AND ITS INHABITANTS 145 



Upper Devonian, completes the evolutionary movement. The 

 Mississippian is a time of expansive evolution of the stego- 

 cephalians, while the establishment of the reptiles was the 

 result of aridity toward its close, hence synchronous with the 

 Ouachita uplift. The culmination of the wide disturbances at 

 the close of the Pennsylvanian brought on aridity and glacia- 

 tion in the early Permian, with the beginnings of the warm- 

 blooded stocks, birds and mammals, although the first recorded 

 bird at 7 is Upper Jurassic and the first known mammals at 6 

 Upper Triassic. Aridity has been regarded as the chief incen- 

 tive to dinosaurian evolution, the actual record at 5 beginning 

 in the Lower Trias. The differentiation of the Sauropoda, a 

 response to the expansion of the amphibious aquatic realm, may 

 have been Triassic, but in greater probability the subsidence 

 which followed the Palisade diastrophic movement late in 

 the Triassic may be nearer to the actual time. They first 

 appear in the Lower Jurassic, but do not culminate until early 

 Comanchian, shortly after which they suffer extinction from 

 unknown causes, geologically speaking. The other dinosaurs 

 gradually increase in numbers, size, and specialization^ until 

 the close of the Cretaceous, when, with the movements which 

 culminate in the Laramide revolution, they plunge into extinc- 

 tion. 



The curve of mammalian evolution is perhaps the most 

 striking of all, consisting as it does of minor fluctuations which 

 may represent accident of preservation and recovery rather 

 than periods of evolutionary expansion, as is equally true of the 

 dinosaurian curve. Then with the Laramide revolution and 

 the coincident dinosaurian extinction comes the rapid mamma- 

 lian deployment, the archaic mammals, first known in the basal 

 Eocene at 9, becoming fewer and less important, although 

 increasing in specialization until their final extinction in the 

 Middle Oligocene. With the fluctuating climate of the north- 

 ern area come the modernized forms, first known in the Lower 



