448 



THE CHANGING GENERATIONS 



dinosaurs disappeared some 70 million years before the coming of man 

 has been no deterrent to science-fiction writers, who have created a wide- 

 spread impression that dinosaurs and early man were contemporaries. 

 Another misconception is that all dinosaurs were immense ferocious 

 carnivores; some of them were, it is true, but the largest were ponderous, 

 slow-moving herbivores, and there were a great many medium-sized and 

 small dinosaurs, some no larger than chickens. Finally, even the name 



dinosaur is misleading, for dinosaurs 

 were not a single group but belonged 

 to two distinct and rather distantly 

 related archosaur stocks, the Sau- 

 rischia and the Ornithischia. 



The most obvious difference be- 

 tween the two groups of dinosaurs 

 lies in the structure of the pelvis. In 

 the Saurischia the three bones of 

 each half of the pelvis formed a tri- 

 radiate structure, as in the primitive 

 archosaurs, with the pubis projecting 

 downward and forward from the ace- 

 tabulum. In the Ornithischia the 

 pubis had swung back until it lay 

 along the lower margin of the ischium, 

 as in the birds (whence the name of 

 the group); but as a support to the 

 belly a new bony process developed 

 from the front edge of the base of 

 the pubis, so that the pelvis became 

 four- instead of three-pronged. 



Fig. 28.12. The pterosaur mural by Con- 

 stantin Astori. A large tailless type 

 (Pteranodon) is in flight above; tailed 

 forms (Rhamphorhynchus, etc.) appear 

 below. (Courtesy American Museum of 

 Natural History.) 



Among the Saurischia there were two 

 great divisions, the first of which included 

 the bipedal carnivorous dinosaurs, or ther- 

 apods. In the Triassic period these were 

 mostly small, swift-running types, rather lightly built. In one line (Fig. 28.16, 

 Struthiomimus) the claws of the forelegs were lost, the fingers became grasping 

 organs, and the teeth disappeared; it has been conjectured that the members of 

 this group fed upon the eggs of other dinosaurs. The main stock grew increasingly 

 large and culminated in the enormous Tyrannosaurus of the Cretaceous period— 

 the largest carnivore that ever ruled the lands. This beast stood 19 feet high, 

 reached a length of 47 feet, with a skull 4 feet long, and is estimated to have 

 weighed approximately 10 tons. Its powerful armament of teeth and the great 

 claws of its three-toed hind feet were obviously adapted for attacking large, thick- 

 skinned or armored prey— doubtless the large Cretaceous ornithischian dinosaurs. 



