MESOZOIC ERA: THE AGE OF REPTILES 



555 



The skin was smooth, without scales. The sharp, flaring teeth show 

 that the creatures lived on animal food, possibly on small fish or some 

 of the cephalopods which were so numerous in the seas of the time. 

 Judging from the shape of the body, they probably swam slowly, 

 depending upon stealth rather than speed in capturing their prey. 



Fig. 515. — Restoration of Plesiosaurus. (Courtesy, American Museum of Natural 



History.) 



It has been shown by a study of the neck vertebrae that the neck was 

 too stiff for very quick movements, but would, nevertheless, be of 

 great assistance both in capturing prey and in enabling an animal 

 quickly to reach the surface for air. Within the body cavity of some 

 skeletons, a large number of polished pebbles have been found — 

 in one case a peck of them — from the size of a hen's egg to that of 

 a baseball. These " gizzard stones " were doubtless of use in grind- 

 ing the food, which was swallowed whole. If the plesiosaurs fed, to any 

 extent, on the shelled cephalopods, some such apparatus must have 

 been extremely useful. Plesiosaurs ranged from the Triassic to the 

 end of the Mesozoic and reached their greatest size in the Cretaceous, 

 and, perhaps, their greatest abundance in the Jurassic. They were 

 not closely related to the ichthyosaurs and were probably descended 

 from a different race of land reptiles. 



Mosasaurus (Sea Lizards). — As the ichthyosaurs disappeared in 

 the Upper Cretaceous, their place was taken by the mosasaurs (Figs. 

 516, 517), long, slender reptiles, with a scaly skin like that of modern 

 snakes, which attained a length of 35 feet or more, although usually 

 smaller. The heads were pointed and provided with sharp, stout, 

 pointed teeth. The jaws were so constructed as to make it possible 

 for the animal to swallow an object of almost the diameter of itself. 



