ENVIHONMENT OF THE TITANOTHEEES 



81 



2. Arboreal: Primates, many Carnivora, and some Insec- 



tivora and Rodentia. Out of 1,007 specimens, belong- 

 ing to 46 genera, 13 genera (184 specimens) were 

 certainly arboreal and 11 genera (485 specimens) 

 were probably arboreal. 



3. Terrestrial (cursorial and ambulatory) : Some of the 



carnivores and all the ungulates (17 genera, 314 

 specimens). Also some lizards and chelonians. 



4. Fossorial: Certainly fossorial, 3 genera (S specimens). 



Some of the insectivores may also have been fossorial. 



5. Amphibious: One insectivore (Pantolestes) (1 genus). 



Probably certain carnivores. 

 Water animals: 



6. Fresh- water: Numerous crocodiles, aquatic turtles, 



fish, and fresh-water mollusks. 



7. Marine: No marine animals. Contrast this lack of 



types with the types of fish in the preceding Green 

 River formation. 



The Bridger life thus included many arboreal, 

 terrestrial, and aquatic forms, the last mostly reptiles, 

 fishes, and invertebrates. The slow-moving, ambula- 

 tory quadrupeds form a relatively large proportion of 

 the mammals, but the cursorial types, such as the 

 Equidae (Orohippus), are relatively rare; also the fos- 

 sorial types. The Bridger life seems to be that of a 

 partly forested flood plain. The remains of large 

 mammals are so numerous as to indicate abundant 

 open, gladed areas, comparable to the partly forested 

 and partly open delta regions along certain rivers of 

 modern time. 



The foot structure of the Bridger quadrupeds gives 

 less certain evidence of an open plains country, 

 favorable to cursorial types, than that of the Wasatch 

 (lower Eocene) quadrupeds of the same region. 



No impressions of leaves from the Bridger forests 

 have been discovered. It is probable that the forests 

 of Green River type, described on pages 72-73, per- 

 sisted into Bridger time and that the climate then 

 was warm-temperate, almost subtropical. 



The faunal history of the Bridger as a whole shows 

 a gradual reduction in the number of archaic mammals 

 of Mesozoic stock and a steady increase in the number 

 of their competitors among the modernized mammals, 

 the numerical relations between these two groups in 

 upper Bridger time being as follows: 



Genera Species 



Archaic mammals 15 35 



Modernized mammals 57 146 



Duration oj the Bridger epoch. — Matthew (1909.1), 

 following the earlier geologists, believes that the 

 lacustrine conditions in Green River time arose from 

 the uplift of the Uinta Mountain range, which blocked 

 the basin and caused the formation of the great lake 

 in which the material that formed Green River shale 

 was laid down. As the river gradually cut its way 

 through the east end of the Uinta Range the lake 

 gave way to the broad Bridger flood plain, in which 

 was deposited the volcanic ash washed down from 

 the slopes of the Uinta Mountains to the south, and 



the deposit was worked over and sorted by the streams 

 that flowed across the plain. The Bridger Basin was 

 subject to intermittent overflow, which gave rise to 

 large but shallow lakes of clear water. If we should 

 assume that the Bridger formation occupied one- 

 tenth of total estimated Eocene time — 90,000 to 

 100,000 years — the fossiliferous beds, which are 1,100 

 feet thick, have accumulated at an average rate of 

 12 inches per century. This estimate would allow 

 110,000 years for the deposition of the Bridger forma- 

 tion exclusive of the "white layers " formed at intervals 

 when deposition was arrested. The titanothere re- 

 mains of the Bridger indicate a long period of evolu- 

 tion, but not so long as that of the Chadron (lower 

 Oligocene). 



Chief localities and exposures of the Bridger formation in ike 

 Bridger Basin 



Bridger E: 



Uppermost exposures: 



Sage Creek Mountain. 

 Henrys Fork Table. 

 Twin Buttes. 



Bridger D: 



Upper exposures: Level 



Twin Buttes D 1-5 



Spanish John's Meadow D 1-5 



Cat Tail Spring D 1-5 



Henrys Fork, Burnt Fork post office D 1-5 



Henrys Fork, Lone Tree post office D 1-5 



Summers Dry Creek D 1-5 



Henrys Fork Hill D 1-5 



Sage Creek Spring D 1-5 



Lane Meadow D 1-5 



Bridger C: 



Lower exposures: 



Henrys Fork, Lone Tree post office C 4-5 



Lane Meadow C 3-5 



Spanish John's Meadow C 3-5 



Henrys Fork Hill C 3-5 



Twin Buttes C 1-5 



Dry Creek C 1-5 



Henrys Fork, Burnt Fork post office C 1-5 



Church Buttes, third bench C 1-3 



Bridger B: 



Upper exposures : 



Cottonwood Creek Typical B 4^5 



Millers ville, 6 miles southeast of B 4-5 



Cottonwood Creek, middle of B3 



Grizzly Buttes B 3 



Church Buttes B 2-3 



Lower exposures: 



Cottonwood Creek B2 



Grizzly Buttes Typical B 2 



Exposure B, 5 miles south of Granger B 2 



Millersville B 1-2 



Cottonwood Corral, Blacks Fork ,. B 1-3 



Exposure A, 5 miles south of Granger B 1 



Church Buttes B 1 



Bridger A: 



Hams Fork Bluff; Granger to Opal, 25 miles. 



Mouth of Big Sandy Creek {IPalaeosyops -fontinalis 



zone). 

 Big Muddy exposures between Carter and Granger. 

 Blacks Fork Bluffs, east of Granger. 



