The Ordovician period
After the die out at the end of the Cambrian, during the beginning of the Ordovician, there was an increase in the variety of animals and plants, with many new species entering the fossil record. Chordates became fish, plants colonized the land, corals began forming reefs, and the cephalopods came into their own. At the end of the period, wild climatic shifts caused the second most severe extinction event in the history of life.
Here are a couple of articles about the Ordovician period.