oligocene

English Thesaurus

1. from 40 million to 25 million years ago; appearance of sabertoothed cats (noun.time)

instance hypernym	:	epoch,
definition	:	a unit of geological time that is a subdivision of a period and is itself divided into ages (noun.time)

part meronym	:	tertiary, tertiary period,
definition	:	from 63 million to 2 million years ago (noun.time)