ENVIEONMENT OF THE TITANOTHEEES 



77 



Wasatch Heptodon-Coryphodon-EoMppus zone through 

 the Wind River Lamhdotherium-Eotitanops-CorypTiodon 

 zone upward into the Oreodon zone of Ohgocene time. 

 This is the only undoubted Eocene-Oligocene sedi- 

 ment thus far determined in the Rocky Mountain 

 basin region. Its total thiclmess is 1,080 feet, and it 

 represents relatively slow sedimentation. There is a 

 single period of erosional unconformity at the end of 

 the upper Eocene. 



The life of the Wind River beds of this section is 

 distinctly of upper Wind River ("Lost Cabin") time, 

 corresponding with Wind River B and Huerfano A, 

 for it includes the titanothere Lambdotherium popo- 

 agicum, a CorypTiodon, two species of Equidae {Eohippus 

 craspedotus and E. venticolus), and two species of 

 Heptodon (H. calciculus and H. ventorum), which are 

 characteristic of closing Wasatch time. The presence 

 of remains of garpikes and crocodiles in this fauna 

 shows that these deposits were fiuviatile and indicates 

 that Wind River shales were of flood-plain origin, 

 though they include many channel fillings of coarse 

 arkose. 



We thus have glimpses of a faunistic period broadly 

 corresponding with the lower Ypresian of France, cer- 

 tainly extending from Wyoming to Colorado, and 

 probably spreading much more widely in the Rocky 

 Mountain and the adjacent Plains region. Though it 

 includes surviving members of the older Wasatch life 

 and incoming members of the succeeding Bridger life, 

 the Wind River and Huerfano life stands directly 

 intermediate between these; in fact, the representa- 

 tives of archaic families destined to become extinct 

 and those of modernized families destined to populate 

 the earth are very nearly balanced, including 21 genera 

 (30 species) of archaic mammals and 22 genera (36 

 species) of modernized mammals. 



Simultaneously with the decline of the coryphodons 

 the uintatheres reappeared in the genus Bathyopsis, 

 ancestral to the giant Uintatherium, which character- 

 izes Bridger C and D. 



THIRD FAUNAI PHASE (MIDDLE AND UPPER EOCENE) 



CORRELATION OF AMERICAN ZONES WITH EUROPEAN 



STAGES 



There is strong evidence of uniform and favorable en- 

 vironment and persistent evolution throughout middle 

 and upper Eocene time in the Rocky Mountain basin 

 region. The changes show progressive modification 

 and adaptation rather than breaks by migration or 

 extinction. Both the archaic and the modernized 

 families increased in size and variety. The surviving 

 archaic mammals appear to have flourished and in- 

 creased, especially in size and muscular power. Near 

 the very end of Eocene time only two new famihes of 

 quadrupeds appear, the ancestral camels (Camelidae) 



and the oreodonts (Oreodontidae), whereas in western 

 Europe new families repeatedly appear from the south. 



east, and north. The general correlation of the Euro- 

 pean stages and the American zones is given on page 78. 



