PALEONTOLOGY AND GENETICS 



fossil calculated from the half-life. This method has proved very useful 

 in the study of late Pleistocene and Recent remains. It has been checked 

 against historical objects of known age, such as wood from Egyptian 

 pyramids, and has been found reliable. 



FOSSrilZATION 



Any remnant of living forms from the remote past may be regarded as a 

 fossil. While there are many ways in which fossils may be formed, most 

 of them involve the burial of the dead organism. As more and more sedi- 

 ment is laid down above, the depth of the fossil increases, and hence, in 

 a general way, the oldest fossils may be expected to be found in the 

 deepest layers of the earth's surface, while the more superficially located 

 fossils may be regarded as recent in origin. Thus, ideally, one ought to be 

 able to read the story of life in the correct historical sequence by examin- 

 ing the fossil record in sequence from the deepest to the most superficial 

 strata. 



But the vast majority of plants and animals are not fossilized after 

 death. Decay and destruction ordinarily await the dying organism. Pred- 

 ators and scavengers may not only eat away the soft parts of the body, 

 but they may also break up skeletal structures beyond all hope of preser- 

 vation or recognition. It is only the unusual instance in which the organism 

 is rapidly buried or in some other way protected from scavengers and 

 from oxidation that a fossil may be formed. Thus the fossil record, even 

 if completely known, would have to be very fragmentary, because the 

 majority of organisms never take the first step toward fossil formation. 



Much the most common method of fossilization is burial in the sedi- 

 ments which are continually deposited on the floor of the oceans and of 

 other large bodies of water. When aquatic organisms die, they may fall 

 into deep sedimentary deposits in which the bodies are protected from 

 scavengers and from oxidation. The soft parts of the body gradually decay 

 and are carried away by the seepage of water. Bones and other hard parts 

 may remain as such, or they may be replaced particle by particle by min- 

 erals in the water. As this process continues, the layer of sediment which 

 is being deposited grows ever thicker, and its lower portions gradually 

 harden into rock, the sedimentary, stratified rock which is characteristic 

 of the beds of marine or aquatic deposits everywhere, and which is the 

 hallmark of ancient seas in areas which are now drv land. However, this 

 is not the only means by which organisms may be buried. Dust storms 

 can have the same effect, and will be effective in causing fossilization of 

 terrestrial organisms. Again, volcanic ash may also rapidly bury organisms 

 and thus preserve them as fossils. Pompeii, which was buried by volcanic 

 ash from Mount Vesuvius in 79 a.d., has been extensively investigated in 

 modern times. Whole families, together with their domestic animals, have 

 been preserved as cavities in the ash, from which casts can be made. 

 Desert forms may be dried out by the hot, dry, desert winds, then buried 

 under the shifting sands. 



Some special methods of burial are also occasionally effective. If a 



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