xv. 19 EVOLUTION OF REPTILES 429 



mostly, if not all, marine. The largest flying birds alive today are the 

 albatrosses, which use their great weight to gain height with the 

 increasing velocity of the wind a few feet above the sea (p. 460). It is 

 possible that the pterodactyls used a similar method of soaring. They 

 were presumably unable to compete with the birds, however, and died 

 out at the end of the Cretaceous, along with so many other reptiles. 



19. Conclusions from study of evolution of the reptiles 



Many of the conclusions that have been drawn from study of verte- 

 brate evolution in the water also apply to the forms that have come 

 on land. The fossil record leaves no doubt that almost all the popula- 

 tions have changed very markedly. Few forms of reptile alive today 

 are closely similar to any found in the Permian or Triassic periods. 

 Sphenodon has shown relatively less change than most others ; it may 

 be significant that it is found in an isolated island region (but see 



P- 772). 



The data are not sufficient to show the rate of evolutionary change. 



We cannot be sure whether it has been constant or even continuous, 



but particular types are found only from a limited range of strata and 



there is little evidence that any terrestrial form remains unchanged for 



more than a few million years, at most. Each type is successful for a 



while and then the niche that it fills becomes occupied by another 



type, either descended from the first or, more usually, from some 



related stock. Thus the earliest large land herbivores were probably 



the pareiasaurs; these were replaced by other reptilian types such as 



the herbivorous mammal-like reptiles, and later the sauropods (in so 



far as these were terrestrial) and various types of ornithischians ; then 



perhaps by the hadrosaurs in the more watery habitats and the stego- 



saurs, ankylosaurs, and ceratopsia on drier ground. Finally, all these 



gave place to the earliest mammalian herbivores, which were in turn 



replaced by others (p. 776). 



Throughout early tetrapod evolution there is a tendency to return 

 to the water, perhaps under some pressure of competition from 

 descendants on land. This is marked among reptiles, where besides 

 the chelonians and ichthyosaurs and plesiosaurs there are the phyto- 

 saurs and crocodiles, and among Squamata the mosasaurs and tylo- 

 saurs, not to mention the sea-snakes. 



The large size of many reptiles has been one of their most striking 

 features, but it is, of course, not true to say that there is a strong 

 tendency for size to increase in all reptile groups. While many 

 have become enormous, others, such as the lizards, have produced 



