132 EVOLUTION OF THE EARTH 



this may have been just enough to give the final death blow to 

 an expiring race. 



Final extinction of dinosaurs. In the later Cretaceous the 

 amphibious habitat was again widespread, if indeed it was 

 ever seriously diminished after its first expansion in the Juras- 

 sic, for here we find the final efflorescence of dinosaurs. The 

 Sauropoda and Stegosauria had gone, but in their place were 

 unarmored duck-billed dinosaurs or trachodons, fairly rapid 

 runners ashore but, judging from webbed feet and compressed 

 tail, as quick as crocodiles when circumstances forced their 

 retreat to the waters. Heavily armored dinosaurs, the nodo- 

 saurs (ankylosaurs) were present, as were the horned Ceratop- 

 sia, some of which were highly grotesque beings. And to main- 

 tain the balance of power there were carnivores, both small and 

 great, the latter the mightiest beasts of prey which ever walked 

 the earth. Then comes their dramatic extinction, the world 

 over, although they may have lost their world-wide dominance 

 some time before, as we only know these late Cretaceous forms, 

 doubtful Patagonia excepted, from a few European localities 

 and from western North America ; but there they were in the 

 climax of their grandeur and there is little save the tendency 

 to overspecialization once more to warn the observer of their 

 coming dissolution. But so far as our records go, not one 

 dinosaur of all the hosts that were survived the Mesozoic, for 

 undoubted post-Cretaceous rocks have not yielded a fragment 

 of their remains. 



Why they became extinct no one knows. Our chart shows 

 a lowering of temperature toward the close of the Cretaceous, 

 and to such climatic changes reptiles are highly sensitive. 

 However that may be, the great Laramide revolution which 

 marks the close of the Mesozoic must have brought in a long 

 chain of attendant events in consequence of which the dinosaurs 

 perished. Of all factors of which we have knowledge, the 

 draining of the low-lying coastal lands, with a consequent 



