A Fungi Lichened to a Tree

A Fungi Lichened to a Tree: When Giant Mushrooms Roamed the Earth

fossil

Back 416 million years ago, the world was a very different place. The complex menagerie of life we are surrounded by on a daily bases, all the extant plants and animals we’ve ever known, didn’t exist in its current form; instead, it was supplanted by something much more primitive and certainly more alien to us. If you were to somehow find yourself standing in the middle of a Devonian landscape, you might find it hard to believe you were still on Earth. The first land plants were just beginning to their slow crawl onto shore, and instead of the verdant, luscious plants we have today, these were small, creeping, and leafless, obtaining a height of no more than two or three feet. They grew in dense clumps, which likely aided them in growing upright, since they were mainly composed of thin bifurcating stems. This would have seriously impeded walking for any sort of bipedal organism like you and me, but at the time this wasn’t a problem, because there were no large land animals; instead, the tiny forests of Rhyniophytes and Zosterophylls were inhabited by small invertebrates. These included early relatives or our modern-day spiders, small mites, crustaceans, and centipedes (Paul and Kenrick, 2004). But there was something else that towered above this vernal panoply, by far the biggest thing on land for millions of years: a giant mushroom!

The taxonomic history of this mushroom is varied and interspersed with several misidentifications, vitriolic backlash from a prominent botanist, and one humorous attempt at obfuscation. Fossils were first discovered in a secluded area of Gaspe Bay, Quebec in 1843. In an area called Seal Cove, W.E. Logan, tasked with mapping the Gaspe Bay for possible coal and minerals, unearthed several fossils that dated back to the Devonian. These remained in his possession for some twelve years until they sparked the interest of a Paleontologist, J.W. Dawson. Dawson was particularly intrigued by a specimen that appeared to be composed of rotting wood, with what looked like fossilized fungal hyphae eating away at it. After taking a trip to Seal Cove to see the fossils for himself, he named the specimen Prototaxites (first yew) loganii (after its discoverer), believing that the wood was an early relative of modern conifers (Dawson, 1859; Heuber, 2001).

While sharing his discoveries at a conference in London, Dawson handed away specimens to whoever showed an interest in the fossils he had collected. He gave sections of Prototaxites to a respected botanist of the time, William Carruthers, who pointed out a few incongruences in his observations. While Dawson seemed to take these comments in stride, he refuted them in a later publication, prompting Carruthers to write his own scathing article on Prototaxites in which he renames the organism, under the assumption that it was actually a giant alga, while also severely criticizing Dawson. At one point, in response to Dawson’s observation on what appeared to be a fibre in the fossil, Carruthers writes, “If Dr. Dawson knew anything whatever about a vegetable cell, and the formation of the spiral fibre in its interior, he would not have written such nonsense…” (Carruthers, 1872).

Once the scientific community at large became aware of the existence of these fossils, other theories emerged about the possible identity of Protaxites as well, including that it might have been a type of lichen (Burnie et al., 2012). Dawson remained steadfast in his interpretation, however, for many years, but in 1889, he apparently suffered a change of heart, admitted Protaxites was a poor choice of a name, and denied ever having associated the fossils with conifers, despite having directly done so in his published papers (Dawson, 1859; Heuber, 2001).

The scientific community at large now generally accepts that Protaxites was a giant fungus, but it took a good 158 years to come to this conclusion (Hueber, 2001). The fungal hyphae seen by Dawson weren’t eating away at the wood of a conifer; they were actually tightly woven masses of mycelium that made up the body of the fungus. It could reach up to 26ft in height! This maybe isn’t all that surprising, given that the largest organism in existence today is a fungus that stretches about 3.73 square miles underground in Oregon (Schmitt & Tatum, 2008). But the upright growth pattern of Prototaxites is strange for a fungus of its size. It is believed to have been saprophytic, meaning that it obtained nutrients from the decaying matter of the plants that grew and died at its base, and possibly from rivers that deposited minerals in floodplains (Heuber, 2001). Protaxites existed for about 50 million years (250X as long as humans have been around). By the late Devonian, however, Prototaxites likely became outcompeted by larger plants, including the large tree lycophytes (Lepidodendron) and the giant horsetail ferns, both of which are now extinct and deserving of their own story! Coming soon!

 

Burnie, D., Cleal, C., Crane, P., & Thomas, B. (2009). Devonian. In Prehistoric Life (pp.114-115). New York, New York: DK Publishing.

Carruthers, W. (1872). On the history, histological structure, and affinities ofNematophycus logani, Carr. (Prototaxites logani, Dawson), an alga of Devonian age. The Monthly Microscopical journal, 8 (4), 160-172.

Dawson, J. W. (1859). On fossil plants from the Devonian rocks of Canada. Quarterly Journal of the Geological Society of London 15: 477-488.

Hueber, F. M. (2001). Rotted wood-alga-fungus: the history and life of Prototaxites Dawson 1859. Review of Palaeobotany and Palynology, 116, 123-158.

Kenrick, P., & Davis, P. (2004). Fossil plants. Washington [D.C.: Smithsonian Books in association with the Natural History Museum, London.

Schmitt, C. L. & Tatum, M. L. (2008). The Malheur National Forest, location of the world’s largest living organism (the humongous fungus). United States Department of Agriculture. http://www.fs.usda.gov/Internet/FSE_DOCUMENTS/fsbdev3_033146.pdf

 

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