224 



THE ORIGIN AND EVOLUTION OF LIFE 



(Ornithischia) . The small Triassic ancestors of this great 

 group of herbivorous, ornithischian dinosaurs also gave rise 

 to a number of secondarily quadrupedal, slow-moving forms, 

 in which there developed various forms of defensive and offen- 

 sive armature. Of these the Jurassic stegosaurs exhibit a 

 reversed evolution in their locomotion since they pass from a 

 bipedal into a quadrupedal type in which the armature takes 



Fig. I02. Offensive and Defensh'e Energy Complexes. 



The carnivorous "tyrant" dinosaur Tyrannosaiirus approaching a group of the horned 

 herbivorous dinosaurs known as Ceratopsia. Compare frontispiece. 



The Ceratopsia are related to the armored Stegosaurus and to the armorless, swift-moving 

 Iguanodontia. Restoration by Osborn in the American Museum of Natural History, 

 painted by Charles R. Knight. 



the form of sharp dorsal plates and spiny defenses, the exact 

 arrangement of which has been recently worked out by Gil- 

 more. Doubtless when this animal was attacked it drew its 

 head and limbs under its body, like the armadillo or porcu- 

 pine, and relied for protection upon its dorsal armature, aided 

 by rapid lateral motions of the great spines of the tail to ward 

 off its enemies. During the progress of Cretaceous time these 

 stegosaurs became extinct, and by the beginning of the Middle 

 Cretaceous two other herbivorous types are given off from the 

 predentate stock. 



The first of these are the aggressively and defensively 

 horned Ceratopsia, in which two or three front horns evolved 



