Answer: Fungus

Long before trees became the lofty and sky-dominating organism they evolved into, there was another towering organism dotting the temperate plains of the globe: giant fungi. Millions of years before Earth would turn into a lush and largely forested biosphere, the tallest organisms around were fungus of the genus Prototaxites.

420 to 370 million years ago, these towering fungi grew three feet in diameter and twenty six feet tall, by far the largest plant around. Although they eventually died out with the rise of larger trees and expanding forests, the Prototaxites have an interesting connection to the trees that replaced them. There is evidence that for millions of years before the trees took over, organisms were boring into the large trunks of the fungi, a skill they took with them with the rise of the trees; early tree-boring organisms, then, cut their evolutionary teeth on huge fungus.