124 THE AGE OF MAMMALS 



and the Big Horn traversed by parallel 44°, east of the Wasatch and west 

 of the Big Horn ranges respectively, both vast flood plain and lacustrine 

 basins surrounded by low mountain ranges. It is significant that at Evan- 

 ston in the Big Horn (Fig. 36) and in the Wind River (Fig. 38) the mammals 

 are found chiefly in or near the so-called "Red Beds." These beds may be 

 an indication of the prolonged exposure of these sediments to the air, or 

 of erosion from the reddish rocks of the Trias. The basins were formerly 

 considered great lake basins, but the river, flood plain, and lagoon theory 

 now prevails. Loomis (1907) ' carefully analyzed the entire mammalian 

 fauna of the Coryphodon Zone with reference to its bearing on the physio- 

 graphic conditions in these old mountain ranges. He shows that, judging 

 by the apparent adaptations to various modes of life, the total known species 

 of the vertebrate fauna are divided as follows: aerial 3 per cent, cursorial, 

 terrestrial, and arboreal 75 per cent, amphibious 12 per cent, aquatic 10 per 

 cent. We may imagine that this small percentage of species of truly 

 aquatic animals, such as crocodiles, fishes, and turtles, mingled their remains 

 with those of the prevailing land animals by becoming stranded or inclosed 

 in lagoons far from the rivers. The bones of terrestrial animals may have 

 been exposed on the sunny fiats. The light-limbed horse Eohippus, 

 probably typical of a plains or partly open country, alone makes up 32 per 

 cent of the total collections. All the other perissodactyl or odd-toed 

 ungulates were light-limbed, including the lophiodonts (Heptodon), primi- 

 tive titanotheres (Lambdotherium) , the surviving archaic condylarths {Phe- 

 nacodus). The feet of all these animals indicate dry rather than swampy 

 ground conditions, because they are more slender than those of the modern 

 tapir. On the other hand, the coryphodons were certainly marshy-land 

 dwellers, and perhaps partly amphibious, or stream dwellers, although this 

 is far from demonstrated. The presence of rivers of considerable size is 

 indicated by the large lepidostean fishes, or garpikes (Clastes), and by the 

 river-living turtles {Trionyx). 



Wasatch Life of the Wyoming and New Mexico Region 



Surviving archaic mammals. — Taken altogether, the prevailing resem- 

 blances of this older fauna of the Coryphodon Zone are with the mammals 

 found in the Sparnacian and Lower Ypresian of Europe, but far closer com- 

 parisons are necessary than any which have been made hitherto. 



Of the smaller Herbivora, no signs of the Plagiaulacidse or any other 

 marsupials have been discovered; opossums (Didelphyidae) were probably 

 living in the forests of this region, however. Of the condylarth ungulates, 

 Phenacodus is the most famous. The discovery by Wortman ^ of the 



• Loomis, Origin of the Wasatch Deposits. Amer. Jour. Sci., May, 1907, Ser. 4, Vol. XXIII, 

 pp. 356-364. 



^ See Cope, E. D., The Vertebrata of the Tertiary Formations of the West. Rept. U.S. 

 Geol. Surv. Terr., Vol. Ill, 1883 (1884), PI. LVII and text. 



