224 VERTEBRATE ZOOLOGY 



fare among the dinosaurs themselves; another, the destructive 

 slaughter, not of adults but of young, possibly while yet in the 

 egg, by small blood-thirsty mammals; yet another, change of climate, 

 either by the diminution of the necessary heat without which no 

 reptilian race may thrive, or of the moisture with an accompanjdng 

 change of vegetation. These are all conjectural causes of extinction; 

 but this we know, that with the extensive changes in the elevation of 

 land areas which marked the close of the Mesozoic, came the draining 

 of the great inland Cretaceous seas along the low-lying shores of 

 which the dinosaurs had their home, and with the consequent re- 

 striction of old haunts came the blotting out of a heroic race. Their 

 career was not a brief one, for the duration of their recorded evolution 

 was thrice that of the entire mammalian age. They do not represent 

 a futile attempt on the part of nature to people the world with crea- 

 tures of insignificant moment, but are comparable in majestic rise, 

 slow culmination, and dramatic fall to the greatest nations of antiq- 

 uity." 



Pterosauria. — ^The pterosaurs represent a series of experiments 

 in aviation on the part of the reptiles. They varied greatly in size 

 from tiny forms comparable with our sparrows to flying dragons with 

 a wing-spread of twelve feet. That they were good flyers, able to 

 venture far out over the sea, is indicated by the fact that their remains 

 are found mingled with those of the marine mososaurs miles away 

 from the Mesozoic shore lines. They were scarcely flyers in the 

 strict sense, but must have been effective gliders or soarers; for they 

 did not possess the powerful musculature necessary for active flight. 

 In Pterodon (Fig. 124, D), one of the largest of the flying reptiles, the 

 head is prolonged into a great keel-like structure, used as a balancing 

 mechanism. Other types had a long tail with a terminal rudder-like 

 expansion much like that of an aeroplane. 



The pterosaurs or pterodactyls might appear superficially to be well 

 suited to be the ancestors of the birds, but anatomically they are 

 quite unsuited for this role. They represent simply a highly specialized 

 adaptive radiation, that was short-lived and met with utter extinc- 

 tion during the Upper Cretaceous. They furnish a final chapter in the 

 remarkable adaptive radiation of the Mesozoic reptiles. 



