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Those Ancient Giant Fungi Weren’t Fungi After All
Those Ancient Giant Fungi Weren’t Fungi After All
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0:00
If you strolled along a shoreline 420 million years ago,
0:03
you would tower over just about every living thing in sight.
0:07
You’d be stepping over tiny plants and bugs,
0:09
most of which wouldn’t even come up to your ankles.
0:11
That is, until you stumbled across one of the world’s first giants:
0:14
an enormous tree-trunk-like organism
0:17
so big that you couldn’t wrap your arms around it.
0:20
This is Prototaxites, and I can tell you it’s not a tree.
0:24
But I also can’t tell you what it is.
0:26
That’s because scientists have spent more than a century
0:28
and a half trying to classify these ancient giants,
0:32
and they still haven’t settled on an answer!
0:34
Over the decades it’s been called “plant”, “algae”, and “fungus”,
0:37
but one hypothesis is gaining traction.
0:40
Prototaxites might be something completely different—
0:43
an entirely unknown lineage of life that
0:46
could deserve a Kingdom all its own.
0:49
[♪INTRO]
0:51
In 1843, a Canadian geologist named William E. Logan collected a
0:56
bunch of plant fossils from a place called Seal Cove in Quebec.
1:00
They were around 400 million years old,
1:02
making them some of the oldest land plants in the fossil record.
1:05
A decade later, Logan passed some of these fossils along
1:08
to another Canadian geologist, John William Dawson,
1:11
who was particularly interested in what appeared
1:13
to be a chunk of fossilized wood from some sort of small tree.
1:17
After close examination, Dawson decided
1:19
that it was indeed the wood of an ancient tree.
1:22
More than likely a partially rotted ancient log,
1:24
judging by the presence of what seemed to be fossilized fungi.
1:28
He identified the tree as a conifer related to modern yews,
1:31
which belong to the scientific genus Taxus.
1:34
He gave this “proto-yew” a fitting name: Prototaxites.
1:38
Dawson was pretty proud of his discovery, but it wasn’t long before
1:41
another scientist came along to rain on his Prototaxites parade.
1:44
In the 1870s, a British botanist named William Carruthers took a look
1:48
at the fossil and proclaimed that the microscopic structure was all
1:52
wrong for wood. Instead, he identified it as some sort of ancient algae.
1:56
Being a pair of prominent scientists in the late 1800s,
1:59
the two men naturally treated their academic disagreement
2:02
with the appropriate level of politeness and civility.
2:05
In 1871, Dawson called Carruthers’ identification an “entire mistake,”
2:09
a “blunder” that could “only be excused
2:11
by defective observations or imperfect specimens.”
2:14
In 1872, Carruthers countered, “If Dr. Dawson knew
2:18
anything whatever about a vegetable cell,
2:21
he would not have written such nonsense.”
2:23
In the 1880s, more excavations in Quebec uncovered very
2:26
tree-trunk-shaped, full cylindrical fragments of Prototaxites,
2:31
which Dawson considered excellent
2:32
support for his conifer identification.
2:35
Meanwhile, Carruthers tried to officially change the name of
2:38
Prototaxites to distance it from Dawson’s coniferous theory.
2:41
Both men were pretty rude, and—
2:43
in a satisfying bit of scientific karma—
2:45
they were both wrong.
2:46
We’re now pretty certain that Prototaxites is neither tree nor algae,
2:51
but we still don’t know what it actually is!
2:54
To this day, scientists continue to debate the identity of these very
2:57
confusing fossils, but they mostly have better manners about it.
3:01
You see, for most of Earth’s history,
3:02
the continents were pretty much empty of life.
3:05
It wasn’t until a little more than 400 million years ago
3:07
that enough species emerged from the sea
3:10
to establish complex ecosystems on land.
3:13
We think Prototaxites was a purely land-lubbing
3:16
creature because it’s found almost exclusively in sites
3:19
along ancient shorelines and other terrestrial habitats.
3:22
The only exceptions are the occasional specimens
3:24
found among ocean fossils, which probably
3:27
represent individuals that got washed out to sea.
3:29
So in those early ecosystems,
3:31
Prototaxites really stood above the rest… literally!
3:34
Some of the fossils are massive chunks of trunk
3:37
more than a meter across and over eight meters long!
3:40
These were by far the largest living things on land in their time.
3:44
They were the Earth’s first terrestrial giants,
3:47
towering above their centimeter-scale neighbors.
3:49
They also would have served as concentrated nutritional hubs.
3:52
Some fossils have bore-holes made by hungry ancient arthropods,
3:56
and it’s likely that Prototaxites was an
3:59
all-you-can-eat buffet for a whole variety of little critters.
