Giant Fossil Fungus

An interesting determination, from Eurekalert:

"Scientists at the University of Chicago and the National Museum of Natural History in Washington, D.C., have produced new evidence to finally resolve the mysterious identity of what they regard as one of the weirdest organisms that ever lived.
"Their chemical analysis indicates that the organism was a fungus, the scientists report in the May issue of the journal of Geology, published by the Geological Society of America. Called Prototaxites (pronounced pro-toe-tax-eye-tees), the organism went extinct approximately 350 million years ago.
"Prototaxites has generated controversy for more than a century. Originally classified as a conifer, scientists later argued that it was instead a lichen, various types of algae or a fungus. Whatever it was, it stood in tree-like trunks more than 20 feet tall, making it the largest-known organism on land in its day.
"'No matter what argument you put forth, people say, well, that’s crazy. That doesn’t make any sense,' said C. Kevin Boyce, an Assistant Professor in Geophysical Sciences at Chicago. 'A 20-foot-tall fungus doesn’t make any sense. Neither does a 20-foot-tall algae make any sense, but here’s the fossil.'
"The Geology paper adds a new line of evidence indicating that the organism is a fungus. The fungus classification first emerged in 1919, with Francis Hueber of the National Museum of Natural History in Washington, D.C., reviving the idea in 2001. His detailed studies of internal structure have provided the strongest anatomical evidence that Prototaxites is not a plant, but a fungus.
"'Fran Hueber has contributed more to our understanding of Prototaxites than anyone else, living or dead,' said Carol Hotton, also of the National Museum of Natural History. 'He built up a convincing case based on the internal structure of the beast that it was a giant fungus, but agonized over the fact that he was never able to find a smoking gun in the form of reproductive structures that would convince the world that it was indeed a fungus,' Hotton said." ...
"Now Boyce, Hotton and their colleagues have produced independent evidence that supports Hueber’s case. The team did so by analyzing two varieties—isotopes—of carbon contained in Prototaxites and the plants that lived in the same environment approximately 400 million years ago.
"The metabolism of plants is limited by photosynthesis. Deriving their energy from the sun and their carbon from carbon dioxide in the air, any given type of plant will typically contain a similar ratio of carbon-12 to carbon-13 as another plant of the same type. 'But if you’re an animal, you will look like whatever you eat,' Boyce said. And Prototaxites displayed a much wider variation in its ratio of carbon-12 to carbon-13 content than would be expected in any plant.
"Geological processes can alter the isotopic composition of fossils, but Boyce and his colleagues conducted tests to verify that the carbon isotopic composition of the specimens they analyzed stemmed from organic rather than geologic factors.
"As for why these bizarre organisms grew so large, 'I’ve wondered whether it enabled Prototaxites to distribute its spores widely, allowing it to occupy suitable marshy habitats that may have been patchily distributed on the landscape,' Hotton said."