TITANOTHEHES OP ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



amblypod Coryphodon, the horse EoMppus, the tapir 

 Systemodon unite to define the Systemodon-Coryphodon- 

 EoMppus life zone of the lower Eocene. 



It is through these zonal resemblances in the 

 mammalian life, and more rarely in the plant life, 

 that relatively sure estimates are made of the time 

 during which the sediments containing certain fossils 

 were deposited, irrespective of such geologic data as 

 whether the sediments are thick or thin, whether they 

 are products of erosion or of volcanic eruption, whether 

 they were deposited in still water or in rapidly moving 

 water, or whether they are composed of clay, sand, 

 gravel, conglomerate, or tuff. The life zone, when 

 adequately defined, is an absolutely reliable means of 

 time correlation as distinguished from other means — 

 physiographic, geologic, or lithologic. 



Similar sediments. — It is true that in the Rocky 

 Mountain region there prevailed at times over wide 



mentation in one region (for example, the Cypress 

 Hills, Saskatchewan) and with excessively slow sedi- 

 mentation on river flood plains in another region (Chey- 

 enne and White Rivers, S. Dak.), or with a fall of 

 volcanic ash in still another region (Beaver Divide, 

 Wyo.). 



Evolution of mammals a stable process. — ^By com- 

 paring all the events in the history of the American 

 continent for which the records afforded by geology 

 and paleontology harmonize with others afforded by 

 paleontology alone we reach the conclusion that one of 

 the most uniform, the most stable geographically, and 

 the most widespread is the evolution of mammalian life. 

 This evolution proceeds more or less uniformly in 

 Europe, in Asia, and in North and South America. The 

 apparently sensitive protoplasm (body substance) and 

 germ plasm (hereditary substance) are far more stable 

 and far more uniform in their progressive evolution 



Former land orea^ Former migration areas Known fossil areas 



Figure 5. — Map showing areas throughout the world in which remains of titanotheres have been 

 found (solid black) and areas in which titanotheres were probably in migration during Eocene 

 and Oligocene time (oblique lines) 



Titanotheres have been found in the northwestern United States, the Gobi Desert (Mongolia), Burma, and southeastern 



Europe. 



areas similar physiographic, climatic, and eruptive 

 volcanic conditions, as, for example, during what we 

 designate Fort Union time, Wasatch time, upper 

 Bridger time. During such periods of uniform con- 

 ditions the geologic evidence is concordant or harmoni- 

 ous with the paleontologic evidence afforded by life 

 zones, and doubtless any paleobotanic evidence that 

 may be found must also be concordant. In basal 

 Eocene (Fort Union) time, for instance, the forests, 

 the mammals, the reptiles, the climate, the physiogra- 

 phy of the chief areas of sedimentation of the whole 

 Rocky Mountain region were all more or less similar, 

 and in this particular epoch these several means of time 

 correlation afford more or less harmonious evidence. 



Unlilce sediments. — Such similar sediments, however, 

 become increasingly rare in the continental deposits 

 of Eocene and Oligocene time. A single life zone, 

 such as the Titanotherium zone, may be contempo- 

 raneous with violent fluviatile action and heavy sedi- 



than the surface of the earth. For this reason they 

 form superior data for time correlation. This is one 

 of the chief generalizations that have grown out of the 

 long series of observations and studies of the correla- 

 tion of Tertiary geologic events in America and Europe 

 that were specially made in the preparation of this 

 monograph. 



Life zones of the titanothere epoch. — By the method 

 of determining geologic time by discriminating life 

 zones the whole epoch of the evolution of the titano- 

 theres has been subdivided into titanothere zones, 

 distinguished not only by successive genera and species 

 of titanotheres but by corresponding changes in all the 

 environmental forms of life. Each of these life zones 

 probably represents a very long period of time, for in 

 each there was a very considerable evolution of the 

 titanotheres as well as of other forms. These zones 

 (17-9; see table, p. 9), named in descending order, 

 are as follows: 



