On: Long Before Trees Overtook the Land, Earth Was Cov...

[retrogradeorbit]

Actually reading the article, came across:

"So science is messy, and despite more than a century of digging, we still don’t really know, for sure, what these huge spires that dominated the ancient Earth really were."

So despite the link-bait title, we still don't know and they may not be giant mushrooms.

[lmm → retrogradeorbit]

Sure, we're not certain, but giant mushrooms are our best guess.