Eon

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In general usage, an Eon is a very long period of time. Geologists refer to a Phanerozoic Eon which is about 550 million years long and covers the period of time during which animals with hard shells that fossilize well wave been abundant. Less frequently, they refer to a Precambrian Eon that covers the four billion years of Earth history prior to the appearance of hardshelled animals.