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http://en.wikipedia.org/wiki/Oligocene

Third epoch of the Paleogene Period The Oligocene [ a ] is a geologic epoch of the Paleogene Period that extends from about 33.9 million to 23 million years before the present ( 33.9 ± 0.1 to 23.04 ± 0.05 Ma ). As with other older geologic periods, the rock beds that define the e…