80 



TITANOTHEBES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



Some of these "white layers" have been traced over 

 many square miles. They are composed of tuffaceous 

 shale and marl or of calcareous shale and are in places 

 filled with fresh-water shells. They mark periods 

 during which the deposition of volcanic dust was less 

 rapid, when the Bridger Basin was temporarily base- 

 leveled and the waters rose into wide, shallow playa 

 lakes, in which sedimentation was slow. That these 

 four relatively thin "white layers," which are vari- 



FlGURE 



-Geologic section of the entire Bridger formation in the 

 Bridger Basin, Wj'o. 



Shows the division by the four chief "white layers" and the main divisions by three principal 

 zones — A, Palaeosyops fontinalis zone; B, Palaeosyops paludosus-Orohippus zone; C and 

 Utniatherium zone. 



ously known geographically as the "Cottonwood 

 white layer," the "Burnt Fork white layer," the 

 "Lone Tree white layer," and the "upper white layer," 

 correspond with long periods of geologic time is shown 

 by the marked faunal differences that separate them, 

 which indicate that extensive migration occurred 

 before and after the deposition of each of these layers, 

 but especially the first, which separates the lower 

 from the upper Bridger life zone. 



Life environment in Bridger time. — From observa- 

 tions made by Hay (1905.1, pp. 327-329) while he was 



collecting fossil turtles in the Bridger in 1903, he con- 

 cluded that the Bridger deposits were almost solely 

 the result of fluviatile and flood-plain action, that this 

 basin was a nearly level country, which was probably 

 covered with vegetation and well forested. The dis- 

 tribution of fossil remains in all parts of the Bridger 

 area indicates that the animals lived near the places 

 where they became buried and that they were chiefly 

 such as may inhabit well-wooded regions. The river- 

 channel beds, which are composed of coarse ma- 

 terials, show that streams with rapid currents 

 traversed the basin. These streams were bor- 

 dered by swamps in which were formed beds of 

 impure lignite, or by fresh-water bays in which 

 the shells of fresh-water mussels accumulated. 

 The finer deposits indicate shallow, muddy 

 bays, in which the remains of the larger quad- 

 rupeds are occasionally found in positions 

 indicating that they had been mired in a 

 standing posture. The old stream channels 

 have yielded remains of several species of 

 bowfins (Amiidae), garpikes (Lepidosteus) , 

 and siluroids. Crocodiles were numerous and 

 diversified. The reptiles suggest that the 

 climate was Floridian, or south temperate, and 

 we may picture a partly open, partly forested 

 country, somewhat similar to the existing 

 bayou region of the Mississippi Delta of 

 Louisiana. Analysis of the Testudinata by 

 Hay (1908.1) has also afforded a clear idea of 

 the physiographic conditions in Bridger time. 

 The soft-shelled river turtles (Trionychoidea) 

 were represented by at least 25 species, and 

 there are now in the world only 26; the 

 Bridger rivers and brooks fairly swarmed 

 with these creatures, some of them equal in size 

 to the largest existing Asiatic species. There 

 are indications of 4 species of the family 

 Emydidae (order Cryptodira), as compared 

 with the 12 species now living in the Missis- 

 sippi Valley. The genus Baptemys, of the 

 same order, has its nearest relatives at present 

 in Central America, and a third genus 

 (Anosteira) is reported by Lydekker in the 

 upper Eocene of England. The presence of 

 : and D, extcusive stretches of land is indicated by the 

 true land tortoises (Testudinidae) of the genus 

 Hadrianus, including giant tortoises nearly 3 feet 

 long, which probably lived on dry lands bordering 

 the sluggish Bridger streams. The ancient Lower 

 Cretaceous order Amphichelydia is also represented 

 here by four species belonging to two genera. 



The environmental adaptations of the animals of 

 the Bridger Basin were classified by Matthew (1901.1, 

 pp. 309, 310) as follow?: 

 Land animals: 



1. Aerial: Remains of birds rare and fragmentary, as in 

 nearly all geologic formations. 



