430 million years ago, towering life-forms known as Prototaxites emerged from the ground, reaching heights of up to 26 feet and growing trunks up to 3 feet wide. When the first Prototaxites fossil was unearthed in 1843, it was mistaken for an ancient rotting conifer. But trees didn’t yet exist during the Silurian period, so what exactly was this thing taking over the ancient supercontinent of Gondwana?

That argument was never truly settled. Most scientists made Prototaxites out to be a sort of humungous fungus, but the field as a whole was never certain. That, however, may have just changed.

Back in 2007, the carbon isotopes in fossils convinced scientists this really was a fungus, since they appeared to be evidence that Prototaxites behaved like fungi by leeching off other living organisms for oxygen. Videos with clickbait titles like When Giant Fungi Ruled the Earth soon spread through the internet. But now, a new analysis by researchers at the University of Edinburgh is showing otherwise.

When paleobiologist Corentin Loron and his colleagues examined Prototaxites fossils from what is now Aberdeenshire, Scotland, they found evidence that it was anatomically and chemically differed from fungi in too many ways to be considered fungal. The problem—it also belonged with nothing else. It was apparently not a plant, animal, or fungus.

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“No extant group was found to exhibit all the defining features of Prototaxites,” Loron said in the study, suggesting that “it is best considered a member of a previously undescribed, entirely extinct group of eukaryotes.”