Earth's history
(a) Earth's history is divided into eons, eras, and periods. The Ediacaran period was the final period of the Proterozoic Era which ended in the Cambrian period of the Phanerozoic Era. (b) Stages on the geological time scale are represented as a spiral.
Source
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"OpenStax College, Biology. November 14, 2013."
http://cnx.org/content/m44660/latest/?collection=col11448/latest
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