272 ENVIRONMENT OF VERTEBRATE LIFE, ETC. 



It is certain from the interpretation of structure that a great preponder- 

 ance of the fauna was palustrine in habit. This is certainly true of the 

 Amphibia and equally true of most of the Reptilia. Certain of the reptiles 

 were adapted to other conditions, as the very swift, perhaps arboreal, 

 Areoscelis, and Edaphosaurus, whose structure and the method of occurrence 

 leads to the belief that it was an upland animal. The fauna as a whole 

 migrated with the extension of its peculiar habitat, following the retreat of 

 the swamps and marine littoral probably to the westward, and came to an 

 end with the habitat. The upland members of the fauna, many of which 

 have undoubtedly left no traces of their existence, lingered upon the higher 

 lands and may have moved about freely, even penetrating back to the 

 eastern side of the continent. 



It is at least suggestive that Edaphosaurus, an upland form, is found 

 farthest from the locus of the greatest abundance of the genus, but an equally 

 significant fact, if we could understand it, is the presence in the same locality 

 (Bohemia) of Cricotus, or a closely related form, the most aquatic of the 

 Amphibia. 



The only consideration that offers a possibility of reconciling these 

 apparently contradictory bits of evidence is that Edaphosaurus was capable 

 of wide migration because of its land habits, and Cricotus was equally capable 

 because of its power as a swimmer. If Cricotus had the intolerance of salt 

 water so characteristic of the Amphibia, and there is no evidence to the 

 contrary, there must have been a peculiarly favorable geographical arrange- 

 ment that permitted these two forms of such different structure and habits 

 to reach the same locality after such a long journey. If Cricotus was 

 tolerant of salt water, it is strange that no remains have been found associated 

 with the widely distributed Mesosaurus. 



The great majority of the fauna seem, so far as we can tell, to have been 

 greatly favored by their environment, but to have been closely restricted 

 by it and quite unable to adjust themselves to changing conditions. 



The close of Permo-Carboniferous conditions and the beginning of 

 Triassic conditions was accomplished within the limits of the "red beds," 

 and certainly within the limits of any known occurrence of air-breathing 

 vertebrates, by a steady increase in the aridity of the climate, with a con- 

 sequent decrease in the area of aqueous habitats and an increase in the 

 salinity of such as remained, which were largely desiccating pools. 



To anyone who has studied the red beds of late Paleozoic and early 

 Mesozoic time the similarity in the structure of the two is beyond question, 

 but the evidence of gradually increasing aridity is equally obvious. The 

 Permo-Carboniferous beds do not contain any large amount of salt or 

 gypsum or other evidence of extreme aridity in the lower middle portions — 

 the equivalents of the Wichita and Clear Fork formations — where the fauna 

 occurs, but such evidence increases in the upper portion and in the Triassic 



