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From about 420 million to 370 million years ago, a mysterious genus of creatures named Prototaxites grew large trunks up to 3 feet (1 meter) wide and 26 feet (8 meters) in height. Scientists have long debated whether these were some kind of weird ancient trees, but a 2007 study concluded they were fungi, not plants.

"A 6-meter fungus would be odd enough in the modern world, but at least we are used to trees quite a bit bigger," study author and paleobotanist C. Kevin Boyce told New Scientist in 2007. "Plants at that time were a few feet tall, invertebrate animals were small, and there were no terrestrial vertebrates. This fossil would have been all the more striking in such a diminutive landscape."