Answer:

miocene

Answer	Crossword Clue
MIOCENE	Tertiary epoch
MIOCENE	Epoch with big birds
MIOCENE	Long-ago epoch

MIOCENE	Epoch in which grazing mammals became widespread
MIOCENE	Geological epoch
MIOCENE	Period of the Age of Mammals.
MIOCENE	Geological period.
MIOCENE	Division of the tertiary system.
MIOCENE	Geological period
MIOCENE	A Tertiary epoch
MIOCENE	Geological epoch when mastodons appeared

MIOCENE	A geological epoch
MIOCENE	When mammals appeared
MIOCENE	Epoch that produced grazing animals