CHAPTER XI. 



DEVELOPMENT AND FATE OF VERTEBRATE LIFE IN THE 



PERMO-CARBONIFEROUS IN RELATION TO ITS 



ENVIRONMENT. 



The study of the development of vertebrate life in North America during 

 late Paleozoic time emphasizes the changes from a long period of slow 

 evolution in a singularly monotonous environment through a period of 

 rapid expansion in a diversified environment to final extinction. As has 

 been repeatedly intimated in the course of this work, the chief directing 

 influence in the sudden expansion was a decided climatic change, accom- 

 panied by physiographic changes, induced by an alteration in the level of 

 the surface of the continent. 



The basis for any study of the vertebrate fauna in relation to the en- 

 vironment must be a study of the morphology of the forms involved. 

 Previous publications of the Carnegie Institution of Washington by the 

 author and others have summarized this as far as our present information 

 permits. The material in the preceding pages summarizes our knowledge of 

 the environment in a similar way. 



In attempting an analysis of the response of the vertebrates of the late 

 Paleozoic to the changing environment it is necessary to begin the study 

 with at least the middle of Pennsylvanian time, in order to understand the 

 conditions which fixed upon the animals the homoplastic characters which 

 were developed in the Permo-Carboniferous radiation. There can be no 

 question that the radiation of the fauna began with the development and 

 spread of "Permo-Carboniferous conditions" in the middle Conemaugh 

 time, not at the beginning of Dunkard time as has been commonly assumed. 

 It is therefore of the utmost importance to keep clearly in mind the great 

 change in the inorganic environment which came with the development of 

 Permo-Carboniferous conditions. 



Fortunately for the simplification of the work, the conditions during 

 early Pennsylvanian time were singularly uniform over large areas. 1 David 

 White has repeatedly emphasized the equability of the humid climate. 

 The work of David White, Stevenson, and others has emphasized the topo- 

 graphic uniformity of the slowly sinking coal basins, in which the filling 

 maintained a nearly constant level. Under such conditions the ultimate 

 food-supply, vegetation, would be fixed in kind though abundant in quan- 



1 See analysis of an environment on p. 40. 

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