Читать книгу The Pleistocene of North America and its vertebrated animals - Oliver Perry Hay - Страница 15

Table of Contents

It is necessary to determine, if possible, where the boundary line shall be drawn between the Pliocene and the Pleistocene. Room must be made for the first interglacial, the Nebraskan, and its fauna. How long this first glacial stage continued we do not know. Chamberlin and Salisbury have indicated (Geology, vol. III, p. 420) that in a rough way the dates from the present of the culmination of the various glacial stages, except the Nebraskan, taken in order backward, may be represented by the geometrical series 1, 2, 4, 8, 16. That is, if the Illinoian stage had its culmination 150,000 years ago, that of the Kansan occurred 300,000 years ago; if the Nebraskan should fall in the same series, it culminated 600,000 years ago; and it and the succeeding Aftonian interglacial held sway as long as all the rest of the Pleistocene put together. It would be rash to assert that this first glacial did last so long; but we see the possibilities. In a personal communication Professor Frank Leverett writes that he estimates that the Kansan culmination took place at not less than 400,000 years ago and the Nebraskan at 500,000. This, as the present writer estimates, would leave for the Nebraskan itself somewhere near 40,000 or 50,000 years. Some changes in the life of the Pleistocene must have been wrought during those years.

The glacial deposits of the Nebraskan stage are not as well known as one might wish. They appear to be in general overlain by the later drifts and are observed mostly where streams have cut through both the overlying drift and the Nebraskan. The old drift found in New Jersey is thin and of no great extent. Moreover, we can hardly expect to find fossil vertebrates in the drift itself. We must therefore depend on studies of supposed Nebraskan fossils found mostly outside of the glaciated area and make comparison of them with earlier and later faunas. If we shall discover collections of Nebraskan vertebrate animals, we may be sure that they will differ from those of the first interglacial, the Aftonian. We may be pretty certain that they will include autochthonous genera of the late Tertiary, which may be missing from the Aftonian, together with at least a few genera from South America and others from Asia.

Now, have any formations and included fossil vertebrates been found which may be fitted into the Nebraskan interval?

In this stage the writer places the beds which Cope designated the Idaho formation (Cope, Proc. Acad. Nat. Sci. Phil., 1883, p. 135). Since Cope’s time several new species have been added to his list from this formation. In 1917 (Bull. Dept. Geol. Univ. Calif., vol. X, p. 432), Dr. J. C. Merriam published a list of the fossils, except fishes, which had been secured up to that time. The list of species referred to the Idaho formation is as follows:

 Equus idahoensis.

 E. excelsus?

 Protohippus?

 Rhinoceros, probably Aphelops (Teleoceras) fossiger.

 Mastodon mirificus.

 Cervus, possibly new. Smaller and more slender than C. canadensis.

 Procamelus, size of P. major.

 Tragocerus? horn-core of antelope.

 Ischyrosmilus n. sp.

 Morotherium leptonyx.

 Castor, possibly n. sp.

 Olor, size of O. paloregonus.

 Graculus idahoensis.

In this collection the presence of horses of the genus Equus, of Cervus, Morotherium, and Castor, is strongly suggestive of the Pleistocene. The type of Mastodon mirificus was found in Pleistocene deposits of probably Aftonian age. Although rhinoceroses are supposed to have become extinct before the end of the Pliocene, this supposition may be an error. The list of Blanco vertebrates is a short one, and the absence of a genus from it is not decisive. One drawing of a seine in the sea-waters of Florida would furnish inadequate materials for conclusions about the fish fauna of that coast.

The Thousand Creek fauna (Merriam, Bull. Dept. Geol. Univ. Calif., vol. X, p. 429), which to the present writer appears of about the same age as the Blanco, contains a species of Teleoceras. The genera Protohippus and Procamelus might be supposed to have continued their existence and evolution until interrupted by an age of ice and by competitors from Asia.

In 1917 (Bull. cit., vol. X, pp. 255–266) Merriam and Buwalda published a short list of fossils which they had collected along the Columbia River in Washington State. A horse was found which was referred to Equus or Pliohippus; also two camelids, one of which was thought to be near Pliauchenia. Merriam concluded that the evidence on the whole favored the Pleistocene. The list will fit into the Nebraskan without difficulty.

In 1889 (Amer. Naturalist, vol. XXIII, p. 253), Professor E. D. Cope published a list of fossil mammals collected in the “Oregon desert,” apparently somewhere in the region of Silver Lake or Summer Lake. The list is as follows:

 Canis sp. indet.

 Elephas or Mastodon.

 Holomeniscus or Auchenia.

 Aphelops sp. indet.

 Hippotherium relictum.

 Equus sp. indet.

