Historical Geology Understanding Our Planet\'s Past

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gluelike substance secreted from their bodies.The organisms ranged from the

Precambrian as early as 3.5 billion years ago to the present, although many did

not become well established until the Cambrian or later. Because of their lack

of a hard shell, amoebas and parameciums did not fossilize well.The radiolar-

ians, which are well represented in the fossil record, usually have siliceous

Figure 19Foraminiferans
from the North Pacific
Ocean.
(Photo by R. B. Smith,
courtesy USGS)

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Historical Geology

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