PAST DISTRIBUTION OF ANIMALS 



539 



bearing animal tracks or the imprint of soft-bodied organisms may be 

 buried by other deposits and the evidences be preserved as fossils. 



These facts make it evident why fossils of marine animals are most 

 abundant and why animals which are most likely to fall into rivers and 



Fig. 371. — Geological time scale. (Based upon data given in Schuchert's "Historical 



Geology.") 



be carried down to the sea and buried in mud are more likely to be 

 represented in fossils than other terrestrial forms. Since decay in very 

 dry climates is slow and animal remains may dry up before decaying, 

 such remains may easily be covered by wind-blown deposits and thus be 

 preserved, especially if the area is later covered by oceanic water, and the 

 deposits converted into rocks. 



