180 INTRODUCTION TO EVOLUTION 



side, a firmly braced tripod for attachment of the legs to the body was 

 achieved. 



The pelvic girdle of the Ornithischia differed from that of the Saurischia 

 mainly in the structure of the pubis. This bone possessed two prongs (Fig. 

 9.5), one extending anteriorly, the other posteriorly and ventrally, parallel 

 to the ischium. The arrangement of the posterior prong resembled that of 

 the pubis in birds, hence the name Ornithischia ("bird hips"). 



Saurischia 



The Saurischia include the dinosaurs most like their thecodont ancestors. 

 These are the Theropoda, which, like the thecodonts, were carnivorous and 

 bipedal. They ranged in size from small animals to Tyrannosaurus, the 

 largest carnivorous land animal that ever lived (Fig. 9.6). This Mesozoic 



FIG. 9.6. Tyrannosaurus attacking the horned dinosaur, Tricerafops. (Restorations by 

 Charles R. Knight; courtesy of the American Museum of Natural History.) 



menace was about 50 feet long and 18 to 20 feet tall as he stood on his 

 tremendous hind legs. The forelegs were tiny in proportion to the 8- to 

 10-ton bulk of the creature; each foreleg retained but two functional dig- 

 its, armed with hooked claws. The lower jaw was hinged to the huge skull 

 in a manner to give the animal a mouth of inordinate gape. The jaws were 



