Simplify the following text for a Grade 9 native English-speaking person.
Original: The Miocene boundaries are not marked by a single distinct global event but consist rather of regional boundaries between the warmer Oligocene and the cooler Pliocene .
Simplified: The Miocene boundaries are not set at any particular world wide event . They are set at regional boundaries between the warmer Oligocene and the cooler Pliocene epochs .