The world's tallest fungus, according to a new paper, was the giant Prototaxites, which towered over the Silurian and Devonian landscapes from around 350 to 420 million years ago.
Fossils for the over 26-foot-tall giant have led to scientific debate over the years, with some experts believing it was a huge plant, algae or lichen. The new study, however, published in the latest Proceedings of the Royal Society B, presents evidence that it was a fungus that likely derived at least some of its hefty nutrition load from bacteria, algae and moss.
The ancient fungus didn't closely resemble any modern species, but it looked surprisingly like a familiar man-made structure.
Each of the ancient enormous fungi "formed large trunks with little evidence of branching, so they would have looked like telephone poles of various sizes," co-author Kevin Boyce told Discovery News.
Like modern fungi, most of the organism would have consisted of an underground network of thread-like structures called hyphae that handle feeding, explained Boyce, an associate professor in the Department of Geophysical Sciences at the University of Chicago.
"The above-ground structure -- what actually gets preserved as a fossil -- would have been involved in spore dispersal, like a mushroom," he added. "For that purpose, the higher off the ground you can get, the farther spores will travel on the wind."
He and his colleagues first began to suspect Prototaxites was a fungus and not a plant after they analyzed carbon isotopes in its fossils. These remaining carbon atoms can suggest what existing, and once-living, organisms ate. Boyce explained that since plants derive their energy from the sun and its carbon from carbon dioxide in the air, the carbon isotope signatures for plants tend to look the same.
"But if you're an animal," he said, "you will look like whatever you eat."
Eight surprising fossil findsWhile Prototaxites was definitely not an animal, its carbon isotopic composition displays much wider variation than what is seen in plants. For the new study, to investigate this conundrum, Boyce and co-author Erik Hobbie of the University of New Hampshire traveled to Lyman Glacier in the Cascade Mountains of Washington state to study a modern fungus called Arrhenia obscurata.
Hobbie explained to Discovery News that "recently deglaciated modern landscapes," such as this part of Washington, "share characteristics with the ancient Silurian/Devonian landscape that was being slowly colonized by land plants for the first time." These shared characteristics include sparse vegetation, a high abundance of fine-grained sediments, and abundant shallow floodplains and pools.
As suspected, the fungus Arrhenia, like Prototaxites, displayed wide isotopic variability. Hobbie said that's because "both would have been feeding on dead organic matter in the sediment."
Patricia Gensel, a professor in the Department of Biology at the University of North Carolina at Chapel Hill, told Discovery News that the new "findings need to be taken seriously." But she believes the case hasn't been closed yet on another hypothesis, proposed by researcher Linda Graham, that the mysterious fossils actually were rolled liverwort mats.
Carol Hotton, a research associate in the Department of Paleobiology at the National Museum of Natural History, told Discovery News that she "whole-heartedly seconds the results" of the new study, but that "it still leaves us with the problem of trying to figure out how Prototaxitesactually functioned as a fungus."
Less controversial is the belief that Prototaxites went extinct at a time when the first forests and terrestrial animal life were spreading and diversifying.
"A Prototaxites trunk would have presumably taken a long time to grow and the ecological stability that would require would have become less available as ecosystems and ecological succession became more complex," Boyce said, adding that burrows in the last known specimens indicate "that something had also figured out how to eat it."
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