MESOZOIC ERA 185 



those of Mesozoic age. Why this mass extinction of creatures which had 

 been successful for so long? 



Many answers to the question have been proposed. Most of them relate 

 in some way to the drastic geologic changes which marked the close of the 

 Mesozoic. At this time the Rocky Mountains were formed, an occurrence 

 which resulted in extensive changes in the flat interior of North America. 

 The inland seas disappeared and the swampy lowlands became greatly re- 

 stricted. This change must have aftected adversely the many dinosaurs 

 which lived amphibious lives or preyed upon those that did. The plant life 

 changed; herbivorous dinosaurs may not have been able to modify their 

 food requirements correspondingly. The climate became colder. Dino- 

 saurs, like modern reptiles, probably had little ability to control their body 

 temperatures. It is noteworthy that the few reptiles which today live in 

 cold climates must undergo long periods of dormancy — hibernation. Per- 

 haps dinosaurs could not hibernate successfully. A contrast in this respect 

 is afl'orded by mammals, already present in Mesozoic times. Their greater 

 metabolic activity and accompanying warm-bloodedness were great assets 

 in a world grown colder. Despite their somewhat larger brains and higher 

 metabolism the mammals of the time were too small to have constituted 

 a direct menace to the ruling reptiles. Perhaps, however, they contributed 

 to the decline of the latter by eating their eggs. 



Why did the dinosaurs become extinct? We have indicated that many 

 factors doubtless contributed to their downfall. They were well adapted for 

 life in the Mesozoic world, but they were not adaptable enough to meet 

 the altered requirements for successful living in the Cenozoic world. 



Conquest of the Sea 



Not all the reptiles of this golden age of reptilian life were dinosaurs. 

 Several groups of reptiles returned to the sea for a home. Of these aquatic 

 reptiles two are shown in Fig. 9.3: plesiosaurs and ichthyosaurs. Both de- 

 veloped a somewhat fusiform body shape, but the plesiosaurs had long 

 neck and tail, while the ichthyosaurs had a fishlike absence of neck and a 

 fishlike fin on the tail. The limbs of plesiosaurs were paddlelike, those of 

 ichthyosaurs were more like the paired fins of fishes in external appearance. 

 The ichthyosaurs even resembled fishes to the extent of developing a dorsal 

 fin (Figs. 3.5, p. 30, and 9.3). Plesiosaurs probably swam rather slowly by 

 an oarlike action of their limbs. Ichthyosaurs must have propelled them- 

 selves by undulation of the body, as does a fish (pp. 56-58), using the 

 limbs as rudders. Thus the ichthyosaurs were much the more rapid and 



