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