ERA PERIOD 



EVOLUTIONARY SEQUENCE 



Cambrian 



450,000,000 



Only 



aquatic 



animals 



Invertebrates 



550,000,000 



1,450,000,000 



S Pre-Cambrian 



Lower 

 Pre-Cambrian 



2,000,000,000 

 plus 



Fig. 20.1. The geologic column. At the right are shown the approximate times of origin ol 

 the major groups of animals, and suggestions of the variations in their relative abundance 

 during different periods. The figures given under Years refer in each case to the time 

 elapsed since the beginning of the period. The Cenozoic periods Paleocene through Pliocene 

 are sometimes referred to as the Tertiarv, the Pleistocene and Recent periods together 

 constituting the Quaternarv. (Adapted from O. D. von Engeln and K. E. Caster. Geoloay. 

 copvright 1952 bv McGraw-Hill Book Co., Inc., printed bv permission.) 



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