A history of land mammals in the western hemisphere

by William Berryman Scott

In the following table of the South American Cenozoic, the assignment of the ages to their epochs is largely tentative, especially as regards the more ancient divisions, and represents the views generally held by the geologists of Europe and the United States; those of South America, on the contrary, give an earlier date to the ages and stages and refer the older ones to the Cretaceous instead of the Tertiary.

CENOZOIC ERA (South America)

Quaternary period	{	Recent epoch
	{	Pleistocene epoch—Pampean Beds, Brazilian caverns
Tertiary period	{	Pliocene epoch	{	Monte Hermoso age
			{	Catamarca age
			{	Paraná age
	{	Miocene epoch	{	Santa Cruz age
			{	Patagonian age
	{	Oligocene epoch	{	Deseado age (Pyrotherium Beds)
			{	Astraponotus Beds
	{	Eocene epoch	{	Casa Mayor age (Notostylops Beds)

The Pleistocene and Pliocene deposits are most widely distributed over the Pampas of Argentina, but the former occur also in Ecuador, Brazil, Chili, and Bolivia. The other formations cover extensive areas in Patagonia, and some extend into Tierra del Fuego.

We have next to consider the methods by which past geographical conditions may be ascertained, a task which, though beset with difficulties, is very far from being a hopeless undertaking. As has already been pointed out, it is perfectly possible for the geologist to determine the circumstances of formation of the various kinds of rocks, to distinguish terrestrial from aquatic accumulations and, among the latter, to identify those which were laid down in the sea and those which were formed in some other body of water. By platting on a map all the marine rocks of a given geological date, an approximate estimate may be formed as to the extension of the sea over the present land for that particular epoch. It is obvious, however, that for those areas which then were land and now are covered by the sea, no such direct evidence can be obtained, and only indirect means of ascertaining the former land-connections can be employed. It is in the treatment of this indirect evidence that the greatest differences of opinion arise and, if two maps of the same continent for the same epoch, by separate authors, be compared, it will be seen that the greatest discrepancies between them are concerning former land-connections and extensions.

The only kind of indirect evidence bearing upon ancient land-connections, now broken by the sea, that need be considered here is that derived from the study of animals and plants, both recent and fossil. All-important in this connection is the principle that the same or closely similar species do not arise independently in areas between which there is no connection. It is not impossible that such an independent origin of organisms which the naturalist would class as belonging to the same species may have occasionally taken place, but, if so, it must be the rare exception to the normal process. This principle leads necessarily to the conclusion that the more recently and broadly two land-areas, now separated by the sea, have been connected, the more nearly alike will be their animals and plants. Such islands as Great Britain, Sumatra and Java must have been connected with the adjacent mainland within a geologically recent period, while the extreme zoölogical peculiarity of Australia can be explained only on the assumption that its present isolation is of very long standing. The principle applies to the case of fossils as well as to that of modern animals, and has already been made use of, in a preceding section, in dealing with the ancient land-connections of North America. It was there shown that the connection of this continent with the Old World and the interruptions of that connection are reflected and recorded in the greater or less degree of likeness in the fossil mammals at any particular epoch. Conversely, the very radical differences between the fossil mammals of the two Americas imply a long-continued separation of those two continents, and their junction in the latter half of the Tertiary period is proved by the appearance of southern groups of mammals in the northern continent, and of northern groups in South America.

Inasmuch as the connection between North and South America still persists, the geology of the Isthmus of Panama should afford testimony in confirmation of the inferences drawn from a study of the mammals. Of course, the separating sea did not necessarily cross the site of the present isthmus; it might have cut through some part of Central America, but a glance at the map immediately suggests the isthmus as the place of separation and subsequent connection. As a matter of fact, isthmian geology is in complete accord with the evidence derived from the mammals. Even near the summit of the hills which form the watershed between the Atlantic and the Pacific and through which the great Culebra Cut passes, are beds of marine Tertiary shells, showing that at that time the land was completely submerged. This does not at all preclude the possibility of other transverse seas at the same period; indeed, much of Central America was probably under the sea also, but the geology of that region is still too imperfectly known to permit positive statements.

When several different kinds of testimony, each independent of the other, can be secured and all are found to be in harmony, the strength of the conclusion is thereby greatly increased. Many distinct lines of evidence support the inference that North and South America were completely severed for a great part of the Tertiary period. This is indicated in the clearest manner, not only by the geological structure of the Isthmus and by the mammals, living and extinct, as already described, but also by the fresh-water fishes, the land-shells, the reptiles and many other groups of animals and plants.

The distribution of marine fossils may render the same sort of service in elucidating the history of the sea as land-mammals do for the continents, demonstrating the opening and closing of connections between land-areas and between oceans. The sea, it is true, is one and undivided, the continental masses being great islands in it, but, nevertheless, the sea is divisible into zoölogical provinces, just as is the land. Temperature, depth of water, character of the bottom, etc., are factors that limit the range of marine organisms, as climate and physical barriers circumscribe the spread of terrestrial animals. Professor Perrin Smith has shown that in the Mesozoic era Bering Strait was repeatedly opened and closed, and that each opening and closing was indicated by the geographical relationships of the successive assemblages of marine animals that are found in the Mesozoic rocks of California and Nevada. When the Strait was open, the coast-line between North America and Asia was interrupted and the North Pacific was cooled by the influx of water from the Arctic Sea. At such times, sea-animals from the Russian and Siberian coasts extended their range along the American side as far south as Mexico, and no forms from the eastern and southern shores of Asia accompanied them. On the other hand, when the Strait was closed, the Arctic forms were shut out and the continuous coast-line and warmer water enabled the Japanese, Indian, and even Mediterranean animals to extend their range to the Pacific coast of North America. A comparison of the marine fishes of the two sides of the Isthmus of Panama shows an amount and degree of difference between the two series as might be expected from the length of time that they have been separated by the upheaval of the land.