STRUCTURE OF THE VERTEBRATES 333 



water, are preserved as impressions in the sand, every structure 

 being sometimes shown with perfect accuracy. And in this class 

 of fossils would be placed the footprints of long extinct reptiles. 



Petrifaction is the slow infiltration of water bearing dissolved 

 mineral matter which gradually replaces the structure of the 

 plant or animal. It is not difficult to understand how the cell 

 spaces of a tree might be filled with silica as the protoplasm 

 oozed away; and later, as the walls decayed, these in turn be 

 replaced with quartz, so that thinly ground sections show the 

 perfect woody structure. It is more difficult to understand how 

 animal tissues were so perfectly preserved. Some crinoids (Phy- 

 lum Echinodermata) have been silicified, and sections show the 

 delicate ampullae in position, as well outlined as in the living 

 animal. 



Conditions for Fossilization. The calcareous shells of sea 

 living animals are most abundant, for these simply remained 

 buried in the sand or mud of the sea. If that particular region 

 of the coast were sinking, new layers of sand, mud, or limestone 

 would be laid above and the shell surrounded by potential rock. 

 The shell would be preserved; or, more frequently, the lime dis- 

 solved away and the space filled with mineral matter. This 

 latter makes a natural cast of the shell which is as perfect as 

 the preserved animal. 



Fish also would be preserved by falling to the bottom and 

 being quickly covered before the body had disintegrated or been 

 eaten. The layer of mud and water would prevent much of the 

 bacterial action, and the imprint would be saved indefinitely. 

 The essential is quick burial to prevent separation of parts, and 

 the packing down of the superimposed stratum of earth. 



Land animals are much less frequently fossilized, for condi- 

 tions are not so propitious. Most animals at death are eaten by 

 others, or the winds and rain scatter the bones so that they 

 decay before being preserved. The destruction by carnivorous 

 animals is veiy important. jNIammalian fossils show the grooves 

 made by the gnawing teeth of rodents; and a herbivorous dino- 

 saur (American Museum of Natural History) has vertebrae with 

 grooves which exactly fit the tooth spacing of a carnivorous 

 dinosaur found close by. Birds, as would be expected, are the 

 rarest of fossils, most of those found being water living varieties. 



