68 



TITANOTHEEES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



A very significant fact, clearly presented in tlie table 

 on page 67, is that these small, light-limbed, cursorial 

 ungulates appear not simultaneously but at successive 

 horizons. At the lowest level are the horses (Eohip- 

 pus) ; at a higher level the pseudo tapirs (Systemodon) ; 

 at a still higher level the lophiodonts (Heptodon) ; and 

 then, toward the end of the lower Eocene, the titano- 

 theres {Lambdoiherium) . 



Figure 45. — Generalized section through Upper Cretaceous and basal and 

 lower Eocene deposits near Pumpkin Buttes, Powder River Valley, AVyo. 

 (No. 12, fig. 3.5) 



Adapted from C. H. Wegemann (1917.1). 



Though the results of our observations may be modi- 

 fied by further discoveries the successive rather than 

 simultaneous appearance of these advancing waves of 

 perissodactyl migration is what a study of modern 

 migrations should lead us to expect. All these animals, 

 as shown elsewhere in this monograph, have similar 

 cursorial foot structure, which indicates extensive areas 

 of dry land and open meadow, in which the small, 

 defenseless Herbivora could easily escape the attacks of 

 the Carnivora. 



Habitat of Wasatch mammals. — The conditions that 

 prevailed in Wasatch time have been determined very 

 interestingly by Loomis in his "Origin of the Wasatch 

 deposits" (1907. 1, pp. 356-364). In adaptation to 

 various habitats the known species of vertebrates are 

 divided as follows: Aerial, 3 per cent; cursorial, 

 terrestrial, and arboreal, 75 per cent; amphibious, 12 

 per cent; aquatic, 10 per cent. The light-limbed 

 horse, Eohippus, typical of a plains 

 or partly open country, alone makes 

 up 32 per cent of the total collections 

 from the Systemodon zone ("Gray 

 Bull" horizon). All the other odd- 

 toed ungulates are light-limbed, in- 

 cluding the tapiroids {Systemodon), 

 lophiodonts (Heptodon), and primitive 

 titanotheres {LamhdotJierium) , as well 

 as the surviving archaic condylarths 

 (PJienacodus and Edocion). The feet 

 of all these animals indicate dry rather 

 than swampy or forested land, because 

 they are more slender than those of 

 the modern tapir. On the other hand, 

 the coryphodons were certainly marsh 

 dwellers and perhaps in part stream 

 dwellers. The small percentage of 

 species of truly aquatic animals, such 

 as crocodiles, fishes, and turtles, whose 

 remains are mingled with those of the 

 prevailing land animals, probably be- 

 came stranded in lagoons far from the 

 rivers. The presence in the rivers of 

 rather large fishes is shown by the re- 

 mains of the large Clastes. Remains 

 of river-living turtles (Trionyx) have 

 also been found in the Wasatch. 



LOWER EOCENE FAUNAL ZONES 

 ZONE 6: EOHIPPUS-CORYPHODON ZONE 



[Second Wasatch life zone. Big Horn B; lower Sparnacian 

 of Europe] 



Below the EoMppus-CorypTiodon 

 zone in the Clark Fork Basin of Wyo- 

 ming lies the first Wasatch life zone 

 {PJienacodus - Nothodectes - CorypJiodon 

 zone) described on pages 64-66. 

 Near the head of the Big Sand 

 Coulee, on the Clark Fork of the Yellowstone, which 

 adjoins the Big Horn River basin on the west, 

 is a series of about 200 feet of red-banded shales, 

 which overlie the Phenacodus zone ("Clark Fork 

 beds," transition basal Eocene) and contain a mam- 

 malian fauna that is radically different from that of 

 the underlying "Clark Fork." These beds (the 

 "Sand Coulee beds " of Granger) mark the first appear- 

 ance in the Rocky Mountain basin region of four 

 modernized orders of mammals — the lemuroids. 



