When Giant Fungi Ruled


420 million years ago, a giant rose along
the river banks of what would become North America. It feasted on the dead, growing slowly into
the largest living thing on land. This towering colossus wasn’t animal, plant,
or mineral. Instead, it belonged to an unlikely group
of pioneers that ultimately made life on land possible — the fungi. If you’re anywhere on land right now — like,
not on a fishing boat or a floating oil rig — then all of the life that you see around
you is there, because of fungi… … fungi like the small, unassuming little
decomposer known as Tortotubus protuberans. It’s one of the earliest known fungi in
the fossil record, dating back 440 million years, to the early Silurian Period. Tortotubus grew near coastlines and rivers
on the supercontinents of Gondwana and Laurentia — in regions that would eventually become
New York, Saudi Arabia, Sweden, and Scotland. But back then, life in these places was anything
but lush. On land, signs of life were sparse — there
were bacteria, algae, early plants like liverworts, and possibly some of the first terrestrial
arthropods. But if it weren’t for fungi, things probably
would’ve stayed that way. Because, even though they weren’t very big,
early fungi like Tortotubus had a superpower — the ability to break down almost anything,
using digestive enzymes. Fungi eat by releasing enzymes that break
down organic matter. This allows them to take the nutrients they
need from the material, while helping the rest to decompose. But! These enzymes can be so powerful that they
can eat into solid rock. Given enough time, and some help from erosion,
many fungi can weaken rocky earth and eventually form fertile soil that plants can sink their
roots into. So big, complex life on land — like vascular
plants — probably wouldn’t have been possible if Tortotubus and its fungal predecessors
hadn’t prepared the land first. Once plant life started to spread and diversify
on land in the Devonian Period, fungi were there to help again. Some scientists think that the first land
plants used symbiotic fungi in their roots to help them gather and transport nutrients. In fact, most plants still do this today. If you pull a plant out of the soil, you’ll
probably see a lot of dirt sticking to little white hair-things on the roots. Those hair-things are actually bundles of
nutrient-absorbing tendrils — called hyphae — from fungi living symbiotically with the
plant. And these fungal structures are incredibly
important for making arable soil that plants can grow in. They help stabilize soil, retain moisture,
and hang on to nitrogen. So without fungi, soil would just be … non-nutritious
dirt. But ya know what’s weird? Despite their starring role in making life
on land possible, ancient fungi haven’t been studied very much by scientists. That’s probably why, back in the 1800s,
when paleontologists discovered a fossil that was eight times taller than the tallest plants
of its time, no one expected it to be a fungus. Geologists first discovered this enormous
fossil in 1843, during a coal survey in Gaspé Bay, Canada. It was more than 8 meters tall, shaped kind
of like a tree trunk. And it came from a layer of earth that dated
back 420 million years, when the Silurian Period gave way to the Devonian — a good
20 million years after Tortotubus first appears in the fossil record. And for a long time, researchers were totally
stumped by this specimen. Stumped? Is that supposed to be some kind of tree trunk joke? The fossil remained unstudied in a museum
collection for years before a Canadian paleontologist named John William Dawson had found more,
new specimens in the 1850s, and tried to classify them. Dawson thought this thing might be a primitive
conifer tree with some sort of fungus growing on it. So he named it Prototaxites, which means “first
yew,” after the tree he thought it most resembled. Then, in 1872, botanist William Carruthers
wrote a paper saying that Prototaxites couldn’t possibly be a plant. He said it was probably a giant mound of algae,
possibly kelp. And, to his credit, English botanist Arthur
Harry Church also studied Prototaxites in 1919, and he said it probably was a fungus. But nobody really paid much attention to him at the time which is…too bad. So, based on Carruthers’ findings, Prototaxites
was filed under “probably weird algae” for more than a century. Until the 1990s, when Francis Hueber, a curator
at the Smithsonian Museum of Natural History, took yet another look. And he focused on a feature of Prototaxites
that had long puzzled scientists: these strange rings. They looked kind of like growth rings, like
what you find in modern trees. But the rings in Prototaxites were lopsided
and not always concentric. So Hueber examined them under the microscope
and found long, tube-shaped cells that were similar to the hyphae found in modern fungi. He concluded that Prototaxites wasn’t a
plant at all, but rather a member of Team Fungus. And this was the same conclusion of another
study done in 2007, that looked at the chemical makeup of the fossils. Now, how can chemistry tell you whether something’s
a fungus? The answer — as always! — is carbon. We talk about carbon a lot around here, because
carbon is the stuff of life. And different kinds of living things can be
identified by how much carbon, and of what kinds, a fossil contains. The thing that researchers focus on here is
the ratio of various isotopes of carbon, like carbon-12 and carbon-13. Plants have pretty consistent ratios of Carbon-12
to -13, because they all get their carbon from the same place — the CO2 in the air. But things that eat instead of photosynthesize
— like animals and fungi — pick up carbon isotopes from their food, so their ratios
can vary a lot. And the 2007 study compared the isotopes in
several fossils of Prototaxites from different eras and found that those ratios changed radically
over time, probably as its food sources changed. This means Prototaxites was not a photosynthesizer,
but an eater — it got its food from other living things. Now, there’s still a chance that this enormous
not-plant-thing was something else, like a lichen. Despite looking like weird plants, lichens
are actually fungal hybrids that house their own algae or photosynthetic bacteria. So far, no one’s come up with a good way
to prove whether Prototaxites was a pure fungus or a lichen. But, either way, it was a giant in its day. And it marks the peak of fungi’s reign over
life on land. When they first appeared in the Early Devonian,
the fungal spires of Prototaxites towered over everything else that grew, burrowed,
and crawled around them. Like the humble Tortotubus, Prototaxites fed
on dead stuff. But unlike its tiny predecessor, the giant
fungus sent out huge networks of hyphae in all directions, sending food back to its central
pillar. And in turn, it may have been a source food,
and shelter, for early invertebrates. Scientists have even found little “bore-holes”
and tunnels that look like insect burrows in some of its fossils. All told, the heyday of the giant fungus spanned
70 million years — a short time that saw a lot of change. When Prototaxites first arrived on the scene,
420 million years ago, vascular plants had just begun to colonize the land. But by the time they vanished 350 million
years ago, the first trees started to tower over the fungi that had paved the way for
them. No one’s sure why the giant fungus went
extinct. I for one am bummed that I’ll never get
to see one. Some speculate that it grew too slowly to
recover from being chewed on by invertebrates all the time Others suggest that the rise of land plants
brought too much competition for nutrients. Either way, ever since the demise of Prototaxites,
for hundreds of millions of years, fungi have continued to thrive, but with a much lower
profile. Today they’re mostly found in the dark,
close to the surface, and even underground. But the world we live in was made possible
by these fungal pioneers — with their ability to digest rock to create soil, and to derive
life from death. Thanks for putting the fun in fungi with me
today! Now, what do you want to know about the story
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