A history of land mammals in the western hemisphere

by William Berryman Scott

In the subjoined table the periods and epochs are those which are in general use throughout the world, the ages and stages are those which apply to the western interior of North America, each region, even of the same continent, requiring a different classification. The South American formations are given in a separate table, as it is desirable to avoid the appearance of an exactitude in correlation which cannot yet be attained.

CENOZOIC ERA

Quaternary period	{	Recent epoch
	{	Pleistocene epoch = Glacial and Interglacial stages.
Tertiary period	{	Pliocene epoch
	{	Miocene epoch
	{	Oligocene epoch
	{	Eocene epoch
	{	Paleocene epoch

Continuing the subdivision of the Tertiary period still farther, we have the following arrangement:

TERTIARY PERIOD (North America)

Pliocene	{	Upper		Wanting
	{	Middle		Blanco age
	{	Lower	{	Thousand Creek age
			{	Snake Creek age
			{	Republican River age
Miocene	{	Upper		Loup Fork age
	{	Middle		Deep River age
	{	Lower		Arikaree age
Oligocene	{	Upper		John Day age
	{	Lower		White River age
Eocene	{	Upper		Uinta age
	{	Middle		Bridger age
	{	Lower	{	Wind River age
			{	Wasatch age
Paleocene	{	Upper		Torrejon age	}	Fort Union
	{	Lower		Puerco age	}

This is a representative series of the widespread and manifold non-marine Tertiary deposits of the Great Plains, but a much more extensive and subdivided scheme would be needed to show with any degree of fullness the wonderfully complete record of that portion of the continent during the Tertiary period. A much more elaborate table will be found in Professor Osborn’s “Age of Mammals,” p. 41. There are some differences of practice among geologists as to this scheme of classification, though the differences are not those of principle. No question arises concerning the reality of the divisions, or their order of succession in time, but merely as to the rank or relative importance which should be attributed to some of them, and that is a very minor consideration.

Much greater difficulty and, consequently, much more radical differences of interpretation arise when the attempt is made to correlate or synchronize the smaller subdivisions, as found in the various continents, with one another, because of the geographical differences in contemporary life. Between Europe and North America there has always been a certain proportion of mammalian forms in common, a proportion that was at one time greater, at another less, and this community renders the correlation of the larger divisions of the Tertiary in the two continents comparatively easy, and even in the minor subdivisions very satisfactory progress has been made, so that it is possible to trace in some detail the migrations of mammals from the eastern to the western hemisphere and vice versa. Such intermigrations were made possible by the land-bridges connecting America with Europe across the Atlantic, perhaps on the line of Greenland and Iceland, and with Asia where now is Bering Strait. These connections were repeatedly made and repeatedly broken during the Mesozoic and Cenozoic eras down to the latest epoch, the Pleistocene. By comparing the fossil mammals of Europe with those of North America for any particular division of geological time, it is practicable to determine whether the way of intermigration was open or closed, because separation always led to greater differences between the faunas of the two continents through divergent evolution.

Correlation with South America is exceedingly difficult and it is in dealing with this problem that the widest differences of opinion have arisen among geologists. Through nearly all the earlier half of the Tertiary period the two Americas were separated and, because of this separation, their land mammals were utterly different. Hence, the lack of elements common to both continents puts great obstacles in the way of establishing definite time-relations between their geological divisions. Only the marine mammals, whales and dolphins, were so far alike as to offer some satisfactory basis of comparison. When, in the later Tertiary, a land-connection was established between the two continents, migrations of mammals from each to the other began, and thenceforward there were always certain elements common to both, as there are to-day. In spite of the continuous land between them, the present faunas of North and South America are very strikingly different, South America being, with the exception of Australia, zoölogically the most peculiar region of the earth.