CHAPTER XXI 

 CENOZOIC ERA : AGE OF MAMMALS. TERTIARY PERIOD 



Comparison of the Life at the Close of the Mesozoic and the Begin- 

 ning of the Cenozoic. — The Age of Reptiles apparently came abruptly 

 to a close, and the Age of Mammals began. In the last stage of the 

 Upper Cretaceous (Lance) the dinosaurs were in the " climax of their 

 specialization and grandeur." The bulky Triceratops (p. 548) with 

 his great horned head, the amphibious duck-bill dinosaur (Trachodon, 

 p. 544) as well as other armored dinosaurs, roamed about in the Rocky 

 Mountain region. At the same time lived the swift and powerful 

 Tyrannosaurus (p. 540) which doubtless preyed upon some of these 

 herbivorous relatives. Associated with these great reptiles were 

 small mammals (p. 564) of lowly organization and of small size. " One 

 of the most dramatic moments in the life history of the world is the 

 extinction of the reptilian dynasty which occurred with apparent 

 suddenness at the close of the Cretaceous, the very last chapter in 

 the Age of Reptiles/' (Osborn.) This does not mean that the reptiles 

 were wiped out of existence by some great cataclysm, but that, as 

 measured by geologic time, the wane was rapid. What cause or 

 causes produced this great result cannot be stated definitely. 



(1) Change in vegetation has often been called in to account for the 

 extinction of various groups of animals, but we find much the same 

 vegetation after the extinction of the dinosaurs as when they were 

 abundant and at the summit of their specialization. Such trees as 

 the fig, banana, sequoia, ginkgo, oak, and sycamore passed from one 

 period to the other without alteration. This being the case, a change 

 in food, unless under exceptional conditions, could not have been a 

 cause of dinosaurian extinction. Moreover, since the vegetation re- 

 mained so nearly the same at the critical time, it is not probable that 

 the climate had been greatly modified. (2) It has also been suggested 

 that the cause of their extinction was their inability to compete with 

 the more agile and intelligent mammals, and the fact that their young, 

 not having the maternal care of these higher vertebrates, were easily 



572 



