PROBLEM 2. We Leiwn of Vrehistoric Livhig Things from Fossils 535 



I'n;. 4S5 A Litinbriaii iiiideruiatcr scene. The larij^e anhnal near the center is a giant 

 trilabite. M'hat others can you recognize? (Smithsonian scientific series, inc.) 



in the latter pait of this second era there 

 is some evidence that life existed, prob- 

 ably simple one-celled plants and animals. 

 The rocks of the third era contain only 

 a few fossils but it seems clear that sea- 

 weeds and various kinds of very simple 

 sea-dwellincT invertebrate animals lived 

 in this third era (Proterozoic). It will 

 help to study the diagram, Figure 483. 

 Note that five sixths of the earth's his- 

 tory was already over w hen the fourth 

 era began. But that was far from recent. 

 It was 550,000,000 years ago. 



The fourth era (Paleozoic). Examina- 

 tion of Figure 483 shows that this, too, 

 was a long era. Much of our continent 

 seems to have been under water in the 

 early periods. The only fossils in the old- 

 est Paleozoic rocks are those of water- 



dwelling plants and animals. It is be- 

 lieved that there was no soil covering the 

 rocks since no fossils of land-living plants 

 or animals have been found. Seaweeds 

 were plentiful and, judging from the large 

 number of fossils, the oceans and inland 

 waters were teeming with jellyfish and 

 sponges, animals like our modern star- 

 fish, corals of many kinds, and snails. 

 There were more complex invertebrates, 

 too, shrimplike forms. In one early pe- 

 riod of this era (the Cavibrian period) the 

 most common animal type was one which 

 has since disappeared from the earth, the 

 trilobite (try'lo-bite). It is classified in 

 the lobster group (Crustacea). Some spe- 

 cies were two inches long; others reached 

 a size of t\vo feet or more. Trilobites 

 were so numerous and of so many kinds 



