EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHBRES 

 Geologic succession and distrihution of the species of Eocene titanotheres — Continued 



273 



SECTION 3. THE LOWER EOCENE TITANOTHERES 



ANCESTRAL TITANOTHERES OF THE lAMBDOTHERIUM ZONE 

 OF WYOMING AT THE END OF LOWER EOCENE TIME 



The sudden appearance of two forms of titanotheres 

 in the region now linown as Wyoming and Colorado 

 at the end of lower Eocene time in the Lambdothe- 

 rium zone is a very striking fact. The two forms are 

 LamidotJierium, relatively small, swift, with slender 

 limbs, very abundant (upland type); and Eotitanops, 

 larger, with medium-sized limbs, less abundant (low- 

 land type). 



No trace of the ancestors of either of these animals 

 has thus far been found in the immediately underlying 

 Heptodon zone and earlier beds, although the remains 

 of horses (Eohippus) occur there in abundance. The 

 evidence favors the theory that the titanotheres mi- 

 grated into the ancient mountain region of North 

 America near the end of early Eocene time. The 

 beds in which they first appear belong to what is called 

 the Lamhdotherium zone, because of the great abun- 

 dance of the remains of this delicately formed titano- 

 there, which is found there in numbers exceeded only 

 by the remains of horses. In the typical Wind River 

 formation Granger, in his collections made from the 

 Lamhdotherium zone in 1905 and 1909, assembled out 

 of a total of 727 specimens remains of 191 Eohippus, 

 111 Lamhdotherium, and 14 Eotitanops. 



The localities of the Lamhdotherium zone known up 

 to the year 1912 were as follows: 



Thickness 

 in feet 



Wyoming, Wind River Basin, Lost Cabin section 400 



Wyoming, Big Horn Basin, Tatman Mountain section 325 



Wyoming, Beaver Divide, Green Cove section 265 



Colorado, Huerfano Basin, Garcia Canyon 400 



The three sections in Wyoming are of nearly uni- 

 form thickness throughout. The geology of the Lamh- 

 dotherium zone in Colorado is described in Chapter II. 

 There is also considerable uniformity in the size and 

 character of the remains of Lamhdotherium. Most of 

 the remains are referred to a single species, L. popoagi- 

 cum, which, however, appears to split up into several 

 subspecies. We Icnow only one phase in the evolu- 

 tion of this animal. Other phases await discovery. 



PHYSIOGRAPHIC ENVIRONMENT AT THE END OF LOWER 

 EOCENE TIME 



It is especially interesting to picture the geographic, 

 climatic, and biotic conditions surrounding these early 

 titanotheres. The picture may be drawn partly from 

 the study of the rocks in which their remains occur 

 and partly from the remains of the numerous mammals 

 that are found with them in these Wind River deposits. 

 The complete geologic relations of the deposits of the 

 Lamhdotherium zone have been presented in Chapter 

 II. We comment here chiefly on the geographic and 

 climatic features of the period. 



WIND RIVER BASIN, WYOMING 



In lower Eocene time {Lamhdotherium zone) the 

 Wind River Basin was a broad flood-plain valley with 

 mountain barriers to the north, west, and southwest 

 and an easterly drainage. (Sinclair and Granger, 

 1911.1, pp. 87-103, 105.) The materials of which 

 the beds are composed came from these surrounding 

 mountains. The fine material consists of highly 

 colored clay, in places banded alternately red and 

 blue, interstratified with pale greenish-buff and yel- 

 low-brown sandstone in more or less continuous 



