The fossil Prototaxites has mystified scientists for over a century. Renowned for its enormous size, the fossil is also known for its enigmatic history: At various times, it has been classified as a conifer, alga, lichen, and even fungus. But new evidence may have resolved the conundrum. Scientists at the U of C and the National Museum of Natural History are saying that one of the most mysterious fossils ever unearthed is that of a 20-foot-tall fungus.

“The problem has been that, although the fossils are enormous, they are constructed only of very small interwoven tubes.... That filamentous anatomy is consistent with lots of different things,” said C. Kevin Boyce, an assistant professor of paleontology at the University, in an e-mail interview. “[T]here is nothing particularly diagnostic about it to then help you choose between all of those options.”

The researchers tried to shed some light on this controversy by analyzing the organism’s chemistry, according to the research they published in the May issue of the journal Geology. They looked at an element all plants need to undergo photosynthesis and derive energy: carbon. The team examined the ratio of the isotopes carbon-12 and carbon-13 in the fossils.

This isotopic ratio tends to be similar among plants of the same type, since they share similar carbon sources and biochemical properties. In contrast, consumers, such as animals or fungi, can have different isotopic ratios “because they will look like whatever they eat,” Boyce said.

The researchers found that Prototaxites had too much variation in its ratios to be a tree, lichen, or alga, leaving fungus as the most likely classification.

While the idea of a 20-foot-tall fungus may seem unrealistic, the Earth was a very different place when Prototaxites lived, from 350 to 420 million years ago. There were no land vertebrates yet, only worms and wingless insects. The fungus towered over everything else on land, since the tallest trees grew only a couple of feet high.

Boyce noted that fungal colonies still exist, spanning many acres. “The difference is that all of the biomass is below ground, with only little mushrooms poking up to disperse spores. The unusual thing about Protoaxites is having such a large above-ground fruiting body,” he said.

But something like Protoaxites probably couldn’t survive in the modern world because it was a slow-growing organism requiring a high level of environmental stability. “It presumably wouldn’t have been very happy with large animals, forest fires, or ecological changes,” Boyce said.

Boyce said that while the debate about Prototaxites isn’t over, the team’s data is “pretty conclusive and resolves issues that have persisted despite a century of anatomical studies.” He now hopes to conduct research into how Prototaxites may be related to modern fungi. “We haven’t addressed that at all,” he said.

The paper was co-authored by Boyce; Carol Hotton and Francis Hueber of the National Museum of Natural History; Andrew Knoll of Harvard University; and Marilyn Fogel, George Cody, and Robert Hazen of the Carnegie Institute of Washington.