What it was
A tree you would recognize. It had a trunk of real wood, much like a conifer's, flat leafy branches, and roots that reached down into the rock. It grew more than 20 meters tall, seven stories, and the biggest perhaps ten. Within a few million years it was growing from the tropics almost to the poles.
What its roots did
They spread about ten meters out from the trunk and pushed into cracks in the rock, splitting it open. Broken rock, fallen leaves and fungi make soil. Before trees, soil was a few centimeters thin. Under these forests it grew to about a meter deep.
What it may have cost
Rain washed nutrients from that broken rock into the sea, where algae bloomed, died and used up the oxygen in the deep water. Forests are the leading suspect for the crises that ended the armored fish. Scientists still argue about how much of the blame they carry.