RIVER OF LIFE 51 



to salt water. But they are not alone. Some of the reptiles 

 and their mammalian descendants have returned to their 

 ancestral home after an intervening period of evolution on 

 land, and are conspicuous members of present-day life in the 

 sea. Others, returning to the oceans from the land, flourished 

 for varying periods before they finally became extinct. 



The struggles of the lobe fishes to survive in the face of stag- 

 nant or drying watercourses or swamps led to the evolution 

 of amphibia, the first true land-living creatures. They were still 

 tied to the water, for without it their eggs cannot develop. 

 By Permian time, however, as the continents which began to 

 rise above the oceans foreshadowed the Appalachian mountain 

 growth, not only had some of the amphibia successfully left 

 the water behind but the early reptiles that arose from them 

 had burst into a new profusion of life. A great variety of these 

 bizarre creatures now took over the new land kingdom. Some 

 were fierce, sharp-toothed carnivores, such as Dimetrodon, 

 with a fin along its back like the sail of a sailfish. Others with 

 crushing teeth fed on shellfish; while still others fed largely on 

 plants. A few had already begun to adapt themselves once 

 more to life in the rivers and seas, and one small group, the 

 theridonts, had begun to undergo the evolutionary changes 

 that led to the mammals. 



The next 50 million years was the great age of reptiles, the 

 Mesozoic era, when 30-ton dinosaurs, giant stegosaurs, 75-foot 

 brontosaurs or thunder lizards, and frightful two-legged tyran- 

 nosaurs, each for a time held sway. Returning to their ancestral 

 home other reptiles developed into ichthyosaurs, large por- 

 poise- or sharklike creatures that gorged themselves on fishes 

 and on the squidlike belemnites. The voracious appetite of 

 these fierce carnivores is gauged by the fact that in one fossil 

 ichthyosaur alone the remains of nearly 200 belemnites, have 

 been found. 



The ichthyosaurs were soon joined by numerous seagoing 

 turtles, including Archelon, which measured over 1 1 feet from 



