PALEONTOLOGY AND GENETICS 



Lastly, organisms mav be petrified, that is, their actual tissues may be 

 replaced, particle by particle, by minerals in solution in the waters of the 

 locality. The principal minerals utilized in this type of fossilization are 

 iron pyrites, silica, calcium carbonate, and other carbonates. The most 

 widely known example of petrifaction is afforded by the petrified forests 

 of southwestern United States; however, animal remains may also be 

 petrified. Generally, this preserves only the hard parts of the body, but 

 occasionally soft parts are so well preserved that even fine details of cells 

 can be made out in thin sections. Most of the fossils from sedimentary 

 rocks are of this type: the original material has been replaced by minerals 

 from the surroundintj; medium. 



Types of Fossils. With respect to what is preserved, there are several 

 types of fossils. The whole organism mav be preserved, but this is very 

 rare, and is known only for Cenozoic fossils. The best examples are insects 

 in amber, some of the mammals from Rancho La Brea, and the mammoths 

 and other mammals which have been found frozen in the arctic. These 

 are, of course, the ideal fossils. But because of their great scarcity, and be- 

 cause they are all of recent origin, thev are less important for paleontology 

 as a whole than are less complete and less satisfactory types of fossils. The 

 vast majority of fossils are petrified or carbonized, and only the hard parts 

 of the organism are preserved. Thus the bones of vertebrates, shells and 

 spicules of invertebrates, and the woody parts of plants are commonly 

 fossilized. Such fossils are, because of the nature of their formation, almost 

 always incomplete. But even beyond this, they are commonly found as 

 broken fragments. 



A fossil, however, need not include any part of the original organism. 

 It may be a mere footprint or leafprint which has chanced to escape de- 

 struction while the mud or sand in which it lay hardened into stone. Such 

 impressions may subsequently be filled with a hard mineral, thus forming 

 a cast of the original structure much as a sculptor might. Soft-bodied or- 

 ganisms are preserved in this way as readily as are those with hard skeletal 

 structures, and many organisms are known only from the casts and impres- 

 sions which they have left. Even the feces may be fossilized and studied 

 under the name of coprolites, and these may yield valuable information 

 about the food habits of extinct animals. 



Incompleteness of the Fossil Record. One of the important aspects of 

 the fossil record is its incompleteness. One reason for this has already 

 been pointed out: the majority of organisms never take the first step to- 

 ward fossilization. They are destroyed by predators or scavengers, or else 

 they lie exposed to the elements and deteriorate. But many other factors 

 also contribute to the incompleteness of the fossil record. Of the several 

 methods of fossilization mentioned above, only one, burial in marine or 

 fresh-water sediments, could be expected to occur with great regularity. 

 Because of this, the majority of fossils are found in such sedimentary 

 rocks, and this consequently results in poor representation of terrestrial 

 forms. 



The record is also biased by the fact that generally only the hard parts 

 of the body are fossilized. In some instances, such parts are taxonomically 



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