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COMMUNITY ECOLOGY: 



of its first full development in the Miocene. Finally, 

 The Subtropical Scrub, perhaps a Neotropical-Ter- 

 tiary derivative, is mentioned in reference to its period 

 of occurrence. 



Lafe Cretaceous. The late Upper Cretaceous fea- 

 tures of North America were so unlike modern land- 

 scapes that they might represent a scene from another 

 planet (Figure 19.12). The land was more restricted 

 and mostly very low, and North America consisted 

 of two subcontinents, one western and the other 

 eastern. Physiographically, the eastern United States 

 featured the Appalachians, which were highly de- 

 graded but much more conspicuous than now. In 

 the west submerged and lowland areas were uplifted 

 and accompanied by vulcanism, giving rise to the 

 Rockies and intermontane plateaus. This probably 

 reflected early Upper Cretaceous intrusion of many 

 batholiths from the tip of Baja California up into 

 Alaska. In general, these batholiths still form the 



Figure 19.12 tote Cretaceous poleogeography, including the distri- 

 bution of geofloros and mojor mountains. Arclo-Tertiory to the North 

 and Neotropical-Tertiary to the South. 



backbone of the western coastal mountains. They 

 are involved in the Canadian Coast Ranges, the 

 northern Rocky Mountains, the Klamath Mountains, 

 the Sierra Nevada Mountains, the southern Califor- 

 nia Coast Ranges, the Transverse Ranges, the Pen- 

 ninsular Ranges, and Baja California mountains. 

 When these batholiths formed, each led to some 

 elevation of the land and mountain formation, mostly 

 by folding and faulting. 



Perhaps the most amazing aspect of North Ameri- 

 can geology was the separation of our continent by 

 a continuous arm of the ocean. This gigantic channel, 

 the Mesocordilleran Geosyncline, went from the Gulf 

 of Mexico to the mouth of the Mackenzie River, was 

 about 12° longitude wide, and separated the western 

 and eastern subcontinents. 



The climate was subtropical up to southern 

 Alaska, tropical over most of the United States, and 

 temperate probably to the North Pole. There appears 

 to have been no significant climatic zonation; hence, 

 elevations probably were low to moderate. The pres- 

 ence of corals, tropical marine animals whose limits 

 of distribution now extend to an 18° C. barrier, 

 fairly well indicates the end of the tropics and the 

 start of the temperate zone. Fossils of Late Cretace- 

 ous corals have been found to 53° N. latitude, thus 

 indicating that the seas were much warmer than they 

 are now. 



The Upper Cretaceous often is called "The Time of 

 the Great Dying." Either during the Upper Cre- 

 taceous or toward its end most reptiles became ex- 

 tinct, hence some ten to twenty million years saw the 

 removal of animals that dominated the earth for over 

 one hundred million years. Of the birds, the earliest 

 types were gone, but toothed birds plus certain mod- 

 ern types — ducks, grebes, and pelicans — are known 

 to have existed. While reptiles were becoming scarce, 

 birds were expanding. 



Of the five mammalian orders present in the Lower 

 Cretaceous, only the small, vaguely rodentlike, 

 herbivorous multituberculates survived. However, 

 even these most persistent of mammal groups (Juras- 

 sic origin) disappeared in the Eocene. In addition. 

 Upper Cretaceous deposits display the first appear- 

 ance of two orders, the first marsupials, much like 

 present opossums, and the first insectivores, much 

 like present shrews. 



Post-Cretaceous development of reptiles and birds 

 can be summarized here. It appears that all modern 

 groups of reptiles were present during Cretaceous 



