464 



VERTEBRATE LIFE AND ORGANIZATION 



Figure 23.15. Pteratwdon, one of the flying reptiles, or pterosaurs, that lived dur- 

 ing tlie late Mesozoic era. (Courtesy of the American Museum of Natural History.) 



reaching a lethal point. As explained earlier, prolonged high tempera- 

 ture destroys most enzyme systeins. 



A bii)e(lal gait naturally Ireed the front legs from use in terrestrial 

 locomotion. The front legs became reduced in many dinosaurs, but in 

 one group of archosaurs they were converted to wings. The wings of the 

 flying reptiles (order Pterosauria) consisted of a membrane of skin sup- 

 ported by a greatly elongated fourth finger (Fig. 23.15). The fifth finger 

 was lost, and the others probably were used for clinging to cliffs. The 

 hind legs were very feeble, and the animal must have been helpless on 

 the ground. Certain pterosaurs became very large, one having a wing 

 spread of 25 feet. 



Most of the archosaurs became extinct toward the end of the Meso- 

 zoic, but the reason for this is not entirely clear. Perhaps the pterosaurs 

 succumbed in competition with birds, which also evolved from primitive. 



