452 



THE CHANGING GENERATIONS 



their roughened edges formed a grinding surface. One group, the orni- 

 thopods, remained bipeds; the other three groups reverted to the quad- 

 rupedal state. 



Some of the ornithopods or bird-footed dinosaurs were small, light, apparently 

 arboreal creatures, but most were larger forms from 10 to 35 feet in length that 

 browsed on shrubs and trees, or lived in swamps and fed on aquatic plants. Among 



Fig. 28.17. Heads of crested dinosaurs. A, Parasaurolophus; B, Corythosaurus; C, Lambeo- 

 saurus. The peculiar crests contain the long, looped nasal passages, which are thought to 

 have served as air storage chambers permitting the animals to remain longer submerged. 

 (Redrawn from Colbert, The Dinosaur Book, by permission American Museum of Natural 

 History.) 



Fig. 28.18. Heads of horned dinosaurs. A, Chasmosaurus; B, Styracosaurus; C, Triceratops. 

 These are some of the highly specialized end types of a line that began in the early Cre- 

 taceous with small forms that had no horns and only the suggestion of a frill or neck 

 shield. (Redrawn from Colbert, The Dinosaur Book, by permission American Museum of 

 Natural History.) 



the semiaquatic types were the duck-billed and crested dinosaurs of the Cre- 

 taceous (Figs. 28.16 and 17), which had birdlike horny beaks, immense numbers 

 of teeth (as many as 2,000 in the mouth at one time with replacements as they 

 wore out), and in some forms webbed feet for swimming. The stegosaurs were 

 four-footed Jurassic dinosaurs about 20 feet long, with a tiny head, a double row 

 of upright triangular plates down the middle of the back, and a powerful tail 

 armed toward the tip with four heavy spikes. The Jurassic ankylosaurs were built 

 like tanks, their large depressed bodies armored above with bony plates and 



