THE TRIASSIC PERIOD 401 



The reptiles adopt many roles of life. There seems to be no 

 doubt that the ancestors of the early reptiles were the amphib- 

 ians. We may think of them, then, as originally inhabitants 

 of marshes and inland bodies of water. Reptiles adapted for 

 such a partially aquatic life were rather common in both the 

 Permian and Triassic periods, as their fossil remains attest. 



The terrestrial type becomes prominent. Some of the 

 reptiles came to spend more and more time on land, and even- 

 tually became fitted for living wholly under such conditions. 

 Some walked on all fours, as do our modern cattle and many 

 other mammals, but the more agile varieties apparently were 

 leapers, using their powerful hind legs and stout tails after 

 the manner of the kangaroo. The majority of these swifter 

 forms seem to have preyed upon other animals, and for this 

 purpose their teeth were sharp and strong. They played the 

 role of our modern beasts of prey, such as the tiger and the 

 wolf, although in a manner no doubt peculiar to themselves. 



Reptiles find a place in the sea. The tempting source of 

 food which the fishes and mollusks of the sea presented was 

 early appropriated by other branches of the reptilian stem. 

 Two main types were represented in the Triassic period. Of 

 these the Plesiosaurs were large, flattened saurians with long, 

 slender necks and short heads. Their legs eventually became 

 mere paddles like those of the modern sea turtles. The sus- 

 picion that they fed partly upon mollusks, for which they 

 probably delved in the shallows with their long necks, is 

 strengthened by the finding within their bodies of smooth 

 pebbles, thought to be gizzard-stones used to pulverize the 

 food swallowed. 



Of all the reptiles none were better fitted for living ex- 

 clusively in the open sea than the Ichthyosaurs (fish-reptiles). 

 They had acquired the form of fishes themselves (Fig. 421), 

 with the long powerful tail fin, the short neck, and long jaws 

 set with sharp teeth. These animals seem to have been 

 almost exclusively fish eaters. Most aquatic reptiles, for 

 example the turtles, lay their eggs in sand along the shore ; 



