Animals and Plants of Past and Their Records 611 



ing waters. In this manner the form of the original skeleton is preserved 

 but not its natural structure. 



In petrifaction, more or less of the original materials of the organism 

 have undergone a certain amount of mineralization. In this case, the 

 plant or animal materials have decayed in waters which contained large 

 amounts of lime, silica, iron oxides, iron pyrites, or other dissolved sub- 

 stances. These chemicals replaced the original materials of the organ- 

 ism, sometimes faithfully retaining the original shape, size, and even 

 minute details of structure of the former organism. Usually, the older 

 the fossil in time, the greater the degree of mineralization. The harder 

 parts of an organism are most frequently preserved. Shells, teeth, tusks, 

 bones, and the harder, woody parts of plants are most frequently petrified 

 by mineralization. We may find petrified wood which shows the minute 

 structures just as they existed in the living trees but in which the walls of 

 the cells are formed of the mineral silicon instead of the original cellulose. 

 In this process, as each particle of cellulose disappeared, its place was 

 accurately taken by a particle of silicon, thus retaining the minute details. 



Carbonization of the original materials usually takes place in animals 

 which possess chitinous skeletons. This also occurs in some plants. In 

 this case, the organism loses oxygen and nitrogen, thus increasing the 

 relative percentage of carbon. Even when plant materials are carbon- 

 ized, they may afford valuable information regarding their original struc- 

 ture. 



Trails and impressions (imprints) are the "fossils of living organisms," 

 while other records are of dead organism. Many animals may leave their 

 trails and imprints, but only vertebrate animals with feet can leave foot- 

 prints. In all cases the records must be left in soft materials which later 

 become hardened and preserved. Trails and imprints, although forming 

 no part of the organism itself, nevertheless are considered as fossils. One 

 of the most common of impressions is that made by a leaf in soft mud 

 which later hardens and retains the impression. If the material in which 

 the plant or animal impression is made turns to rock, the result is a fos- 

 sil. This type of fossil usually does not give much information concerning 

 the internal structures but much concerning the shape and form of the 

 organism as a whole or of its parts. 



III. CONDITIONS FOR FOSSIL FORMATION 



In order for fossils to be properly formed, there must be a rather 

 rapid burial of the original organism in a locality suitable for fossil 

 formation. This burial usually is accomplished by water-borne sediment. 

 Organisms with hard parts are more likely to fossilize than softer ones. 



