E seems the answer. Extinctions only can mark the end of a period.
I don't know.. what if some animals/plants go extinct today without the meteor, would we mark an end of period, or would we mark it only when one definite catastrophe occurs.
Meteor slamming the earth was a major event because of extinctions. If extinctions did nt happen after slamming, then we can hardly say that slamming marked the end of that period. The significance to slamming came because of extinctions. Hence extinctions could have marked the end of period.
The path is long, but self-surrender makes it short;
the way is difficult, but perfect trust makes it easy.