182 INTRODUCTION TO EVOLUTION 



strength of a pillarlike leg increases in proportion to its cross section, which 

 increases only by squares. Thus, as Romer (1945) has pointed out, if a 

 reptile doubles its length its weight is increased about eight times while the 

 strength of its legs is increased but four times. The largest species of 

 sauropods had weights ranging from 30 to 50 tons. It seems that much of 

 the support for this weight must have been suppHed by water in which 

 the giants spent the greater portion of their lives. 



Brontosaurus, one of the largest dinosaurs, reached a length of about 

 80 feet and weighed some 30 tons. Much of the length is attributable to 

 the long neck and tail (Fig. 9.7). The small head contained a brain dis- 

 proportionately small even for a reptile, a class not noted for its brain de- 

 velopment. 



Ornithischia 



Whereas the Saurischia had their major period of expansive evolution 

 during Jurassic times, living on into the Cretaceous as indicated in Fig. 

 9.3, the other great order of dinosaurs had their greatest period of develop- 

 ment during the Cretaceous. The Ornithischia were on the whole more 

 specialized than were the Saurischia. One indication is seen in the fact 

 that the Ornithischia departed from the thecodont pattern of pelvic struc- 

 ture, while the Saurischia retained this pattern (Fig. 9.5). 



All of the Ornithischia were herbivorous. Their teeth were somewhat 

 leaf-shaped, with serrated edges. Most of the Ornithischia lacked teeth in 

 the front of the mouth. Presumably this toothless region was covered with 

 a horny beak somewhat like that possessed by turtles. 



A majority of the Ornithischia forsook the bipedal gait of their thecodont 

 ancestors, though in most of these secondarily quadripedal forms the dis- 

 proportionate length of the hind legs betrayed their ancestry. Among the 

 bipedal ornithischians the forelegs were never so greatly reduced in size 

 and function as they were among the saurischian bipeds. 



The Ornithischia fall naturally into four groups or suborders: 



1. The Ornithopoda include all the bipedal Ornithischia and some of 

 the quadripedal ones. Best known among them are the bipedal, duckbilled 

 dinosaurs. About 30 feet in length, these dinosaurs had long, powerful 

 hind legs and reduced forelegs. The toothless beak was flattened and wid- 

 ened to form an oversized duckbill, probably used for underwater feeding 

 much as a duck employs its bill. Mummies show us that the skin was cov- 

 ered with small scales and that there was webbing between the toes, indic- 

 ative of life in swampy regions or along the margins of pools or lakes. 



