4 ANIMALS OF THE PAST 



ever since vertebrate life began there have been carniv- 

 orous animals of some kind to play the role of bone- 

 destroyers. Even were there no carnivores, there were 

 probably then, as now, rats and mice a-plenty, and few 

 suspect the havoc small rodents may play with a bone 

 for the grease it contains, or merely for the sake of 

 exercising their teeth. Now and then we come upon a 

 fossil bone, long since turned into stone, on which are 

 the marks of the little cutting teeth of field mice, put 

 there long, long ago, and yet looking as fresh as if made 

 only last week. These little beasts, however, are in- 

 direct rather than direct agents in the destruction of 

 bones by gnawing off the outer layers, and thus per- 

 mitting the more ready entrance of air and water. 

 Plants, as a rule, begin their work after an object has 

 become partly or entirely buried in the soil, when the 

 tiny rootlets find their way into fissures, and, expanding 

 as they grow, act like so many little wedges to force it 

 asunder. 



Thus on dry land there is small opportunity for a bone 

 to become a fossil; but, if a creature so perishes that 

 its body is swept into the ocean or one of its estuaries, 

 settles to the muddy bottom of a lake or is caught on the 

 sandy shoals of some river, the chances are good that its 

 bones will be preserved. They are poorest in the ocean, 

 for unless the body drifts far out and settles down in 

 quiet waters, the waves pound the bones to pieces with 

 stones or scour them away with sand, while marine 

 worms may pierce them with burrows, or echinoderms 

 cut holes for their habitations; there are more enemies 

 to a bone than one might imagine. 



Suppose, however, that some animal has sunk in the 

 depths of a quiet lake, where the wash of the waves 

 upon the shore wears the sand or rock into mud so fine 

 that it floats out into still water and settles there as 



