Simplify the following text for a Grade 11 native English-speaking person.
Original: The Ordovician-Silurian Extinction Events may have been caused by an ice age that occurred at the end of the Ordovician period as the end of the Late Ordovician was one of the coldest times in the last 600 million years of earth history .
Simplified: The extinctions may have been caused by an ice age that occurred at the end of the Ordovician period : the end of the Ordovician was one of the coldest times in the last 600 million years of earth history .