4:01
So at a glance, Prototaxites might sound a lot like a tree,
4:05
but the similarities end with the trunk.
4:07
For one thing, they don’t appear to have had branches like trees do.
4:11
And under the microscope, Prototaxites’ tissues
4:13
do not look like wood—there are none of wood’s
4:16
characteristic structures, such as spiraling or closely-bundled fibers.
4:21
Instead Prototaxites’ tissues are woven layers of tube-like filaments.
4:26
On the other hand, these filaments form concentric
4:29
layers of the trunk, again, reminiscent of tree rings.
4:32
This confusing combination of traits makes it challenging
4:34
to determine how Prototaxites lived its life…
4:37
or even which direction it grew!
4:39
Most artwork depicts Prototaxites standing upright like a tree,
4:43
and it’s often described as reaching high into the sky,
4:46
towering over the rest of the ecosystem.
4:47
Like what we did at the beginning of this episode.
4:50
But some scientists point out that there
4:51
isn’t much evidence to support this.
4:53
Prototaxites trunks have a pretty consistent width throughout, unlike
4:57
tree trunks which taper to help stabilize it against wind and gravity.
5:01
And while there are cases of fossil tree stumps preserved in their
5:04
original upright position, no such fossils are known for Prototaxites.
5:08
So it’s possible that Prototaxites grew horizontally along the ground,
5:12
or perhaps even below ground like the roots of a tree.
5:15
Or some scientists have compared Prototaxites to fungal structures
5:19
called rhizomorphs, root-like tubes of bundled fungus tissue.
5:23
And all this mystery adds to the confusion over what Prototaxites is.
5:28
The debate has outlived Dawson and Carruthers.
5:31
Over the years, scientists have tried classifying
5:33
Prototaxites as just about everything:
5:36
plant, seaweed, green algae, red algae, fungus,
5:39
or even some sort of symbiotic relationship like lichens.
5:42
Gradually, we’ve been ruling them out.
5:44
We’ll tell you everything Prototaxites
5:46
isn’t right after this quick ad break.
5:49
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6:36
Let’s start with what we know for sure: Prototaxites
6:39
is definitely eukaryotic. Its cells are complex.
6:42
And they have walls, which we can see thanks to some
6:45
incredible fossils that preserve its microscopic tissue structure.
6:49
So it can’t be a prokaryotic organism like bacteria.
6:53
Having cell walls also means it can’t be an animal,
6:56
though that’s never been a particularly popular suggestion anyway.
6:59
Plants are eukaryotes, but Prototaxites isn’t a plant.
7:03
Not only is its microscopic structure all wrong,
7:06
as Carruthers so delicately pointed out,
7:08
but it also doesn’t appear to have been photosynthesizing.
7:11
Paleontologists can get clues about how an ancient organism
7:14
got its energy by counting carbon isotopes in the fossils.
7:18
See, if you get your food by eating things,
7:20
the carbon you’re taking in depends on what you’re eating.
7:23
But photosynthesizers … just photosynthesize.
7:25
So their carbon acquisition system is very consistent,
7:28
and they’ll end up with very consistent carbon signals,
7:31
even in their fossils.
7:32
But when researchers examine the carbon isotopes of
7:35
Prototaxites across different fossil sites, they see a lot of variation,
7:39
suggesting Prototaxites was a consumer, not a photosynthesizer.
7:43
This is a pretty strong strike against plants and algae.
7:47
So, like I said before, Dawson and Carruthers were both wrong.
7:50
It also casts doubt on another popular suggestion:
7:53
that Prototaxites was a lichen.
7:55
Lichens are very unique structures, made of fungal tubes
7:58
with algae inside—an inseparable symbiotic relationship.
8:02
The fungi feed on decomposing matter, the algae get
8:05
energy from the sun, and they share the bountiful harvest.
8:08
So if Prototaxites was some sort of giant, lichen-like symbiotic
8:13
structure, a variable carbon isotope might be expected.
8:16
BUT no one has ever found fossil evidence of symbiotic organisms
8:21
living among Prototaxites tissues, even in those extremely
8:25
well-preserved fossils that retain microscopic cellular structures.
8:28
And besides that, Prototaxites just isn’t shaped like a photosynthesizer.
8:33
A good photosynthesizer has a lot of surface area per volume, to
8:37
absorb as much sunlight as possible without feeding too much tissue.
8:41
This is why leaves are leaf-shaped, because being flat
8:43
and thin means they can do lots of light-capturing
8:46
without being very energetically expensive.
