200 



THE ORIGIN AND EVOLUTION OF LIFE 



CYMBOSPONDYLUS 



GEOSAURUS 



REPTiuA TYLOSAURUS 



CRE7ACEOUS 



Fig. 76. Convergent Aquatic Adap- 

 tation INTO Elongate Fusiform Type 

 in Four Different Orders of 

 Amphibians and Reptiles. 



Independently convergent evolution of four long- 

 bodied, free-swimming, swift-moving, surface-liv- 

 ing aquatic types in which the fins and limbs are 

 retained as paddles: Cn'co^Mi, an amphibian; Ty- 

 losaurus, an Upper Cretaceous mosasaur; Geo- 

 saurus, a Jurassic crocodilian; C ymhos pondylus , a 

 Triassic ichthyosaur. A very similar fusiform type 

 evolves among the mammals in the Eocene ceta- 

 ceans (Zeuglodon) , as seen in Fig. 123. Restora- 

 tions prepared for the author, independent of 

 scale, by W. K. Gregory and Richard Deckert. 



passage from a terrestrial phase, 

 through palustral, swamp-living 

 phases into a littoral, fluviatile 

 phase, and from this into littoral 

 and marine salt-water phases; 

 so that finally in no less than 

 six orders of reptiles the pelagic 

 phase of the high seas was inde- 

 pendently reached. 



The role in the economy of 

 oceanic life which is now taken 

 by the whales, dolphins, and por- 

 poises was assumed by families 

 of the plesiosaurs, ichthyosaurs, 

 mosasaurs, snakes, and croco- 

 diles, all flourishing in the high 

 seas, together with families of 

 the turtles, which are the only 

 high-sea reptiles surviving at the 

 present day. Moreover, under 

 the alternating adaptations to 

 terrestrial and marine life, which 

 prevailed during the 10,000,000 

 years of late Palaeozoic and 

 Mesozoic time, several families 

 of the existing orders of reptiles 

 sought a seafaring existence 

 more than once and gave off 

 numerous side branches from 

 the main stem. The adapta- 

 tions to marine life have been 

 especially studied by Fraas, 



