At a time when the tallest trees stood just a few feet high, giant "mushrooms" towered over the landscape.

That's the finding being reported by new a paper appearing in the May issue of the journal Geology.

he study adds to the quest to solve a long-standing scientific puzzle: the true nature of a fossil that was the world's largest organism from about 420 million to 370 million years ago.

Called Prototaxites, the mystery life-form was first reported in 1859 based on samples found in Canada.

The ancient organism boasted trunks up to 24 feet (8 meters) high and as wide as three feet (one meter).

Prototaxites was widespread - its fossils are found all over the globe.

Lead study author Kevin Boyce, of the University of Chicago, said the unidentified monstrosity was a staple in textbooks while he was still in school.

"It's fun because it's kind of a classic specimen that people have worried about for a long time," Boyce said. "It's been an outstanding question for 150 years."

Chemical Clues

Since the fossil's discovery, researchers have speculated that Prototaxites was a type of algae or lichen or even a primitive pine tree.

The idea that the fossil could have been a giant fungus first emerged in 1919.

©Illustration by Mary Parrish, National Museum of Natural History; photograph reprinted from Review of Paleobotany and Palynology, Vol. 116, "Rotted wood--alga--fungus: the history and life of Prototaxites Dawson 1959," by Francis Hueber, p. 146, Smithsonian Institution, Copyright 2001, with permission from Elsevier
Giant stalks of Prototaxites rise above the landscape in an artist's rendition of Earth 420 million to 370 million years ago (top).

New research on the mystery fossil, like this sample found in Saudi Arabia (bottom), backs up earlier theories that Prototaxites was a massive fungus that stood up to 24 feet (8 meters) tall.

Francis Hueber, a co-author of the new study with the Smithsonian Institution, revived the fungus idea in 2001.

Hueber was among the researchers who, as early as the 1970s, were studying chemical signatures called isotopes in the organism.

In plants like today's trees and flowers, which get energy from the sun and carbon from the air, two particular carbon isotopes should be in balance.

In plants and animals that eat other life-forms, the isotope ratio should vary widely.

That fact turned out to be an important clue: The fossil's combination of isotopes revealed the funguslike habit of feeding on decaying organic matter.

In addition, co-authors at the Carnegie Institution in Washington, D.C., found that Prototaxites fossils contain isotopes from primitive plants of the age as well as certain soil organisms.

Occasional stalks probably sprouted from a vast underground network of hyphae - the fungal equivalent of roots - noted the University of Chicago's Boyce.

The fungus probably grew slowly to attain such large sizes, he added.

It was likely aided by a relatively slow turnover of plant types. In today's world, ecosystems can change quickly, with grasslands giving way to forests in the space of a few years.

Size Matters

Patricia Gensel is a professor of paleobotany at the University of North Carolina at Chapel Hill who was not involved in the new study.

She said the new paper proposes a workable solution to one of science's big hangups about Prototaxites being a fungus: Its reproductive parts seem too big by modern fungus standards.

"If the 'logs' of Prototaxites represent a fruiting body [the fungal reproductive organ], it is huge - bigger than any modern individual fruiting body," she said.

The other plants that coexisted with the massive fungus were at most 6.5 feet (2 meters) tall, meaning their remains wouldn't have provided enough nourishment to support fungi with large fruiting bodies.

"It is felt by some that they alone do not provide a sufficient source of carbon for it," Gensel said.

The study shows that the huge fungus had another source of food: soil microbes called crusts.

"Sucking up carbon from microbial crusts would [make large fruiting bodies] possible."