive wear but would not necessarily produce a better mechanism for grinding.
The fulfillment of the new requirements is to be seen in the change from what is known as the low-crowned, browsing type of molar, to the high-crowned, grazing type. Details of the changes that may be traced through millions of years of gradual adjustment become apparent only from the examination of a great deal of fossil material. As compared with earlier types of construction, a modern molar tooth may appear extremely complicated, but the process which brought about the improved quality is very simple. A little discussion of tooth structure, however, is required to make this clear.
A tooth, as everyone knows, is partly imbedded in the jaw, partly exposed outside the gum. In a short-crowned tooth the exposed portion is known as the crown, and the part imbedded in the jaw consists of one or more roots which are comparatively long. The crown is nearly always protected by a thin layer of hard enamel. In a grinding tooth, the working surface has a number of more or less prominent elevations known as cusps. The enamel layer completely covers this surface until wear begins. As the tooth goes into service the signs of use begin to appear; the enamel is soon worn from the tops of the cusps, and the underlying substance, called dentine, becomes exposed. This is far less resistant to wear, and as the enamel continues to be reduced the tooth becomes less efficient as a grinding device, partly because of the smoothing off of the surface, partly because of the relative softness of the inner material which is being exposed in increasing quantity. A very old molar tooth of the low-crowned type has a smooth surface from which almost the last trace of the enamel has been removed. In many prehistoric animals the enamel is of a darker color than the dentine or cement, this difference in color enabling one to see at a glance how the teeth are constructed.
Grazing Type of Molar Teeth
The side view of the bison’s molar and premolar equipment illustrates the elongated construction which is common among grazing animals. In the pattern of the grinding surface may be seen a cross section of the enamel layers. One layer surrounds each tooth while two folded “cylinders” of the same material occupy the interior.
In a long-crowned tooth the roots are usually very short, for much of the crown itself is imbedded in jaw bone, and the longer roots are not required. Growth of the tooth is usually completed after a few years; then as it is gradually worn away it is continuously moved upward by the production of new bone under the roots, which slowly fills the bottom of the socket and continues to provide the necessary support. An equally important difference between the two types of teeth, however, is to be seen in the arrangement of the enamel, the long-crowned type being provided with this durable substance on the inside of the crown instead of having a mere protective cap on the outside.
The more complicated structure was developed from the simpler form by the easy method of deepening certain depressions located between cusps at the top of the tooth. As the crown of the tooth increased its length these depressions remained tucked in, and eventually became deep pits roughly cylindrical in shape. In addition to the enamel and dentine, a third tooth substance, known as the cement, made its appearance at about this time, and we find that quantities of this new material were deposited outside the crown enamel and also inside the enamel walls of the pit, in this way producing a firmly consolidated structure otherwise weakened by deep channels and hollow pockets. The cement differs only slightly from the dentine but is deposited while the uncut tooth is in the gum tissues of the mouth, the enamel and dentine elements being formed earlier in the embryonic tooth before it emerges from the jaw bone.
A tooth constructed by such a process, if cross-sectioned through the crown, will be found to consist of successive layers of hard and softer materials. In living animals the top of the tooth soon wears off and the enamel layers stand in higher relief because of their greater resistance to wear. A roughened surface of excellent grinding quality is thus provided, and as long as the wear continues there remains the same relative amount of enamel to retain the roughness, and resist abrasion.
Among the various types of grazing animals there is a marked difference in the arrangement and form of the enamel layers. Within a species of genus, however, the complicated enamel patterns of the molar teeth are consistently similar. In the case of horses especially, these patterns provide a most helpful key to the identification of extinct forms. The general pattern, in any of the more modern horses, may be understood more readily if the wavy enamel layers be regarded as forming a set of cylinders with deeply crinkled walls. Near the outer border of the tooth, surrounded by a thin layer of cement, is the enclosing cylinder which represents the enamel cap of the old-fashioned, low-crowned tooth. Inside of this is the central mass of dentine which has been penetrated by two of the deep pits previously mentioned. The original enamel cap has been depressed into these pits, forming two inner cylinders which are filled with cement. Instead of being circular in outline, when the cap is worn through at the grinding surface these inner cylinder walls are seen to be wrinkled and folded so as to produce a most irregular pattern. However, if several teeth of the same kind of horse are compared, it will be found that the edges of these cylinders produce figures which are remarkably uniform and characteristic for that species.
American Mastodon (Mastodon americanus)
A true mastodon of the short-jawed type.