The Ordovician -- Silurian Extinction Events may have been caused by an ice age that occurred at the end of the Ordovician period , due to the expansion of the first terrestrial plants , as the end of the Late Ordovician was one of the coldest times in the last 600 million years of earth history .
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 .