When Giant Funguses Ruled the Earth

About 400 million years ago, during the Devonian period, the world was a very strange place.

Green plant life had begun to cover the land surfaces of the Earth, but these plants were small, most of them ferns, and they only grew a foot or two high at most. This was long before the age of the dinosaurs, and there were no animals with backbones anywhere on land.

In this placid setting, there arose a form of life that was stunning in its size and bizarre in its appearance. For more than a century, scientists have puzzled over its fossils, trying to figure out exactly what it was.

The fossils are large mineralized columns, like tree trunks, sometimes reaching eight or more meters (24 feet plus) in length. They have been found scattered around the globe, in places like Canada, Australia and Saudi Arabia.

At first these life forms were thought to be trees – conifers. Their name, Prototaxites (pronounced “pro-toe-tax-eye-tease”) means “early yew tree.” But they had no branches. They were eventually determined not to be “vascular plants,” the term used for higher plants with conducting tissues that allow resources like water, minerals and photosynthesis products to circulate throughout the organism. Later theories characterized Prototaxites as a lichen, or perhaps algae, or fungus.

In 2007, scientists from the University of Chicago and the Smithsonian’s National Museum of Natural History in Washington, D.C., announced they had finally solved the mystery of “one of the weirdest organisms that ever lived”: Prototaxites was indeed a fungus – a giant, tree-trunk-like fungus.

The fungus theory had first been advanced in 1915 and was revived in 2001 by Francis Hueber of the Smithsonian. Hueber’s research has played a large part in the latest determination. The Chicago and Smithsonian researchers found independent evidence supporting Hueber’s contention that the life form was a fungus:

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,” [Chicago’s C. Kevin] 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,” [the Smithsonian’s Carol] Hotton said. The relatively simple Devonian ecosystems certainly seemed to contain nothing to prevent them from growing slowly for a long time. Plant-eating animals had not yet evolved, Boyce said. But even if Prototaxites hadn’t been eaten by the dinosaurs and elephants that came much later, they probably grew too slowly to rebuild from regular disturbances of any kind, Boyce said. “It’s hard to imagine these things surviving in the modern world,” he said. 

So it’s official: The Devonian was the period when giant funguses ruled the Earth.

But wait a minute – someone has come up with another theory challenging the experts.

In 2010, Linda E. Graham of the University of Wisconsin and several colleagues published a paper presenting evidence that the tree-trunk-style fossils of Prototaxites were not towering columns of fungus after all, but rather rolled-up liverwort mats (similar to rolls of grass sod).

As a recent “Catalogue of Organisms” blog post describes the new theory:

In Graham et al.'s estimation, Prototaxites should not be classed with the fungi but with the liverworts. Liverworts are small, often mosslike plants of moist habitats. Members of one group of liverworts, the thallose liverworts, lack any distinction between leaves and stem but grow as a flattened thallus anchored to the ground by rhizoids (rootlets) on the lower surface…. But modern liverworts lack strong supporting tissue and would be pushing to reach an inch in height - how could they have produced the eight-metre columns recorded for Prototaxites? A transverse section of Prototaxites shows a ring structure like that found in a tree trunk. Hueber (2001), who interpreted Prototaxites as a perennial fungal fruiting body, felt that this ring structure also resembled tree rings in indicating discontinuous growth by the organism. Graham et al. (2010) interpret the ring structure differently. They suggest that large mats of thallose liverworts covered the Silurian landscape. These mats could become detached from their substrate by agents such as wind and rain, and start to roll up as they decayed. As they rolled, they would form the large columns that, after being compressed by burial and fossilised, would eventually be identified as Prototaxites.

The blog author, an entomologist, finds this new interpretation “intriguing, if a little difficult to accept outright.”

He adds:

Prototaxites is represented by a reasonable number of specimens (I don't know the actual number, but thirteen species have been named from numerous localities around the world) - were the conditions that would have lead to mat-rolling common enough to have produced that number of fossils? I wonder if it would be worth investigating how Prototaxites specimens compare in abundance to nematophyte specimens and what that might tell us about the likelihood of 'Prototaxites' formation from liverwort mats. Certainly, the only thing that could be more intriguing than the existence of these giant pillars from so early in the earth's history would be if it turned out that they never existed at all.

On a personal note, I’ve inspected large Prototaxites fossils recovered by Saudi Aramco geologists from an area in central Saudi Arabia. They do indeed resemble tree trunks, and one can easily imagine them standing tall, some 400 million years ago, in the region today called Najd, swaying slightly in the wind, dominating the ferny landscape for miles around.

It is much less easy to imagine them as the fossilized equivalent of fruit rollups….