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.