PROBLEM 2. We Learn of Prehistoric Living Things from Fossils 537 



1 



Fig. 488 How one artist pictures a landscape during the age of reptiles. Tyranno- 

 saurus rex and the three-horned dinosaur, Triceratops horridus. (American museum 



OF NATURAL fflSTORY) 



crawled over the earth. There were very 

 few reptiles. Certain kinds of insects ex- 

 isted in enormous numbers, huge dragon- 

 flies with a wingspread of a foot, and 

 cockroaches three to four inches long. 

 Toward the close of the Carboniferous 

 period the earth must have sunk in many 

 of the swampy regions. The plants died 

 and were covered by water and then 

 by sediments that eroded from higher 

 ground. Then chemical changes, together 

 with pressure and heat, changed the sub- 

 merged plants to coal. It took millions of 

 years to make the coal; meanwhile more 

 and more sedimentation and rock forma- 

 tion occurred above the coal beds. This 

 happened more than 200 miUion years 

 ago. 



The Age of Reptiles. This is the fifth 

 (iMesozoic) era. It is believed that it 

 lasted for about 140 million years. The 



huge evergreens of this era can be seen 

 petrified in Arizona. From the fossil evi- 

 dence, there were reptiles of many spe- 

 cies on the land, in the sea, and in the air. 

 A few kinds of small mammals were in 

 existence and some strange birds had ap- 

 peared. Representatives of the other 

 groups, the fishes and the many types of 

 invertebrates were in existence still, but 

 by far the most numerous among animals 

 were the reptiles, both in numbers and in 

 species. Toward the close of the era the 

 dragonlike reptiles known as di?JOsaurs 

 flourished. Some species of dinosaurs 

 were small; others gigantic. The thunder 

 lizard {Brontosaimis) reached a length 

 of sixty-six feet. It apparently waded 

 about in swamps and shallow lakes and 

 must have eaten about 300 pounds of 

 plants daily. Although it had an esti- 

 mated weight of more than 70,000 



