42 



CENOZOIC MAMMAL HORIZONS 



III. THIRD FAUNAL PHASE. 



Absence of fresh Eurasiatic or northern migration — Continuation of sim- 

 ilar environmental conditions — Descendants of the archaic and modern- 

 ized mammals slowly evolving and competing with one another during 

 the lower and middle Eocene — Gradual elimination of the archaic mam- 

 mals — Gradual divergence from the faima of western Europe, and little 

 evidence of faunal interchange — Estabhshment of North American 

 Ungulata- Artiodac tyla. 



First, as to progressive divergence from Europe, it appears that by 

 the middle and upper Eocene stage there were 13 non-European 

 famiUes of mammals in America and 11 non-American families of 

 mammals in Europe, as against 4 European-American families com- 

 mon to the two regions. This independent and divergent evolution 

 was not sufficiently emphasized until suggested by the writer in 

 1899.^ It points to the existence of prolonged geographic or climatic 

 barriers between the two continents. 



Second, as to the continuously uniform conditions in the Mountain 

 Region, Matthew has especially called attention to the prolonged 

 uniformity of life, alike as to families, genera, and species, throughout 

 the Wasatch, Wind River, Huerfano, and lower Bridger depositions. 

 To this uniformity may be added the Uintatherium zone of the 

 Bridger and Uinta basins; in other words, the uniformity extended 

 from the lower to the upper Eocene. The changes are those of modifi- 

 cation and development rather than of breaks in the balance of nature 

 by migration and extinction. 



Our conclusions are as follows: (1) Environment: Uniform and 

 favorable environmental conditions prevailed during this long period 

 in the Mountain Region, with the competition and balance of nature 

 somewhat in favor of the modernized families, all of which persisted, 

 while 5 families of the archaic mammals disappeared., (2) Evolution: 

 Both the archaic (Cretaceous) and the modernized mammals in- 

 creased in size and in variety; the changes are chiefly specific rather 

 than generic. (3) Gains and losses: Two archaic families of Ungulata, 

 Condylarthra-Phenacodontidse and Amblypoda-Coryphodontidse, ap- 

 peared for the last time (Wind River); 1 new archaic family, the 

 Amblypoda-Uintatheriidai, appeared (Wind River); 2 families of 

 archaic Carnivora-Creodonta have disappeared (Wasatch), namely, 

 Palseonictidae, Arctocyonidse;^ the progressive Carnivora-Miacidae 

 are represented by 5 genera; 1 new family of Ungulata-Perissodactyla 



a Osborn, H. F., Correlations between Tertiary mammal horizons of Europe and America, etc.: Ann. 

 New York Acad. Sci., vol. 13, 1900, p. 18. "'Fourth, the Ligurien is widely distinct faunally from the 

 American upper Eocene or Uinta, with which it has been heretofore paralleled. At no period of the 

 Tertiary were the Nearctic and Palffiarctic faunae so widely separated. In fact, a much wider gap exists 

 between western America and Europe in the upper Eocene than in the preceding lower and middle 

 Eocene or in the succeeding lower Oligocene." 



f> Matthew considers that the Arctocyonidae should not be placed among the archaic mammals, but 

 rather that they represent an early branch of the Pro-Carnivora. 



