[ol-i-goh-seen] Geology.
noting or pertaining to an epoch of the Tertiary Period, occurring from 40 to 25 million years ago. See table under geologic time.
the Oligocene Epoch or Series.

1855–60; oligo- + -cene

post-Oligocene, adjective
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World English Dictionary
Oligocene (ˈɒlɪɡəʊˌsiːn, ɒˈlɪɡ-)
1.  of, denoting, or formed in the third epoch of the Tertiary period, which lasted for 10 000 000 years
2.  the Oligocene the Oligocene epoch or rock series
[C19: oligo- + -cene]

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Word Origin & History

1859, "pertaining to the Tertiary period between the Eocene and the Miocene," coined in Ger. (1854), from Gk. oligos "small, little, few" (of unknown origin) + kainos "recent" (see recent).
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Science Dictionary
Oligocene   (ŏl'ĭ-gō-sēn')  Pronunciation Key 
The third epoch of the Tertiary Period, from about 37 to 24 million years ago. During this time there was an increase in volcanic activity, and Australia and South America separated from Antarctica. The climate started to cool and a glacier started to form in Antarctica. Modern mammalian groups continued to develop, and the first cats, dogs, horses, and related mammals appeared. Artiodactyls (even-toed ungulates) took over from the perissodactyls (uneven-toed ungulates) as the dominant medium-sized herbivores. Many types of grass also first appeared at this time. See Chart at geologic time.
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