MESOZOIC ERA: THE AGE OF REPTILES 543 



and are set rather far apart, a shape and arrangement fitting them 

 for biting, but not for mastication. The brain is smaller than the 

 spinal cord. 



The reptiles of this family grew to be as much as 80 feet long and 

 stood 16 or more feet high, and some are believed to have weighed 

 35 to 40 tons. They lived on flat plains, such as those at the mouth of 

 the Amazon to-day, occupied by interlacing streams and small lakes 

 in abandoned river channels ; in a warm climate with luxuriant vege- 

 tation. That the water was fresh is shown by the fossil remains with 

 which they are associated, such as fresh-water plants, and shells, fish, 

 crocodiles, turtles, and other dinosaurs. They went on land occa- 

 sionally but not habitually, since the great weight of the solid bones 

 would impede their movements, thus rendering them less able to 

 escape their enemies. In the water they could swim with ease, 

 propelled by their long tails. Their food was either floating plants, 

 or such as were loosely attached to the bottom or banks; but they 

 probably sometimes cropped foliage growing 20 feet above the water, 

 which their long necks enabled them to reach. The character of 

 the teeth precludes the possibility of hard, tough vegetation, since 

 these are weak and not adapted to grinding. The lack of grinding 

 teeth made it necessary for them to bolt their food, and it is interesting 

 to note the occurrence of polished flint pebbles associated with the 

 remains, which may have been " stomach stones " or " gastroliths," 

 used in grinding the food after it had been swallowed. 



These huge, four-footed creatures were probably descended from 

 the carnivorous dinosaurs, either before or after the latter acquired 

 the bipedal habit. When the carnivorous race became widespread 

 and competition more severe, certain of them probably had a mixed 

 diet at first, which in time became entirely herbivorous. After 

 this change was established, the increase in size was largely a matter 

 of abundance of food and lack of enemies. Although the body in- 

 creased in bulk and changed in structure, the teeth failed to be modi- 

 fied to a great degree, but retained many of their ancestral characters 

 to the end of the race. The unarmored quadrupeds first appeared 

 either at the close of the Triassic or at the beginning of the Jurassic, 

 and survived into the Lower Cretaceous. Their extinction may 

 have been caused by a change in climate; by starvation as the 

 result of the disappearance of the water plants upon which they 

 fed ; by the arrival or development of powerful enemies ; or in 

 other ways. 



CLELAND GEOL. — 35 



