Fish diversify and proliferate. Armoured fish, lobe finned fish, and sharks make their first appearance. All modern land dwelling vertebrates ultimately derive from lobe finned fish -- so this is big news for humans.
Meanwhile... on land. The advent of vascular plants allowed forests and vegetation to grow without immediate proximity to water. Vascular plants quickly colonised the continents, laying their roots down in lands no other plants could use. This rapid expansion led to some major changes in the Earth's atmosphere, and (at the very least) contributed to a large scale mass extinction at the end of the period.