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Francis Hueber of the National Museum of Natural History first suggested the fungus possibility based on an analysis of the fossil's internal structure, but had no conclusive proof.

Boyce and colleagues filled in the blanks, comparing the types of carbon found in the giant fossil with plants that lived about the same time, about 400 million years ago.

If Prototaxites were a plant, its carbon structures would resemble similar plants. Instead, Boyce found a much greater diversity in carbon content than would have been expected of a plant.


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Fungi, which include yeast, mold and mushrooms, represent their own kingdom, neither plant nor animal. Once classified as plants, they are now considered a closer cousin to animals but they absorb rather than eat their food.

Samples of the giant fungi have been found all over the world from 420 million to 350 million years ago

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The discovery of the world's largest fungus - up to 8,500 years old and carperting nearly 10 square kilometres of forest floor - has raised questions about what constitutes an individual organism.

A study of a tree-killing fungus in rugged northeast Oregon, USA, found that a single individual covered an area equivalent to about 1,600 football fields, according to a report in the current issue of the Canadian Journal of Forest Research.

"The fact that an organism like this has been growing in the forest for thousands of years really expands our view of the forest ecosystem and how it works," said Dr Catherine Parks, a pathologist at the U.S. Department of Agriculture who led the study. "From a broad scientific view, it challenges what we think of as an individual organism."

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