What was that? Unraveling a 400-million-year-old mystery

Contradictions and puzzles surround the giant fossil Prototaxites. The fossils resemble tree trunks, and yet they are from a time before trees existed. The stable carbon isotope values are similar to those of fungi, but the fossils do not display structures usually found in fungi. Plant-like polymers have been found in the fossils, but nutritional evidence supports heterotrophy, which is not commonly found in plants. These are a few of the confounding factors surrounding the identification of Prototaxites fossils. Since the first fossil of Prototaxites was described in 1859, researchers have hypothesized that these organisms were giant algae, fungi, or lichens. A recent study by Dr. Linda Graham and her colleagues published evidence in the February issue of the American Journal of Botany that they believe resolves this long-standing mystery (http://www.amjbot.org/cgi/content/full/97/2/268).

Prototaxites existed during the Late Silurian to Late Devonian periods-- approximately 420-370 million years ago (ma). Prototaxites fossils have a consistent tubular anatomy, composed of primarily unbranched, non-septate tubes, arranged in concentric or eccentric rings, giving the fossils an appearance similar to that of a cross-section of a tree trunk. The fossil "trunks" vary in size and may be up to 8.8 m long and 1.37 m in diameter, making Prototaxites the largest organism on land during the Late Siluarian and Devonian periods.

Graham and her colleagues hypothesized that Prototaxites fossils may be composed of partially degraded wind-, gravity-, or water-rolled mats of mixotrophic (capable of deriving energy from multiple sources) liverworts that are associated with fungi and cyanobacteria. This situation resembles the mats produced by the modern liverwort genus Marchantia. The authors tested their hypothesis by treating Marchantia polymorpha in a manner to reflect the volcanically-influenced, warm environments typical of the Devonian period and compared the resulting remains to Prototaxites fossils. Graham and her colleagues investigated the mixotrophic ability of M. polymorpha by assessing whether M. polymorpha grown in a glucose-based medium is capable of acquiring carbon from its substrate.

via Paleontology news: What was that? Unraveling a 400-million-year-old mystery.

Answer from 2007:

C. Kevin Boyce, Assistant Professor in Geophysical Sciences at the University of Chicago, with a slice of Prototaxites, a giant prehistoric fungus. The computer screen shows a chemical map of a magnified portion of the organism's cell walls. (Credit: Lloyd DeGrane photo)

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." ...

- scidaily