612 General and Applied Biology 



Hence, the simpler, softer organisms are rarely fossilized. This explains 

 the absence of many fossils of the earliest plants. The organism must 

 also remain intact a sufficient length of time to permit the fossilization 

 process to take place. The original organism must be sufficiently heavy 

 to settle to the bottom to be eventually covered rather than float on the 

 water. Air must be excluded in order to prevent the oxidation of the 

 organism as well as to prevent bacterial decay before the fossil is formed. 

 During and after formation, the fossil must withstand such natural 

 conditions as the elevation and sinking of the earth's strata, pressure and 

 heat, the erosion processes within the strata, and the slow circulation of 

 waters, especially acidulated waters, through the fossil. Because there 

 are few places on land where materials are being extensively deposited, 

 terrestrial plants and animals have little chance of becoming fossilized 

 unless they are placed in water and eventually covered. The majority 

 of land plants and animals after death will be quickly decomposed on 

 the surface of the earth, thus leaving no extensive records. However, 

 if covered with large quantities of volcanic ashes or lava, sand or dust, 

 earth through landslides or earthquakes, or calcareous materials from 

 calcareous springs, even terrestrial plants and animals may be fossilized 

 to a limited extent. If the structures of an animal or plant are thin, 

 fragile, easily broken, easily dissolved, and easily decayed, there may 

 be little opportunity to form a fossil. 



IV. SIGNIFICANCE OF FOSSILS 



Fossils of certain types may indicate the boundaries and extent of 

 former waters and lands. Fossils also may suggest the types of organ- 

 isms of the past and their probable relationships with modern forms. 

 The character of the fossils included in certain strata of the earth gives 

 clues as to their geologic ages and when those particular sediments which 

 formed these strata were laid down. Hence, certain animal and plant 

 fossils are known as index fossils because through them it is possible to 

 determine particular geologic ages and periods. Certain fossils also dem- 

 onstrate that life has not existed without changes in the past because 

 of the revelations of records of past animals and plants. 



Fossils may give evidences of geographic distribution of organisms of 

 the past and may show where land connections once existed but are no 

 longer present. The Bering Strait between Asia and Alaska is about 35 

 miles wide and has a maximum depth of 200 feet. Studies of the fossils 

 on these two continents show undoubtedly that they were once connected 

 by land which sank beneath the water. It has been suggested that there 



