The Consequences of Evolution - 561 



Fig. 29-7. Part of the !eg of a dinosaur, D/'p/odocus, 

 as uncovered in the Bone Cabin Quarry, Wyoming. 

 (Courtesy of the American Museum of Natural His- 

 tory, New York.) 



(Figs. 29-7 and 29-12); and the study of fossils 

 constitutes the science of paleontology. A few 

 fossils are found in sand drifts, asphalt, 

 amber (petrified resin), and in the ice of 

 arctic regions, but a vast majority of fossils 

 are found in sedimentary rocks. These 

 stratified rocks are formed wider water, by 

 the slow deposition of sand, clay, mud, or 

 lime, which subsequently harden into rock, 

 under the action of high pressure and a 

 series of slow chemical transformations. 



The fossil record, as it is known today, pro- 

 vides only a very fragmentary history of the 

 life of past ages, but this fact is not surpris- 

 ing in view of the peculiar chain of chance 



and circumstance necessary for the forma- 

 tion, preservation, and discovery of fossils. 

 Usually only the hard parts of any creature — 

 such as shell, bone, or wood — are likely to 

 survive as fossils, and since a majority of 

 very early organisms never possessed any 

 skeletal parts, vast multitudes of ancient 

 forms have left only the barest traces of their 

 existence. Also the fossils found in sedimen- 

 tary rock, by the nature of their origin, are 

 derived mainly from aquatic organisms or, 

 more especially, from species that have dwelt 

 in the ocean. Only rarely does it happen that 

 the remains of a land-dwelling form find its 

 way to the oceanic floor, and consequently 

 terrestrial species, on the whole, are rather 

 poorly represented in the fossil record. More- 

 over, sedimentary rocks are deposited only 

 under certain conditions — relatively near the 

 shore, where large amounts of sediment are 

 accumulating, in areas that are sinking — so 

 that the accumulation may reach considera- 

 ble depth. And once formed, fossils are sus- 

 ceptible to destruction. Very deeply buried 

 sedimentary rocks begin to suffer distortion 

 or metamorphosis, as a result of great pres- 

 sure or heat, and these factors may destroy or 

 obliterate a large part of the fossil content. 

 If, on the other hand, such rock is raised and 

 exposed to the eroding action of running 

 water, wind, rain, sleet, frost, etc., again there 

 may be a wholesale destruction of the fossil 

 record. And finally, fossils that escape the 

 vicissitudes of the ages can be discovered 

 during this present age only if they happen 

 to lie at or near the surface of the earth, in 

 some accessible region (Fig. 29-9). Consider- 

 ing all these factors, therefore, it is not sur- 

 prising that the record of pre-existing species 



3fe 



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Fig. 29-8. Total skeleton of a dinosaur, Diplodocus. (Courtesy of the American 

 Museum of Natural History, New York.) 



