THE EVOLUTION OF ANIMAL LIFE 



or traces of ancient animals and plants, usually preserved in the rocks. 

 These rocks were themselves formed by the compaction and cementing to- 

 gether of deposits of silt, mud, sand, or volcanic ash, which had slowly settled 

 out of the air, or out of streams, lakes, and shallow seas. Animals that died 

 and fell into these deposits, or were overwhelmed by them, were buried by 

 the accumulating sediments, often in large numbers. Their soft parts usually 

 disappeared, through the normal processes of decay; but the shells or other 

 skeletal elements of invertebrates, and the bones of vertebrates, were fre- 

 quently preserved in some form or other. Very commonly, their original 

 mineral constituents were leached out by ground water and gradually re- 

 placed by other minerals, to form hard and resistant petrifactions. Such 

 fossils, differing in composition from the surrounding rock, are conspicuous 

 when the rock is broken or eroded away. The shells of mollusks and brachi- 

 opods, and calcareous tubes of various annelids, often became filled with 

 mud or silt upon the death and disintegration of the animal. Having turned 

 to stone, this mud filling remains as an internal mold, after the shell itself 

 has dissolved and disappeared. Similarly, external casts of the bodies of 

 animals remain in sedimentary rock, although the bodies have disappeared 

 from within them. 



Delicate and soft-bodied organisms were rarely fossilized, for obvious rea- 

 sons, but a few impressions of the bodies of large jellyfishes have been found 

 in rocks of very fine structure. The bodies of aerial organisms, such as 

 insects, were sometimes carried to the ground and buried by clouds of fine 

 volcanic ash, or fell into bodies of water, to be entombed in sedimentary 

 bottom deposits. The structure of the rock which these deposits subsequently 

 formed is so fine that the delicate patterns of wing venation of the insects can 

 be perfectly traced in their impressions. Some of the most valuable fossils of 

 insects and other small terrestrial invertebrates have been found embedded in 

 amber, which is the beautifully translucent substance formed by the trans- 

 formation of gums exuded from ancient trees. 



The skeletons of a wide variety of vertebrates and invertebrates have been 

 recovered from peat bogs, and from tar pits such as the famous La Brea Pits 

 near Los Angeles, California (Fig. 20.2). Through countless thousands of 

 years, animals have become entrapped in these pits, and their skeletons have 

 been infiltrated and preserved by the oily tar. Under entirely diflferent 

 circumstances, animals of the Glacial Period were sometimes frozen in ice and 

 snow and their entire carcasses, complete with fiesh and hide, preserved for 

 extremely long periods, as were those of several mammoths found in Alaska 

 and Siberia. 



Traces of animals, as distinct from skeletons and impressions of their 

 bodies, are of common occurrence in ancient sedimentary rocks. These traces 

 include footprints of dinosaurs, found in great profusion in certain deposits 

 in Texas and in the Connecticut Valley, as well as the tracks of worms and 

 other invertebrates in rocks which represent the shores or bottoms of ancient 

 seas (Fig. 20.3). 



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