A history of land mammals in the western hemisphere

by William Berryman Scott

In South America proper nearly the whole of Patagonia was submerged by the transgression of a shallow, epicontinental sea, in which were accumulated the beds of the Patagonian stage, containing an exceedingly rich and varied assemblage of marine fossils, an assemblage which has very little in common with the contemporary formations of the northern hemisphere. It is this lack of elements common to the northern faunas which has led to the long debate concerning the geological date of the Patagonian formation, the South American geologists very generally referring it to the Eocene. However, the occurrence of genera of Cetaceans (whales and dolphins), which are also found in the Miocene of Maryland and Virginia, is very strong evidence that the proper date of the Patagonian is Miocene. A continuous coast-line, or at least an unbroken continuity of shoal-water conditions, seems necessary to account for the similarity of the Patagonian fossils with those of New Zealand and Australia, and that this connection was by way of the Antarctic continent is indicated by the occurrence of similar fossils in the South Shetland Islands, an Antarctic group. On the Chilian coast the Navidad formation, which is believed to be approximately contemporaneous with the Patagonian, has so different a fauna as to point to some kind of a barrier between the Atlantic and the Pacific, and this barrier, Dr. von Ihering holds, was the land-extension from South America to Antarctica.

After some oscillations of retreat and advance, the sea withdrew from Patagonia and the terrestrial accumulations of the Santa Cruz stage were formed. These beds are partly composed of river-deposits, but chiefly of more or less consolidated volcanic ash or tuff, and have yielded a surprising number of beautifully preserved mammals. No other assemblage of South American Tertiary Mammalia is so well known and understood as the Santa Cruz fauna, and the very large number of all but complete skeletons which have been found strongly suggests that many of the animals were buried alive in the showers of volcanic ash. The Santa Cruz fauna is completely and radically different from any of the North American assemblages, and at that time no immigrant from the north had penetrated so far as Patagonia.

In the upper Miocene the Andes stood at a much lower level than they do now; fossil plants, some of them collected at a great height in the mountains, are the remains of a luxuriant and purely tropical flora nearly identical with the vegetation of the modern forests of Bolivia and Brazil. Such a vegetation could not exist at the altitudes where the fossils occur and these demonstrate a great elevation of the mountains since those leaves were embedded. The same mild climatic conditions which prevailed in the northern hemisphere during the Miocene must also have characterized Patagonia, subtropical shells extending far to the south of their present range.

Whatever may have been true of the land-bridge connecting South America with Africa during the early Tertiary epochs, it must have been submerged in the Miocene, otherwise there would not have been the open pathway for the Cetacea of Patagonia to reach the Atlantic coast of North America and vice versa.

5. Pliocene Epoch

North America.—The Pliocene of North America is not nearly so well displayed or so satisfactorily known as the preceding Tertiary epochs, and only of comparatively late years has it been recognized at all upon the Atlantic coast. The Atlantic and Gulf shores had very nearly their present outlines, but with some notable differences. It would seem that the northeastern portion of the continent stood at a higher level than it does now, north Greenland being joined with the islands of the Arctic archipelago and Newfoundland with Labrador, the Gulf of St. Lawrence then being land. From Nova Scotia to southern New Jersey the coast-line was many miles to the east and south of its present position, but the sea encroached here and there upon the shores of Virginia, the Carolinas and Georgia, and southern Florida was mostly under water, as was also a narrow strip of the Gulf coast from Florida to Texas and along the east of Mexico. On the Pacific side of the continent the marine Pliocene is far thicker and more important than on the east coast and in California is largely made up of volcanic materials. Quite extensive disturbances in this region had marked the close of the Miocene, the strata of which in the Coast Range had been violently compressed and folded. An elevation of the land had caused the sea to withdraw from the central valley of California and had restored Lower California to its peninsular conditions, reducing the gulf to the narrow limits which it had had in the lower Miocene and extending southern Mexico to the west and south. British Columbia and southeastern Alaska stood at higher than their present levels and the countless islands of that region were part of the mainland. Bering Strait was closed, for at least a great part of the epoch, and, as a continuous shore-line was thus formed and a way of migration opened, the marine fauna of California and Japan became closely similar.

Fig. 52.—Map of North America during the Pliocene epoch, Bering Strait open. Explanation as in [Fig. 48]. (Modified from Schuchert.)