166 INTRODUCTION TO EVOLUTION 



ing a rather inefficient locomotor apparatus wherein an undue proportion 

 of muscular energy was expended in raising the body off the ground. This 

 inefficiency was probably mitigated by the fact that labyrinthodonts 

 spent a great portion of their lives in the extensive swamps of the period, 

 where water supported much of the weight of the body. Many of these am- 

 phibians were small, only a few inches long. At the other extreme were 

 animals about 1 feet long, and one, known only from its footprints, which 

 probably weighed at least five or six hundred pounds. 



The first reptiles appeared in the Pennsylvanian period. Since fossils of 

 reptiles are exceedingly rare in deposits of the period we shall postpone 

 further discussion of them until our consideration of the next period, the 

 first period having reptiles as prominent members of the fauna. 



Permian Period 



The marine animals of the Permian period were similar to those of the 

 two preceding periods, though changes were occurring gradually. The cri- 

 noids, for example, which had reached a peak of abundance in the Mis- 

 sissippian, had become relatively rare by Permian times. The close of the 

 period saw the last of the trilobites. When the curtain had arisen on the 

 Paleozoic era some 300 million years previously the trilobites had occupied 

 the center of the stage, dominating the scene. It is perhaps fitting that their 

 extinction marked the closing act of the Paleozoic drama. 



Brachiopods, particularly spiny-shelled ones mentioned as abundant in 

 Mississippian seas, continued as prominent members of the marine fauna 

 in the Permian but declined markedly by the end of that period, many 

 forms becoming extinct. 



Contrariwise, the cephalopods with wavy and contorted suture lines, the 

 ammonites, underwent progressive evolution during the Permian. Many 

 new forms appeared, foreshadowing the great expansion of this group dur- 

 ing the following era. 



Turning to life on land, we find that the plants characteristic of the 

 Pennsylvanian period lived on into the Permian. As the period advanced, 

 however, the extensive swamps basking in mild climate disappeared 

 from large portions of the earth. More arid conditions arose, accompanied 

 by cold in winter. In widespread regions of the Southern Hemisphere ex- 

 tensive glaciers were formed. Thus the luxuriant vegetation of the coal de- 

 posits was replaced by hardier plants over much of the earth. True co- 

 nifers became the leading type of forest tree. 



Permian insects were quite unlike those of the preceding period. They 