Cope looked upon this collection as remarkable in that it showed the presence of true horses and camels associated with a rhinoceros. He concluded that the fossils belonged to his Idaho formation. Dr. W. D. Matthew thought that the collection was a mixture of fossils from two formations (Bull. Amer. Mus. Nat. Hist., vol. XVI, p. 321). It may, however, have been made in Nebraskan deposits.

In 1921 (Proc. U. S. Nat. Mus., vol. LIX, pp. 617–638), the writer described a collection of vertebrate remains from Anita, Coconino County, Arizona. These remains were found in a cave in making explorations for copper ore. The list follows:

 Equus occidentalis.

 E. giganteus?

 Mylohyus? sp. indet.

 Procamelus coconinensis.

 P. longurio.

 Antilocapra americana?

 Marmota arizonæ.

 Citellus tuitus.

 Neotoma cinerea.

 Lepus benjamini.

 Brachylagus browni.

 Taxidea robusta.

 Canis nubilus?

 C. latrans?

 Chasmaporthetes ossifragus.

The writer believes that this assemblage of mammals must be referred to the Pleistocene. It will be noted, however, that there are two species of the genus Procamelus. These resemble so much two species, P. major and P. minimus, described by Leidy and Lucas (Trans. Wagner Free Inst., vol. IV, pp. I-XIV, 15–61) from the Alachua clays of Florida, that it seemed at first necessary to identify them as such. The genus Procamelus seems, therefore, to be brought definitely into the early Pleistocene, probably the Nebraskan.

The collections made in the Alachua clays in Florida were obtained in Alachua and Levy counties. On pages 195 and 375 will be found an account of the geological conditions under which the fossils were found, and lists of the species. The essential features are that such supposed Miocene or Pliocene genera as Gomphotherium, Procamelus, Teleoceras, and Aphelops were found associated with the Pleistocene genera Odocoileus, Tapirus, Megatherium, and Equus. This has been explained on the theory that the clays are of Tertiary age and that the Pleistocene species had become mingled with those of an earlier time. At a number of places in Florida where phosphate rock has been mined there have been secured similar associations of early camels, rhinoceroses, horses (Hipparion, Parahippus) with genera belonging undoubtedly to the Pleistocene. This has occurred so often that the writer doubts the correctness of the explanation given. He ventures, therefore, to include in the Pleistocene of the Nebraskan stage the various deposits that have received the names Alachua clays, the Dunnellon formation, and Bone Valley formation. The latter, called also the land-pebble phosphates, is believed by Sellards to be contemporaneous in age with the Dunnellon or hard phosphates, but to have accumulated under different conditions. Both the Alachuan and the Bone Valley formations were referred by Sellards to either the late Miocene or the early Pliocene, with an evident preference for the latter. It seems to have been the presence of the rhinoceroses that most influenced him in his assignment of the deposits; but there were naturally other considerations. He wrote:

The presence of rhinoceroses in the formation is believed to establish definitely the fact that the beds can not be later than the early Pliocene, since rhinoceroses in America apparently did not survive beyond that time (Fla. Geol. Surv., vol. VII, p. 73).

According to Sellards the hard phosphate, belonging to the Alachua (Dunnellon) formation (Fla. Geol. Surv., vol. V, p. 37) resulted from a disintegration of underlying Upper Oligocene deposits and probably the Vicksburg limestone. Through chemical action these rocks were partly dissolved and the residual materials were mixed by local subsidence and by action of streams and later modified by chemical changes.

The land-pebble phosphate of the Bone Valley formation had, Sellards concluded (Fla. Geol. Surv., vol. VII, p. 55), resulted from underlying phosphate marls of Upper Oligocene age. This occurred during a time of general subsidence of sufficient extent to permit marine waters to reach the area covered by the Bone Valley phosphates. The presence of sea-water is indicated by the occurrence of bones of cetaceans.

With regard to the effects of streams and of the chemical action of the water on the rocks, which contributed to the formation of the hard rock phosphate and the production of sinks and caves, it may be remarked that we know of no time when rocks were dissolved and caves formed to the extent that they were during the Pleistocene.

As shown on page 15, various deposits of marine marls along the Atlantic coast are referred by the writer to the Nebraskan. Among these marls are the coquina rock found at St. Augustine and the marine marl underlying the bed at Vero, which contained early Pleistocene vertebrate fossils. These marls are known to extend well inland, being found at Kissimmee, 50 miles from the coast. In some places they are met with at depths of 70 feet (Sellards, Fla. Geol. Surv., vol. VIII, pp. 105–106). Marls of probably the same age occur on the western coast of Florida (Dall, Bull. 84, U. S. Geol. Surv., p. 152). The writer believes that some of these marls may yet be connected with the phosphate beds of the Bone Valley formation.

A figure taken from Sellards (Geol. Surv. Fla., vol. VII, opp. p. 53) may be found on page 377. This illustrates the relation of the Dunnellon and Bone Valley formations to the underlying deposits.