446 



THE CHANGING GENERATIONS 



upon this mode of locomotion, but the problem was most successfully 

 solved by the archosaurs. The earliest archosaurs had lizardlike bodies; 

 the front legs were short, the hind legs long and much modified, the tail 

 massive and the pelvic girdle strong, and the legs had been brought 

 under the body for better support. When resting or walking, all four feet 

 touched the ground, but speed was attained by running on the hind legs, 

 with the fore part of the body raised and balanced at the hips by the long 

 tail. 



From such a stem form there developed during Permian and Triassic 

 times five major lines of archosaurs — the phytosaurs and crocodiles, the 



Fig. 28.10. A Triassic phytosaur, Rutiodon. Some of these crocodilelike animals reached 

 great size. They were an offshoot from the primitive archosaurs, and though relatives were 

 not ancestors of the crocodiles that replaced them. {Courtesy American Museum of Natural 

 History.) 



pterosaurs, two distinct groups collectively known as dinosaurs, and the 

 ancestors of the birds. Some of these continued their evolution as bipeds, 

 though the phytosaurs and crocodiles failed to progress in this direction, 

 and some of the dinosaurs later reverted to a quadrupedal condition. 

 Among the bipeds the front legs had lost much of their original useful- 

 ness. In some groups they became much reduced or almost vestigial, but 

 in others they took on changed functions and were altered into grasping 

 hands, or defensive weapons with spikelike thumbs, or, even more dras- 

 tically, into wings. 



Phytosaurs and crocodiles. During the Triassic a group of primitive 

 aquatic archosaurs became abundant. Known as phytosaurs, they were 

 fish-feeding, crocodilelike creatures (Figs. 25.2 and 28.10), with long 

 slender jaws armed with many sharp teeth and with the nostrils situated 



