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COMMUNITY ECOLOGY: 



Figure 19.14 Eocene poleogeogrophy, including the distribution of 

 geofloras which partially indicate the distribution of major mountains. 

 Arcto-Tertiory to the North, Neotropical-Tertiary to the South, and 

 Madro-Tertiary Geofloro in mid-western United States. 



present orders of mammals. However, few species 

 other than the persistent marsupials and insectivores, 

 which reached their present insignificance in the 

 Oligocene, would be familiar to us; all dominant 

 forms were archaic types. The most numerous orders 

 were the multituberculates, marsupials, insectivores, 

 somewhat squirrel-like primates, creodont carnivores, 

 archaic rodents, and two of the many orders of hoofed 

 mammals. The dominant hoofed mammals were the 

 slim and almost carnivore-appearing condylarths and 

 amblypods; older groups like the creodonts were re- 

 placed by more modern representatives of the orders, 

 so many new types appeared. Among the generally 

 small hoofed types that dominated this time were 

 "dawn horses," tapirlike animals, rhinoceroses, 

 titanotheres, oreodons, and primitive camels. Many 

 of these forms appear to have entered from Asia. 



Oligocene. Geologically, erosion was the most im- 

 portant process in the Oligocene. Western mountains 



seem to have approached degradation to base level. 

 However, this geological picture is difficult to recon- 

 cile with that of the climate and flora. A partial ex- 

 planation might involve the continuation of Eocene 

 vulcanism well into this period. 



The short period of latest Eocene to earliest Oligo- 

 cene seems to have had one of the greatest climatic 

 changes of the Tertiary. Immediately there was much 

 drying and cooling of climate and, as a result, marked 

 southward movement, mixing, and evolution of floras. 

 Moreover, this was just the start of a continuous cool- 

 ing cycle that ended with the great ice ages of the 

 Pleistocene. There were intervals of increased 

 warmth, but the over-all trend was toward the Pleis- 

 tocene and its ice ages. 



The Middle to Upper Oligocene was the time when 

 the western states were first invaded by the Arcto- 

 Tertiary Geoflora, an invasion whose southern limit 

 reached central Colorado (Figure 19.15). However, 



Figure 19.15 Late Oligocene to Early Miocene poleogeogrophy, 

 including the distribution of geofloras. Symbols as in Figure 19.14 plus 

 Subtropical Scrub in present Great Plains area. 



