Even today, fungi are the largest organisms on Earth. There is said to be one (underground, that is) that spans a giant expanse of the Upper Peninsula of Michigan, I believe.
That is a fungal mat, no? I think you're right that it's the largest single organism on earth, but according to this it's in Oregon. This Wikipedia entry points out that "the largest organism" depends on how it is being measured. Either way, "Humongous Fungus" is an apt name.
Yeah, that would be something to see. Imagine the sporing though . . . if it did spore. Like this but way bigger. Contemporary conifers are bad enough, especially in a heavily forested area. Man, last year it was everywhere in my part of New England. Sticky, yellow pine spooge. Everywhere. What I found interesting about the Wikipedia entry on prototaxites was that it mentioned that there was evidence of animals making their homes in the trunks. I know animals do this within trees, but my imagination is just running wild with what that would have looked like. I don't know if you're a gamer, or if you've checked out Minecraft, but my favorite biome is the mushroom biome: Those cow things are called mooshrooms. Anyway, I'm really glad that people study ancient organisms. It really puts into perspective how unlikely our contemporary world is.