Towering Mystery Fossil Was a 'Shroom With a View
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. The 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.
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."
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