DEVELOPMENT OF VERTEBRATE LIFE IN PERMO-CARBONIFEROUS 273 



beds it reaches a maximum. As is well known, there is no great structural 

 break at the top of the Paleozoic series in these regions; in only one place, 

 near Ouray, Colorado, is there known even a considerable unconformity 

 at the base of the Triassic. 



It must be remembered that the deposits of the red beds of late Paleozoic 

 and early Mesozoic are not in any degree continuous. They consist of 

 lenses and small irregular bodies of shale, sandstone, and clay, with small 

 layers of impure limestone, following each other in no persistent sequence. 

 This is so pronounced that a section made at one place is not to be depended 

 upon even within the distance of half a mile. A consideration of the many 

 sections taken in the regions concerned will reveal this condition. 



Despite the similarity of the beds revealing similar conditions of deppsi- 

 tion, the beds carrying Permo-Carboniferous vertebrates are uniformly 

 followed by a barren interval and then by the appearance of typical Triassic 

 forms. Other than the obvious climatic change, there is no suggestion of a 

 cause for the extinction of the fauna, but the climatic change is itself a 

 sufficient explanation. The members of the fauna were closely adapted to 

 every phase of their environment, and, in taking advantage of abundant 

 possibilities, had developed to a high degree of specialization. They had 

 passed the zenith of their development and the group as a whole was in a 

 stage of developmental senility where overdevelopment of certain morpho- 

 logical characters is the common condition. Flexibility in evolution was so 

 far lost that with the advent of a new environment the fauna disappeared. 

 This is quite what would be expected. The only question is whether the 

 fauna was totally extinguished upon the continent and replaced by a new 

 fauna developed elsewhere, or whether some of the less-specialized forms 

 survived to give rise to the Mesozoic types. This question can not be 

 answered at present, but so far as we know there were no survivals in North 

 America. What was going on in the portions of the continent from which 

 we have no record we do not know; it is possible that some of the forms 

 migrated to the continents of the Old World and there perpetuated the 

 fauna in the Triassic. One thing is very definite at present : no form has 

 been found which bridges the gap between the North American fauna and 

 that of the Triassic. The two occur in the same localities, but are separated 

 by a barren space in the geological column, and when the new forms appear 

 they are already well-defined Mesozoic types. So far as we can now tell, 

 the Permo-Carboniferous land vertebrates became extinct upon the North 

 American continent and the Mesozoic forms appeared by migration from 

 an unknown source. Perhaps connecting-links may be found in North 

 America, but at present there is no suggestion that they exist or of where 

 to search for them. 



