27 



as we know it could not occur. As a necessity for life, then, water 

 serves also to limit life, and one of the principal themes that charac- 

 terized the evolution of both plants and animals during Phanerozoic 

 time was the development of structures and biochemical processes 

 that enabled these life forms to spread to the land surface where 

 water was in short supply. Indeed, many of the major evolutionary 

 innovations of the Phanerozoic concerned the relations between 

 water and life, such matters as the developments of lungs and of 

 hardshelled eggs in animals, and the origin of seeds and of specialized 

 pollenation mechanisms in plants, both once novel biological solu- 

 tions to the scarcity of water on land. 



Another important generalization concerning the fossil history 

 of life is its unevenness. Some portions of the record are well docu- 

 mented and understood, but others are nearly unknown. It is a gen- 

 eral, but not a perfect rule, that the older the material, the poorer 

 the record. For markedly different phases of the fossil record, 

 "eons" can be recognized: 



1. The classical fossil phase: The Phanerozoic Eon, extending 

 from about 600 m.y. ago to the present, is far better understood 

 than any of the earlier phases; literally thousands of richly fossilifer- 

 ous deposits of this age are known, units that collectively provide a 

 sound and rather detailed basis for understanding the major aspects 

 of the history of life. 



2. Before the classical fossils: The Proterozoic Eon, extending 

 from about 2500 m.y. ago to about 600 m.y. ago, when the Phanero- 

 zoic began, is understood only in outline; the total fossil record now 

 known consists of three kinds of deposits: (a) There are about a 

 dozen latest Proterozoic (690 to 600 m.y. old) fossiliferous deposits 

 with large organisms primarily as sandstone impressions, (b) There 

 are also about 150 microfossiliferous deposits (fig. III-2), spread 

 somewhat unevenly throughout the eon (very roughly, 10-25 depos- 

 its per 100 m.y. during the later Proterozoic and only 1-5 deposits 

 per 100 m.y. within the earlier portion of the eon). And (c) there are 

 hundreds of limestones and dolomites that contain the structures 

 called stromatolites. These structures are layered, commonly mound- 

 shaped, sedimentary rocks. They were built over time through the 

 growth and metabolic activities of whole communities of micro- 

 scopic organisms; but, with rare exceptions, they do not contain 

 fossil remnants of the bodies of the microorganisms responsible for 



