Crossword Clue:

the beginning of a distinctive period in history

AnswerCrossword Clue
EPOCHDistinctive period of history
SILVERAGESecond-greatest period in the history of something
EONPeriod in the earth's history
EONPhanerozoic ... (current period in the earth's history)
QUATERNARYPertaining to the present period in earth history
PERMIANPeriod marked by the largest mass extinction in earth's history