Extract
Matthew's provisional classification of the freshwater Tertiary of Western America in 1899 was succeeded ten years later by a detailed correlation of the American Tertiary Mammal horizons with those of Europe, published by Osborn and himself in 1909. At that time these authors placed the Pikermi horizon of Europe (Pontian) in the Upper Miocene and the Val d'Arno horizon (Villafranchian) in the Upper Pliocene. They in common with many later writers subsequently adopted a Lower Pliocene and Pleistocene age respectively for the two horizons named. Allowing for this change of view, Osborn's and Matthew's correlation has been substantially followed, with a few modifications due to individual opinion, by all later authorities. The reasons for it were somewhat elaborated by Osborn in his Age of Mammals (1910). It is evident that the Equidae formed the basis for it. The appearance of the genera Anchitherium (as species of Kalobatippus and Hypohippus were then named), Hipparion and Equus in North America were assumed to be approximately contemporaneous with their appearance in Europe. That North America was at any rate the main, if not the exclusive radiative centre for the distribution of the Equidae has been universally acknowledged. Hence Osborn's and Matthew's choice of this group as a standard of age seemed to be fully justified. In accordance with it the first Hipparion beds of North America were regarded as Pontian.