380 



COMMUNITY ECOLOGY: 



Pliocene. Along with the development of grassland 

 there were sharp changes in the nature of the mid- 

 continental to Mexican border Subtropical Scrub 

 Barrier. There is evidence that a highway opened 

 near the Canadian border and allowed some mixture 

 of eastern and western Arcto-Tertiary species. 



Perhaps geomorphic processes largely influenced 

 Madro-Tertiary evolution. Both Miocene and Plio- 

 cene geology featured vulcanism and increasing arid- 

 ity in areas of Madro-Tertiary development. For ex- 

 ample, it now appears that geology contributed to 

 the frequent fires that selected fire-adapted plants 

 now represented by Madro-Tertiary derived wood- 

 land trees and Chaparral shrubs. 



By Upper Miocene times temperature averages 

 were much like those of today. However, the sum- 

 mer maxima were not as high and the winter minima 

 were not as low; therefore, the climate was much 

 milder than it is now. .Still this was real similarity 

 to modern conditions, in that during the Upper Mio- 

 cene there was a shift in the west from past climate 

 of summer showers and winter rains to dry summers 

 and a single winter rainy season. 



The Miocene was the culmination of mammalian 

 development in North America; more kinds existed 

 than during any time of the Cenozoic. This develop- 

 ment can be related to evolution in the grassland and 

 movements from Asia. The carnivores included dogs, 

 ancestral wolves, bearlike dogs, saber tooth and false 

 saber tooth tigers, members of the weasel family, and 

 primitive racoons. Modern types of rodents became 

 more numerous. Rabbits and hares were present. 

 Also, hoofed types became very numerous. Elephant 

 relatives, which originated in Africa and traveled 

 through Eurasia, entered North America. Bizarre 

 types were represented by the common but no longer 

 abundant oreodons and by giant, piglike creatures, 

 all of which disappeared at the end of the period. 

 Deerlike species, rhinoceroses, horses, tapirs, camels, 

 peccaries, and antelope were present. 



Pliocene. Although modern climatic features 

 started in the Upper Miocene and were approximated 

 more closely in the Lower Pliocene, truly modern cli- 

 mate did not prevail until the Upper Pliocene (Fig- 

 ure 19.17). Modern climate was preceded by fluctua- 

 tion and progressively stricter zonation, trends caused 

 mostly by the elevation of greater mountain masses 

 (owing to general uplift of the western mountains) 

 and the previously set pattern of distribution of these 

 western masses. In addition, mountain building (es- 



Figure 19.17 Pliocene poleogeogrophy, including the distribution of 

 geofloros. Symbols as in Figure 19.16. 



pecially the rise of the modern Sierras) caused an in- 

 crease in climatic extremes that were associated with 

 increases in temperature maxima and lowering of 

 temperature minima. For these reasons, desert vege- 

 tation and communities evolved and the over-all na- 

 ture and distribution of plants was much as it is now, 

 though the continent was somewhat differently 

 shaped. Only the more drought-resistant vegetation 

 remained and the Grassland Element became fully 

 established. 



Unfortunately, Pliocene mammalian fossils are 

 rather scarce. Apparently previous North American 

 groups became further modernized and many present 

 genera and species were developed. Some hoofed 

 animals declined (tapirs and camels) and others dis- 

 appeared (oreodons and rhinoceroses), but still 

 others remained abundant or even expanded their 

 numbers (horses and antelope). Also, with the open- 

 ing of the Panamanian land bridge South American 

 marsupials, New World monkeys, primitive rodents, 

 primitive hoofed mammals, and edentates entered 



