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COMMUNITY ECOLOGY: 



MESOZOIC PALEOGEOGRAPHY 



It must be emphasized that the following account 

 is based on biological and geological evidence at hand. 

 Because such evidence is incomplete, many state- 

 ments should be prefaced by qualifying phrases, but 

 this is not done. Also, the evidence frequently allows 

 for variation in interpretation of past events. The 

 accounts here presented could definitely be altered 

 by new fossil evidence. 



At the end of the Paleozoic Era in the Permian 

 Period volcanoes were prevalent in eastern Oregon 

 and California and in western Idaho and Nevada. 

 Some of this vulcanism took place under what then 

 was part of the Pacific Ocean. Also in the Permian, 

 the Appalachian Mountains started growth and 

 probably soon rivaled the Alps in size; but toward 

 the end of the Permian and start of the Mesozoic, 

 growth ceased and erosion prevailed. 



Further history of the Appalachians, and thus of 

 significant eastern North American mountains, can 

 be dispensed with briefly at this point. However, 

 western mountains have a more complex history that 

 will be treated in reference to the sequence of life. 



The Mesozoic saw the culmination of the Appa- 

 lachians as significant mountains (Figure 19.11). 

 Early Triassic erosion was followed by late Triassic 

 rejuvenation, and then by Jurassic peneplaination. 

 This peneplaination prevailed into the Cenozoic when 

 only a chain of 2,000- to 3000-foot mountain rem- 

 nants remained to imply past grandeur. Later in the 

 Cenozoic, the extensive region was elevated, but only 

 about 100 feet, into a shallow fold. Finally, the Ap- 

 palachians gradually were eroded to their present 

 form. Therefore, only the earliest Cenozoic displayed 

 topography sufficient to affect the distribution of 

 flora and fauna. 



A more complex Mesozoic picture existed in the 

 west (Figure 19.11). The Triassic contained the fore- 

 runner of three important geological features that 

 dominated the entire Mesozoic. These were the 

 Pacific Coast Geosyncline in the extreme west, the 

 Mesocordilleran Geanticline in the middle, and the 

 Rocky Mountain Geosyncline at the eastern margin 

 of this western region. The Mesocordilleran Geanti- 

 cline existed roughly in the area of the present Rocky 

 Mountains and was the major highland of the Meso- 

 zoic. To its west, the Pacific Coast Geosyncline was 

 a major basin of deposition throughout the Mesozoic. 

 However, the Pacific Basin progressively grew to its 



greatest extent in the Upper Triassic, declined in the 

 Jurassic, and reached its last significant growth early 

 in the Upper Cretaceous. The decline was the con- 

 sequence of intermittent growth in the coastal moun- 

 tains. The Rocky Mountain Geosyncline was a less 

 important center of deposition until its start of growth 

 in the Jurassic. This growth culminated in continen- 

 tal bifurcation in the Upper Cretaceous. 



Mesozoic Life. The Mesozoic frequently is called 

 the Age of Gymnosperms and Reptiles. Predominant 

 plants were conifers, including many living genera, 

 but ginkgos, ancient cycads, seed ferns, cycadeoids, 

 and modern types of ferns were conspicuous. Later 

 in the Mesozoic cycads and, for a while, members of 

 the now extinct Caytoniales also were common. The 

 forests probably had most of the groups named, but 

 more arid slopes and plains might have featured ferns 

 and cycadeoids. 



The conspicuous reptiles of these times commonly 

 are called dinosaurs. Earliest forms were of small to 

 moderate size, but by the Jurassic some of the true 

 giants were prevalent. The gamut of types truly ruled 

 the world then. Oceans contained shark-like and 

 many bizarre reptile types. Flying types occupied 

 the air. The land contained a great variety of her- 

 bivorous and carnivorous species, including the larg- 

 est land animals ever known. In addition, reptiles 

 gave rise to the first known mammals of the Triassic 

 and first known birds of the Jurassic. However, the 

 end of the Mesozoic was the end for the old reptile 

 line. 



LATE CRETACEOUS TO RECENT 



Part of the Mesozoic, the Upper Cretaceous, must 

 be stressed for full appreciation of present life in 

 North America. This is necessary because the major 

 groups of modern plants already were present at that 

 time. This emphasizes the fact that subdivisions of 

 geological history are based primarily upon the ani- 

 mal record. Because plants appear to evolve before 

 animals and provide the conditions necessary for 

 adaptive radiation of animals, there is a logical basis 

 for our starting with the last of the Mesozoic. 



In the following account of the Late Cretaceous to 

 Recent, much detail is omitted. The Neotropical- 

 Tertiary Geoflora and Madro-Tertiary Geoflora are 

 not treated below the geoflora leval. The Arcto- 

 Tertiary is mostly discussed at the geoflora level. 

 The Grassland Element is recognized from the time 



