426 REPTILES xv. 17- 



sharp spikes. The hind legs were much longer than the front, a relic of 

 bipedal ancestry. The feet carried hoof-like structures. The skull was 

 very small and the brain much smaller than the lumbar swelling of 

 the cord. The teeth were in a single row and small. The ankylosaurs 

 of the Cretaceous were covered all over with bony plates, somewhat 

 in the manner of the mammalian glyptodonts (*Nodosaurus, Fig. 247). 

 Finally, the ceratopsians, such as * Triceratops of the late Cretaceous, 

 developed enormous heads, with huge horns and a large bony frill, 

 formed by extension of the parietals and squamosals to cover the neck. 

 These later Cretaceous animals appear to have lived on dry land and to 

 have walked on all fours, although the bipedal ancestry is shown in 

 the shortness of the front legs. There are several indications that the 

 climate at the close of the Cretaceous was becoming drier and the 

 organization of the giant reptiles became modified accordingly. They 

 survived successfully for a while, but were ultimately replaced by the 

 mammals, perhaps as a result of still further change in the climate 

 (see p. 538). 



18. Order *Pterosauria 



The Triassic archosaurian reptiles gave rise to two independent 

 stocks that took to the air, the pterodactyls and the birds. Both of 

 these appear first in the Jurassic as animals already well equipped for 

 flight, although obviously basically of archosaurian structure. We 

 cannot therefore say anything about the steps by which their flight 

 was evolved and can only speculate about the influences that drove 

 them to take to the air. The early archosaurs were bipedal animals, 

 and the fore-limbs were therefore free and available for use as wings. 

 There has been much speculation about the intermediate stages by 

 which flight was produced. Other reptiles, such as Draco, the flying 

 lizard (p. 407), develop a membrane between the limbs and the body 

 to assist them in making soaring jumps. The flight of pterodactyls 

 and birds may have originated thus or, as suggested by Nopcsa, by 

 the flapping of the fore-limbs during rapid running on the ground, 

 the animals then becoming airborne for longer and longer periods. 



The stages of the evolution of flight may have been different in the 

 two cases, for whereas the birds are obviously bipedal animals and the 

 similarity to such reptiles as * Strnthiomimus and *Ornithomimus is 

 obvious, the pterodactyls probably could not walk on their hind legs 

 and may have used the wing more for soaring than for flapping flight. 

 In spite of great differences there are interesting parallelisms in the 

 structure of the fully evolved fliers of the two groups, for instance the 



