ENVIRONMENTAL CONDITIONS 413 



tions resulted in the carbonaceous deposits of the Lower Mississippian. 

 Following this came a change toward marked aridity, so that the flood- 

 plain and playa deposits of the Upper Mississippian are marked by mud- 

 cracked red shales, cross-bedded red sandstones, deposits of gypsum, and 

 even of salt. Then another backward and farther swing of the pendulum 

 brought in the conditions which permitted the accumulation of conglom- 

 erates, gray shales, and abundant coals of the Lower Pennsylvanian — the 

 true Carboniferous. This was followed by climatic oscillations which led 

 to the greatest of known glacial epochs and, in the Permian, the most 

 intensely arid conditions known in earth history. 



The nature of the geologic record of amphibians indicates that they 

 evolved under climates marked by seasonal dryness and inhabited river 

 plains far from the sea. The abruptness of appearance of well developed 

 sustaining legs and feet points to an origin perhaps as far back as the 

 Lower Devonian, but a rapid expansion and evolution in the Tipper De- 

 vonian. They survived the change to more generally wet conditions in 

 the Lower Mississippian, but showed more convincingly their adaptation 

 to semi-arid continental conditions through the footprint record they 

 have left in the Mauch Chunk shales. The impressions of plants indicate 

 that over the broad river plains of eastern Pennsylvania there flourished 

 each season an herbaceous vegetation of cryptogams following the with- 

 drawal of the river floods, until the advancing seasonal dryness caused it 

 to wither. No traces of an arboreal vegetation have been found, and this, 

 taken in conjunction with other facts, suggests that in the dry season the 

 streams completely vanished, or at least were reduced to rivulets and 

 water-holes unable to afford sufficient underground water to support an 

 arboreal vegetation on the banks. 



The early amphibians therefore were not only adaptations from fluvia- 

 tile fishes, but at first seem to have been restricted to uplands or to river 

 plains, where the seasonal dryness was emphasized — conditions which pre- 

 vented, however, the preservation of their bones. Doubtless less rapid 

 swimmers than their ancestors and probably small in size at the time of 

 their origin, the natural explanation of their absence from the more per- 

 manent floodplain waters would appear to be in the better adaptation of 

 the more powerful fishes for these localities, the better adaptation of the 

 amphibians for theirs, and a closeness of adaptation to environment 

 which at first prevented the intermixing of the two faunas. It appears 

 to have been but a short geological time, however, until the expansive 

 evolution of the higher group, aided by widespread oscillations in cli- 

 matic conditions, adapted them to a life in an environment suitable for 



