INTRODUCTION TO MAMMALIAN PALEONTOLOGY 



there in several of the ancient river drainage basins 

 of Wyoming. While the remains of the animals and 

 plants of the period were accumulating in these sedi- 

 ments the titanotheres and other herbivorous quadru- 

 peds and the carnivores that preyed upon them, as 

 well as the other mammals and invertebrates of the 

 land, of the water, and of the air, were constantly 

 evolving, appearing and disappearing through mi- 

 gration and extinction. Thus where the sediments 



of Front's "gigantic PalaeotJierium" (Menodus gigan- 

 teus) in 1846 to the present time, it has been found 

 that the lower division of this zone is distinguished by 

 the presence of 85 species of vertebrates. The names 

 of the dominant form or forms of each zone are used 

 to designate the several life zones. For the designa- 

 tion of the Titanotherium zone the name of this single 

 genus Titanotherium (Menodus) is used, for it is the 

 most distinctive form in that zone. 



swEETGRAss co.jNTRODUCTION OF ANCESTORS 



(MONT.) 



2 ^I'aunal Period 



g::=a ARCHAIC MAMMALS 

 ONLY 



1-1/ ^^Faunal Period 



CorypTiodon - Ayriblypods 



and. 

 Eohippjis -^vrstSorses 



W$m wm MAMMALS 



Pan.toZcmibda -^-mhlypods 

 Polynhastodan. 



Figure 4. — Successive and overlapping Oligocene and early Eocene formations of the Rocky Mountains 

 The duration of the titanothere epoch is indicated by the arrow. 



are very rich in fossils of all kinds — mammals, reptiles, 

 iishes, and rarely birds — we are able to restore the 

 life that was distinctive of certain more or less con- 

 tinuous phases of geologic sedimentation. These 

 time divisions are designated life zones, as distin- 

 guished from the sedimentary divisions of groups and 

 formations. 



After an exploration of the Titanotherium zone that 

 covered a period of over 70 years, from the discovery 



Many genera persist through several successive life 

 zones. Two genera, the large-hoofed Coryphodon and 

 the small primitive horse EoMppus, persist through 

 four lower Eocene geologic phases or life zones, during 

 which a succession of other species, as well as migra- 

 tions, extinctions, etc., may be clearly observed. It 

 may therefore be necessary to select more than one 

 genus, perhaps as many as three genera, in order to 

 define clearly a certain life zone. For example, the 



