Geology: The Science of the Earth's Crust

by William J. Miller

During the early part of the Cenozoic era birds became still more advanced and numerous, with many modern groups represented. Some of the more primitive types were, however, still left over during the Tertiary, as, for example, a toothed bird, in which the teeth were merely dentations of the bill, thus being the most degenerate of all types of tooth structure.

Mammals comprise the highest class of all animals. They are, of course, all warm blooded and characterized by suckling their young. So far as known, mammal life began in the early Mesozoic era as a branch of primitive reptiles, but they made little progress throughout the era when they occupied a very subordinate position in the animal world. They were few in number, small, and primitive in structure. There is no evidence for the Mesozoic existence of any of the higher forms of mammals, that is, those which give birth to well-formed young which are prenatally attached to the mother by the so-called placentum. “During the eons of the Mesozoic, from late Triassic time until its close, the mammals (including the remote progenitors of humanity) were in existence, but held in such effective check (by reptiles) that their evolutionary progress was practically insignificant. This curb is strikingly illustrated by the wonderful series of tiny jaws and teeth of these diminutive creatures found in the Comanchian (early Cretaceous) of Wyoming, in actual association with the single tooth of a carnivorous dinosaur, many times the bulk of the largest mammalian jaw. The removal of this check resulted (in the Tertiary period) in the speedy evolution of the archaic mammals.” (Schuchert.)

The phenomenal development of mammals during the Tertiary period forms one of the most wonderful chapters in the whole evolution of organisms. Even very early in the Tertiary, many important higher (placental) types of mammals had evolved, and the simpler, more primitive Mesozoic forms became very subordinate. By the close of the Tertiary the higher types of mammals had become marvelously differentiated into most of the present-day groups or types. A very significant feature of the evolution was the steady increase in relative size of brain. The vast numbers of fossil skeletons and bones of mammals found in Tertiary strata is scarcely believable. In our brief discussion we can do no more than describe a few representative examples of the Cenozoic evolution of mammals.

The great diversity of modern placental animals may be suggested by a few examples, as the tiger, dog, horse, camel, elephant, squirrel, hedgehog, whale, monkey, and man. Forms like these, traced back through their ancestors to the very early part of the Tertiary period, gradually become less and less distinct until they cannot be at all distinguished as separate groups, but rather there are ancestral generalized forms which show combinations of features of the later groups. Those early Tertiary generalized placental mammals had four feet of primitive character, with five toes on each foot; the whole foot, which from toe to heel touched the ground, was not adapted to swift running; the teeth were simple (primitive) in type and of full original number (forty-four); the toes were supplied with nails which were about intermediate between real claws and hoofs in structure; and the brain was relatively much smaller and simpler in structure than in most modern mammals.

Fig. 66.—Chart showing the main features of interest in the evolution of the horse family through several million years of the present (Cenozoic) era of geologic time. (After Matthew, American Museum of Natural History.)

The history of the horse family furnishes an excellent illustration of certain evolutionary changes among mammals. Skeletons of many species, ranging from the early Tertiary to the present, have been found in remarkable state of preservation representing every important change in the history of the horse family. A study of the chart will make clear some of the most striking changes which have taken place. The oldest member of the horse family represented on the chart was about the size of a small fox, with four toes and a degenerated fifth toe (splint) on the front foot, and three toes and splint on the hind foot. Since the chart was made a still more primitive form, even more closely resembling the original five-toed ancestor, has been found. Gradually the middle toe enlarged, while the others disappeared except the two splints or very degenerate toes still left in the modern horse. Increase in size of the animal and brain capacity accompanied these changes. Also the teeth underwent notable change, and two originally separate bones (radius and ulna) of the foreleg became consolidated into a single stronger bone.