56 



TITANOTHEEES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



also scattered, but more sparingly, through the earlier 

 Torrejon, Wasatch, and Wind River formations, along 

 with river-borne material derived from the decay and 

 erosion of older rocks. 



SECTION 2. EOCENE AND LOWER OLIGOCENE 

 FORMATIONS AND FAUNAL ZONES 



FIRST FAUUAI PHASE (BASAL EOCENE) 



SEVENTEEN LIFE ZONES 



Largely as the result of explorations and researches 

 made for this monograph, the major Eocene and Oligo- 

 cene type life zones that were recognized by Leidy, 

 Cope, and Marsh up to the year 1900, such as the 

 " Coryphodon beds," " JJintaiherium beds, "Diplacodon 

 beds," and " TitanotJierium beds," have gradually been 

 differentiated, through the work of Osborn, Granger, and 

 Matthew, into 16 known life zones, each distinguished 

 by the presence of a highly varied mammal fauna and 

 by the appearance or disappearance of certain groups 

 of mammals and reptiles. There is also one theoretic 

 life zone, between known upper Eocene and known 

 lower Oligocene time, making 17 in all. Each of these 

 life zones corresponds with a series of sediments rang- 

 ing in thickness from 300 to 600 feet. Many of them 

 correspond with changes in climate, temperature, and 

 forestation, and some of them are clearly defined and 



sharply demarcated from others. A single generic 

 name, such as Coryphodon, rarely suffices to distin- 

 guish them, because many genera and even certain 

 species may survive for long periods of time. 



Each of these faunal zones is defined paleontologic- 

 ally by one or more of the life forms it contains, geo- 

 logically by the locality v/here it is best preserved, to 

 some extent botanically by the flora it contains, and 

 lithologically by the character of its rocks as shown 

 by microscopic analyses. Thus, for example, we have 

 the typical upper Wind River zone — the "Lost Cabin " 

 or LamhdotJierium-Eotitanops-CorypJiodon zone, a rather 

 cumbersome designation, which indicates that only 

 during this period did these three types of mammals 

 exist together. In this zone Lambdotherium is the 

 most distinctive genus. Sediments in different geo- 

 graphic basins are correlated in such a manner as to 

 present the whole life story of the Eocene epoch, as 

 shown in the accompanying diagram. Of the two 

 chief faunistic gaps that have been recognized, that 

 between the Wind River and the Bridger has now been 

 filled by explorations of the Huerfano, so that there 

 remains only that between the Uinta and the White 

 River. These 16 known life zones will doubtless be 

 multiplied to 20 or more by future discovery. They 

 are shown in the following table: 



