Fig. 169.—Plagiaulax Becklesii. Jaw, and pre-molar enlarged, showing flat surface, with ridges.—Purbeck.
Indeed, the recent discoveries in America and in the east of Europe have almost thrown into the shade those researches of Cuvier in the Paris basin which first brought this important fact to light. The Eocene mammals, like the Carboniferous amphibians, the Mesozoic reptiles, and the Cretaceous forests, appear to spring full-grown from the earth, and this at nearly the same time in every part of the northern hemisphere. It has been suggested that they may have come in gradually without our knowledge in the Cretaceous period; but if so, we should have found some of their remains along with those of the Upper Cretaceous plants. But the prevalence of the great reptiles up to the close of the Cretaceous would seem to render the co-existence of large mammals unlikely. It has further been supposed that geological changes in the southern and northern hemispheres may have alternated with each other, so that there may be in the former Cretaceous beds in which the remains of ancestors of the Eocene mammals may be found. But we do not as yet know of such deposits. We may be content, therefore, to suppose that at the close of the Cretaceous there was established somewhere a sort of Eden for the first placental mammals, in which they were introduced and could live unharmed by the decaying monsters of the reptilian age, until the time came when they could increase and multiply and replenish the earth. The nearest approach to such a centre of mammalian life is perhaps to be found in those great American lake basins embedded in the mountains of the West, which have been so well described by Hayden and Newberry, and which have yielded so many animal remains to the researches of Leidy, Marsh, and Cope.
Fig. 170.—Restoration of Palæotherium magnum. Eocene.—After Cuvier and Owen.
The typical deposits of the Early Eocene have long been those of the Basin of Paris, where thick and highly fossiliferous deposits of this age rest on the more or less denuded surface of the Upper Chalk, and have afforded a rich harvest of remains of about fifty species of placental quadrupeds, whose bones have been found in the gypsum quarries of Montmartre. The great majority belong to the Ungulates, or hoofed animals, and the most abundant genera are those called by Cuvier Palæotherium ([Fig. 170]) and Anoplotherium, of which there are several species, and which have affinities with the modern Tapirs on the one hand, and with the Horse on the other. Of the Unguiculate or clawed orders there are carnivorous forms allied to the Hyæna and the Fox, a Bat and a Squirrel; and the Marsupials are represented by an Opossum. Lyell describes a bed of clay associated with the gypsum, in which are numerous footprints, probably produced on the margin of a lake. Many of these might be referred to the Palæothere and its allies; but there are others belonging to quadrupeds yet unknown, and there are also tracks of tortoises, crocodiles, and lizards, and of a large wading bird. Such a bed, perhaps deposited on the margin of a salt lake, resorted to as a “lick” by herbivorous animals, and by the carnivorous species which preyed on them, is well fitted, by the thronging life which it indicates, to teach how little we can know of the actual number and variety of the old inhabitants of the earth.
In England, Eocene beds of the age of those of Paris, occupy the valley of the Thames and the Isle of Wight and neighbouring parts of Hants. They have afforded mammalian fossils similar to those of Paris, though less abundantly, but they are rich in remains of marine animals and of land plants.
Instead of describing the well-known animals of the French and English Tertiaries, from these Eocene deposits upwards, I shall shortly sketch the succession in America, as worked out by Marsh and Cope, with the aid of the admirable summary given by Gaudry of the present state of knowledge with reference to the sequence of mammalian life from its appearance in the Early Eocene up to the present time.[80]