from or referring to the period of time between about 34 and 23 million years ago

Fossils of an especially rich array of Oligocene mammals have been unearthed here. 这里出土了特别丰富的渐新世哺乳动物的化石。

Fossilized leaves may provide information on elevation changes since the end of the Oligocene period. 树叶化石可以提供自渐新世末期以来的海拔变化信息。

the period of time between about 34 and 23 million years ago

The Oligocene was a period of global cooling, volcanic eruptions, and seismic disturbance. 渐新世是一个全球变冷、火山喷发和地震扰动的时期。

The specimen is from a young female that lived in the early Oligocene. 该标本是一个生活在渐新世早期的年轻雌性个体。