450 



THE CHANGING GENERATIONS 



as Brontosaurus) was a heavier creature, only 70 feet long but weighing some 30 

 tons. The giant of them all was Brachiosaurus, known from North America and 

 Africa. Unlike all other sauropods, which showed their bipedal ancestry in their 

 long hind legs and massive tails, Brachiosaurus had short hind legs but long fore 

 legs which, with the very long neck, would have enabled it to look over the top 

 of a three-story building. The body was very stout and the tail short; the whole 

 form suggests adaptation for life in rather deep water. The total length was about 

 80 feet and the weight about 50 tons. 



The structural features of these sauropods are marvels of mechanical efficiency. 

 The legs are massive columns for the support of great weight; the spine is com- 

 posed of great vertebrae so hollowed and perforated as to give maximum strength 



Fig. 28.15. Sauropod saurischian dinosaurs. A, Apatosaurus (Brontosaurus) ; B, Diplodocus; 

 C, Brachiosaurus. Not drawn to scale. (Modified from W. D. Matthew, Schuchert, and 

 Colbert.) 



for the weight of bone used; and the entire skeleton is built on the lines of a 

 double-piered cantilever bridge. Even so, such great creatures must have been 

 clumsy on land, and there is evidence that they were chiefly inhabitants of swamps 

 and streams where their weight was partly water-borne; their peglike teeth, 

 furthermore, seem best fitted for feeding upon soft aquatic vegetation. 



The ornithischian dinosaurs. The second order of dinosaurs, the 

 Ornithischia, arose in the late Triassic but did not undergo rapid evolu- 

 tionary expansion until during the Cretaceous. Its members never at- 

 tained the great size reached by some of the saurischians but are even 

 more interesting on account of the variety of bizarre forms which they 

 exhibit. From their earliest appearance the ornithischians were plant 

 feeders. The front teeth were lost and replaced by horny beaks; the back 

 teeth became more numerous and changed from the primitive conical 

 type to flattened leaflike blades pressed together in long rows so that 



