According to science news, Long Before Trees Overtook the Land, Earth Was Covered by Giant Mushrooms.
So like the Shivering Isles?
Honestly, after reading the article, I had a hilarious mental picture of a bunch of bespectacled men in labcoats hucking grocery-store mushrooms at each other while they argued whether a fungus, lichen, or other similar organism could grow to 8 meters. It’s a valid question, but imagining a reasoned debate around a microscope was too tame. (Although I suppose if the thing’s 8 meters the microscope wouldn’t always be necessary. Stereotypes ftw!)
Seriously though, I love it when scientific data leads people to painting pictures of what the planet looked like before we started writing things down. Barring time travel, we have no way of knowing – and that’s awesome.