ENVrROlSTMENT OF THE TITANOTHERES 



47 



be distinct from those of the true Laramie and to be 

 more closely allied to those of the true Fort Union 

 above. 



The true Fort Union floras of basal Eocene (Thane- 

 tian) age include between 500 and 600 species of trees, 

 which were apparently derived from areas farther 

 north, certainly not from areas farther south. 

 These forests, which were contemporaneous with the 

 Puerco and Torrejon mammals, indicate a climate 

 in the Rocky Mountain region between the fortieth 

 and fiftieth parallels that was far from tropical, yet 

 moderately warm and humid, with mild winters, favor- 

 able to the growth of palm, fig, and camphor trees, 

 as well as other warm-temperate trees, including gink- 

 gos and sequoias. This flora, which is characteristic 

 of the early uplift period of the Rocky and Uinta 

 Mountains in Colorado and Wyoming, indicates a 

 somewhat cooler climate than that of the subsequent 

 lower Eocene (Green River) epoch in the same region 

 and a much cooler climate than the subtropical climate 

 of the South Atlantic States in early Eocene time. In 

 fact, both in the Rocky Mountain region and farther 

 south the American climate became milder and more 

 tropical as the Eocene epoch advanced. 



EOCENE GEOGRAPHY OF WESTERN NORTH AMERICA AND 

 ITS RELATION TO FAUNAI MIGRATIONS 



GEOGRAPHIC DIVISIONS AND THEIR BEARING ON 

 MIGRATION 



The main topographic features of western North 

 America were established between late Cretaceous and 

 middle Eocene time. In late Cretaceous and early 

 Eocene time certain routes of migration connected 

 the animal life of the central Rocky Mountain 

 region with that of Eurasia and probably with 

 that of South America. The key to these routes 

 of migration and to the geographic distribution of 

 these animals is afforded by the results of researches 

 made since 1853 by the geological surveys of the United 

 States and Canada. The foundation of the descriptive 

 geologic history of the Rocky Mountain region is laid 

 in the report of F. B. Meek and F. V. Hayden (Meek 

 and Hayden, 1862.1). 



The entire Cordillera region extends from Bering 

 Strait to the Isthmus of Tehuantepec, a distance of 



4,500 miles, and has an average width of 500 to 600 

 miles. The main geographic divisions of the Cor- 

 dilleran region, named in order from east to west, are 

 the following : 



Rocky Mountain Range, Bering Sea to Colorado, 

 including — 



Front or eastern range, facing the Great Plains. 

 Rocky Mountain basins between the eastern and 

 western ranges, forming the central north and 

 south migration routes of mammals. 

 Westerly ranges, facing the interior plateaus. 

 Central interior plateaus, intermontane belt region 

 (main migration routes of herbivorous mammals) : 

 Northern interior plateaus, Alaska to Washington. 

 Columbia Plateau. 



Nevada-Sonora Plateau (Great Basin). 

 Colorado Plateaus. 

 Mexican Plateau. 

 Pacific mountain system, British Columbia: 

 Sierra Nevada. 



Pacific mountain basins between the Sierra 

 Nevada and the Coast Ranges. Coastal 

 migration routes of mammals. 

 Pacific Coast Range. 



A transverse section of the Cordillera on the 41st 

 parallel exhibits clearly the main confines of these 

 mountain ranges, basins, and plateaus. The great 

 plateaus and the mountain basins may have pre- 

 sented bordering forests and central grassy plains 

 and jungles, interspersed with swamps, marsh lands, 

 rivers, and lakes similar to those in the plateau 

 and mountain (Kenya, Kilimanjaro) region of equa- 

 torial Africa to-day. Migration from north to south 

 or from south to north was possible along three 

 routes. 



Our only knowledge of the late Cretaceous and 

 Eocene mammal life of North America is afforded 

 by the remains of mammals of the Rocky Mountain 

 basins and foothills from Alberta to northern New 

 Mexico. During the Oligocene epoch the life of the 

 Columbia Plateau is revealed in the John Day forma- 

 tion of Oregon. The life of the Great Plains first 

 appears in the lower Oligocene formations in South 

 Dakota, Wyoming, Nebraska, and Colorado, which 

 border the Rocky Mountains on the east. The Eocene 

 mammalian life of the country that stretches east- 

 ward from the Rocky Mountain Front Range to the 

 Mississippi and the Atlantic coast is entirely unknown. 



