184 INTRODUCTION TO EVOLUTION 



horn over each eye and a horn on the nose (Fig. 9.9). They possessed a 

 parrotlike beak and a great frill of bone projecting backward over the 

 neck. While this doubtless served to protect the neck, its principal function 

 was probably to afford attachment for powerful muscles supporting the 

 heavy head. The head in giant ceratopsians constituted an unusually large 

 proportion of the body. The massive structure and armament of the "busi- 



TRICERATOPS 



FIG. 9.9. Head of Tricerafops, a giant horned dinosaur. Skull 

 about 8 feet long. (Restoration by John C. Germann; by permis- 

 sion from The Dinosaur Book, by Colbert, p. 82. Copyright 1951. 

 McGraw-Hill Book Company, Inc.) 



ness end" of a horned dinosaur give us some conception of what was 

 required for successful living in a world inhabited by such carnivores as 

 Tyrannosaurus (Fig. 9.6). 



The ceratopsians were the last dinosaurs to appear on the scene. Their 

 entire evolution was confined to the Cretaceous period. We may note in 

 passing that the famous dinosaur eggs which have received so much 

 publicity were laid by a small, ancestral member of the group, Protocera- 

 tops. Two of the eggs were found to contain bones of unhatched embryos. 



Extinction of the Dinosaurs 



For at least 160 million years the dinosaurs were "lords of all they sur- 

 veyed." Then "suddenly," in the geologic sense, they all became extinct. 

 Not one dinosaur fossil has ever been found in deposits more recent than 



