When fossils of the fungus Prototaxites were first discovered in the mid-1800s in Quebec, Canada, the size of this 20-foot-high, towering, cactus-shaped organism caused scientists to classify it as a conifer. Later, researchers argued it was a lichen, fungus or, possibly, algae.

In 2001, more than 150 years after its discovery, Francis Hueber, a paleobiologist at the Smithsonian’s National Museum of Natural History, examined the internal structure of Prototaxites fossils—an interwoven mesh of tiny tubes—with a regular microscope and a scanning electron microscope. Hueber then published a paper identifying Prototaxites as “an extinct form of fungus with sporophores (structures that release spores) that exceed comparable forms living today and exceed the imagination as well....”

Now, a new study by Hueber, Carol Hotton, also of the National Museum of Natural History, C. Kevin Boyce of the University of Chicago and their colleagues has produced new scientific evidence to support Hueber’s theory and finally resolve the identity of this mysterious organism. The team analyzed carbon-12 and carbon-13 isotopes found in Prototaxites fossils and compared their ratios to carbon isotopes in fossils from plants that lived in the same environment some 400 million years ago.

Deriving energy from the sun and carbon from carbon dioxide in the air, plants living in the same environment will typically contain similar carbon-12 to carbon-13 ratios. Prototaxites fossils, however, displayed a much wider variation in the ratio of carbon-12 to carbon-13 isotopes than would be expected in any plant. Rather, its carbon isotope ratio is more indicative of a ground-dwelling fungus that, as it grows, absorbs carbon from a variety of elements in the soil.

“Prototaxites was the most bizarre and, for the greater part of its existence, the largest and tallest element in the terrestrial floras of the Devonian [period],” Hueber wrote in 2001.

Its humongous size may have enabled it “to distribute its spores widely, allowing it to occupy suitable marshy habitat that may have been patchily distributed on the landscape,” Hotton says.