THE EVOLUTION OF ANIMAL LIFE 



species represented by fossils include only a tiny fraction of the animals and 

 plants that have inhabited the earth. All in all, however, the fossil record 

 constitutes our best source of information about the forms of life existing in 

 past times. 



Aside from the information they yield about the characteristics of individual 

 animals, fossil remains from sedimentary rocks make possible the establish- 

 ment of an orderly chronology for extinct types of animals. In view of the 

 manner in which sediments are deposited in water, it is obvious that the oldest 

 layers, or strata, will be found at the bottom and the youngest at the top. 

 This is true even though entire deposits have been elevated thousands of feet 

 above sea level, subsequent to the time of their original formation. In many 

 places this elevation has occurred without disturbance of the horizontal re- 

 lationships of the strata, and it is possible to recognize not only the fossils 

 but also traces of ancient topography, as when there are signs of beaches, 

 mud flats, or deep waters. In other parts of the earth, particularly in great 

 mountain ranges, strata that were originally horizontal have been folded 

 during their elevation. The arrangement of the layers may be further compli- 

 cated by the occurrence of breaks in the strata, known as faults, and by the 

 horizontal slippage of one side of such a fault to overlie the other. By study- 

 ing the rock outcrops in all parts of the world, with their included fossils, 

 geologists have been able to piece together a series of sedimentary formations, 

 the lowest resting upon earlier igneous rocks. This series constitutes the 

 record of geologic history. The positions of the fossils in these strata indicate 

 the order of appearance, or geologic succession, of the various forms of life. 

 The relative ages of the different deposits are determined by their positions, 

 and the absolute ages of the strata may be calculated, within certain limits 

 of error, by special physical methods. 



In the fossil record as thus set forth chronologically, there is a succession 

 from simpler to more complex and more specialized types of animals (see 

 Fig. 20.1). This sequence is of the greatest significance as evidence for 

 organic evolution. Not only are the animals of the past different from those 

 of the present, but the fossil record begins with forms that are vastly diflerent. 

 These are gradually succeeded by others that become more like modern forms, 

 until they merge with those now living. There are many gaps, and many 

 forms known as fossils have no close relatives among the modern fauna, but 

 the geologic succession shows the grand course of evolution so far as it can 

 be ascertained from the fossil record. 



Special Examples of Geologic Succession. Invertebrates. There are 

 many instances among invertebrate groups, particularly in moUusks and 

 brachiopods, where the succession of fossil remains is comprehensive enough to 

 show in some detail the various courses taken in evolution. A good example 

 is furnished by the ammonoids, a large group of cephalopod mollusks which 

 flourished in Paleozoic times (see p. 388) but are now extinct (Fig. 20.4). 

 The ammonoids can be traced to a common ancestry with the nautiloids, 

 another cephalopod group from which they diverged during the Silurian. 



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