PROBLEM 2. We Learn of Prehistoric Living Things from Fossils 541 



Fig. 493 Skeletons of the modern horse and Eohippus to show comparative sizes. 

 What resemblances and differences can you see? (American museum of natural 

 history) 



ERA 



Psychozoic 1,000,000 years 



Cenozoic 

 60,000,000 years 



Mesozoic 



200,000,000 years 



Paleozoic 



550,000,000 



years 



Proterozoic 



1,000,000,000 



years 



Archeozoic 



2,000,000,000 



years 



AGE OF 



Man 



Mammals 



'"" 



Reptiles 



Amphibians 



Fishes 



Invertebrates 



Simple 

 living 

 forms 



Earliest 

 living 

 things 



\^l 



Fig. 494 The numbers in the left column tell when each era began. The wider a 

 branch at any level the more numerous were the species at that time. Which is the 

 widest branch now? Some grow narrower near the top. Why? 



