INTEODUCTION TO MAMMALIAN PALEONTOLOGY 



9 



formed a considerable part of the work done for this 

 monograph. At first the periods of sedimentation 

 were regarded as broadly equivalent to a similar 

 number of life zones. For example, up to the year 

 1900 the two chief formations, the Wasatch and the 

 Bridger, were treated as containing one fauna each. 

 It was not known that the Wasatch represents five 

 distinct life zones, that the Bridger represents four 

 and perhaps five life zone^, and that the partly con- 

 temporaneous Washakie represents three distinct life 

 zones. The correlation of different areas of sedimen- 

 tation by means of fossils was similarly loose and in- 

 exact. The evidence discovered since 1900 by parties 

 sent out from the American Museum of Natural 

 History proves that there was considerable change of 



environment as well as a great faunal change during 

 Bridger time. The careful recording of the precise 

 geologic level at which every specimen, especially 

 every type specimen, was collected, together with 

 close analysis of lithologic evidence that the rocks 

 afford as to modes of deposition, has worked a com- 

 plete revolution in our knowledge of the history of 

 these mountain basins in Eocene time and of the 

 flood plains in early Oligocene time and has afforded 

 the relatively precise and far more interesting sequence 

 of events that is described in Chapter II. 



Our geologic studies show that from basal Eocene to 

 early Oligocene time there were six great physiographic 

 and climatic epochs of sedimentation, shown in the 

 accompanying table. 



Epochs of sedimentation and life zones from iasal Eocene to early Oligocene time in hasins in the Rocky Mountain 



region 



Physiographic epochs 



6. Lower Oligocene, represented by Chadron formation. Flood plains east of 

 the Rocky Mountains. Sedimentation extremely slow. Moderate rain- 

 fall. Warm temperate climate. 



5. Latest upper Eocene, represented by Uinta formation (Uinta C). Flood- 

 plain basins south of the Uinta Mountains. Sedimentation relatively 

 rapid; fine material. Heavy rainfall, diminishing. 



4. Upper Eocene, represented by contemporaneous deposits in Washakie and 

 Uinta Basins (horizons Washakie B and Uinta B) and probably by upper- 

 most part of Bridger formation, or Bridger E. Violent river and stream 

 action from the north and south sides of the Uinta Mountains. Erup- 

 tions of volcanic dust; coarse material. Heavy rainfall. 



3. Middle Eocene, represented by Bridger formation (horizons Bridger A, B, 

 C, andD). More quiescent flood-plain conditions in the Bridger Basin; 

 eruptions of volcanic dust; intervals of evaporation. Sediments com- 

 posed in part of eroded material, generally laid down on lacustrine 

 deposits. 



2. Lower Eocene, represented by Wasatch, Wind River, and Green River 

 formations. Warm temperate climate of the Green River lake period, 

 and evidently arid conditions in the contemporaneous Wind River sedi- 

 ments. Alternation of arid and fluviatile conditions characteristic of 

 Wind River and Wasatch time. Evidence of open country, favorable to 

 cursorial mammals. 



1. Basal Eocene, represented by the Puerco, Torrejon, and Fort Union forma- 

 tions. Forests, base-leveled areas, flood plains, and swamps widespread. 

 Evidence of somewhat cooler climate. 



17. Titanotherium-Mesohippus. 

 16. Theoretic zone. No fauna discovered. 

 1 5 . Diplacodon-Protitanotherium-Epihippus. 

 14. Eobasileus-Doliohorhinus. 



13. Metarhinus. 



12. Uintatherium-Manteoceras-Mesatirhinus. 



11. Palaeosyops paludosus-Orohippus. 

 10. Eornetarhinus-Trogosus-Palaeosyops fonti- 

 nalis. 



9. Lambdotherium-Eotitanops-Coryphodon. 



8. Heptodon-Coryphodon-Eohippus. 



7. Systemodon-Coryphodon-Eohippus. 



6. Eohippus-Coryphodon. 



5. Phenacodus-Nothodectes-Coryphodon. 



4. Pantolambda. 



3. Deltatherium. 



2. Polymastodon. 



1. Ectoconus. 



The evidence of the existence of these successive 

 climatic, physiographic, and biologic conditions is 

 derived from studies by Berry of the flora; by Hay of 

 the reptiles; by Osborn, Scott, Wortman, Granger, 

 Matthew, Peterson, Douglass, and Riggs of the mam- 

 mals; and by Sinclair and Johannsen of the lithology. 

 These studies, the results of which are in part set forth 

 in Chapter II, show a great advance upon the pioneer 

 studies by Leidy, Marsh, and Cope, which were based 

 chiefly on characters of the skeleton and teeth. 

 101959^29— VOL 1 3 



Our paleontologic division of the strata of the Eocene 

 and lower Oligocene epochs into sixteen known life zones 

 and one theoretic life zone enables us to fix the date of 

 the immigrations of animals into this region, as well as 

 the emigrations and extinctions, with much greater 

 precision than formerly. Remains of titanotheres 

 have been found in the upper eight of the known life 

 zones. 



Extremely important is the realization that the zonal 

 fossil fauna reflects local conditions of sedimentation. 



