EVOLUTION-PAST AND PRESENT 



627 



and thus preserved — flesh, skin, and all — 

 for at least 20,000 years. 



Animals that have fallen into petroleum 

 surface deposits have been preserved in 

 their entirety, although more frequently the 

 flesh disintegrates, leaving only the bones in 

 perfect condition. The most famous case 

 is that of the La Brea tar pits in Los Angeles 

 where elephants, antelopes, bears, lions, 

 horses, and the famous saber-tooth tigers 

 ( Fig. 25-3 ) have been found in great abun- 

 dance. Apparently the animals became 

 bogged down in the viscous tar and were 

 attacked by predators which also met the 

 same fate; hence, the rich fossil find in this 

 one locality. The efforts of man to preserve 

 his own kind by various means has been 

 reasonably successful in that mummies 

 have been found dating back 6,000 years. 



The hard parts of animals become fossil- 

 ized very readily, as attested by the large 

 numbers found in various parts of the 

 world. Such parts as shells and other exo- 

 skeletons of invertebrates (Fig. 25-4) and 

 the endoskeletons of vertebrates are most 

 commonly preserved. Here again ideal con- 

 ditions must prevail if the fossil is to form. 

 The animal must be buried shortly after 

 death, usually by the sinking of its body 

 into the soft mud bottom of a stream or 

 other body of water, and tlien be quickly 

 covered by silt which subsequently be- 

 comes rock by the cementing action of min- 

 erals in the water. Thus the original shape 

 of the animal is maintained even though the 

 organic parts completely disappear at some 

 later time. Ground water containing car- 

 bonic acid dissolves away the shell, leaving 

 a mold which is later filled with minerals 

 that precipitate out as the ground water 

 seeps through. The cast that is formed is 

 an almost perfect replica of the original 

 shell or other hard part. More commonly 

 the replacement is accomplished a little at a 

 time: the most soluble parts are filled in 

 first, the least soluble portions last. This 

 type of replacement often reveals the mi- 



Fig. 25-4. Invertebrates with shells were fossilized in 

 great profusion in ancient times. The shell was usually 

 completely replaced by minerals and the detailed anat- 

 omy of the shell was frequently preserved as shown in 

 this relative of the chambered nautilus. This specimen 

 has been sectioned to show how even the details of 

 the chambers have been retained. 



nute detailed internal anatomy and is re- 

 ferred to as petrifaction. 



Animal products such as eggs and excre- 

 ment have been fossilized, and even tracks 

 have been preserved in great abundance in 

 certain localities, including the southwest 

 part of this country ( Fig. 13-42 ) . Such im- 

 pressions as tracks were made in soft mud 

 which later dried sufficiently hard to with- 

 stand subsequent rains, and were finally 

 filled with new material. Both the old and 

 new deposits became rock and thus the 

 tracks were preserved. Today when these 

 strata are split open, the tracks are as dis- 

 tinct as when they were first made. Rain- 

 drops have also been preserved in this way. 

 Such findings have been invaluable in de- 

 termining the habits of these ancient ani- 

 mals. Tracks, for example, reveal whether 

 the animal walked on all fours or hopped 

 on its two hind legs. This information, to- 

 gether with the fossil remains of the animal 

 itself, gives a rather full picture of animal 

 life in the past. 



Ancient animal life 



We have concrete evidence that life took 

 hold in earnest only during the last quarter 

 of the 2 billion years of earth history. As 

 was pointed out at the beginning of this 