8:49
But Prototaxites is just a big thick trunk.
8:52
There’s no evidence of branches or leaf-like structures
8:54
to spread out and capture the sunlight.
8:56
Even if it did stand up tall, which is a great strategy for catching rays,
9:01
it would have way too much bulk and not enough surface.
9:04
So, if we’ve ruled out plants, animals, algae, and all manner of
9:07
prokaryotes, that really only leaves us with one option: fungus.
9:11
And Prototaxites does seem to check off a lot of fungal features.
9:15
Fungi are eukaryotes, closely related to animals.
9:19
Like animals, they have complex cells,
9:21
but unlike animals, fungi have cell walls. Check!
9:24
Fungi eat stuff, mainly feeding by breaking down
9:27
organic material and absorbing it into their cells. Check!
9:30
Fungi build their bodies of filamentous tubes called hyphae,
9:33
which sometimes weave together into dense,
9:35
complex structures like mushrooms and rhizomorphs. Check!
9:38
And we already know fungi were part of ancient ecosystems, since
9:41
more recognizable fungus fossils are found alongside Prototaxites.
9:45
For a long time, the only major mark against fungi
9:49
seemed to be Prototaxites’ incredible size.
9:52
It would be Earth history’s most humongous fungus by a wide margin.
9:57
Nevertheless, the most popular Prototaxites
9:59
hypothesis in recent decades is that it was some
10:02
kind of giant fungal fruiting body or root-like structure.
10:06
But this theory is far from airtight…
10:08
Every time scientists try to place Prototaxites
10:11
within a known fungal family, it doesn’t quite fit.
10:15
What’s more, researchers are discovering little details
10:17
about Prototaxites’ anatomy and chemistry that just
10:21
don’t line up with any known fungi, modern or ancient.
10:24
So there’s one final possibility. Maybe Prototaxites isn’t a fungus, or a
10:30
plant, or an algae. Maybe it doesn’t belong to any known lineage of life.
10:34
Maybe it’s a totally separate, extinct type of eukaryotic life-form.
10:39
At least that’s the case a 2026 study makes, after
10:42
documenting a whole bunch of these non-fungus features.
10:45
Which, in sum, make it seem more likely
10:47
than ever that Prototaxites isn’t fungal after all.
10:51
In that study, researchers took a very close look at chunks of
10:53
Prototaxites from a site called the Rhynie Chert in Scotland.
10:57
At around 407 million years old, it’s one of the oldest
11:00
known terrestrial communities in the fossil record,
11:03
and it’s loaded with lots of plant and fungus fossils.
11:06
Although Prototaxites is constructed of woven tubes, a lot like fungi,
11:10
the scientists found that the tubes make complex, branching
11:13
intersections unlike anything seen in known fungal species.
11:17
Researchers suspect these structures were sites of gas and
11:20
nutrient exchange, and might have helped Prototaxites efficiently
11:24
manage resources so it could grow to such prodigious sizes.
11:28
This is a really neat insight into the physiology
11:30
of these ancient species, while also nudging them
11:33
another small step farther away from fungi.
11:36
And in a giant leap, when the researchers examined the
11:38
fossil’s chemical composition, they didn’t find any of the
11:42
molecular building blocks you’d expect a fungus to have.
11:45
Specifically, their analysis didn’t turn up any of the evidence
11:48
you’d expect if Prototaxites’ tissues had beta-glucan or chitin,
11:52
both of which are universal fungal cell components.
11:55
Now, you might argue that those molecules could have gotten
11:58
warped by the fossilization process, rendering them
12:01
unrecognizable to chemical analysis.
12:03
But the researchers thought of that. They did the same analysis on
12:06
fossil fungi from the same site, and did find the expected molecules.
12:11
So, unless something strange and unique
12:13
happened to Prototaxites during fossilization—
12:15
something that didn’t happen to anyone else—
12:18
it looks like it’s missing some of the fundamental fungal ingredients.
12:22
In light of these results, some experts suggest that the evidence
12:25
simply doesn’t support identifying Prototaxites as a fungus.
12:29
At this point, the most logical answer might be that
12:31
Prototaxites was completely different, part of an entirely
12:35
unknown and extinct family of complex, multicellular life.
12:39
Of course, this one study won’t end
12:41
a debate that’s been raging for 160 years.
12:44
It’s still possible that Prototaxites is a fungus, just a very odd one.
12:48
But as time goes on, the idea of a fungal
12:50
Prototaxites seems less and less likely.
12:53
So for now, Prototaxites’ classification remains a mystery,
12:56
and leaves a gaping hole in our understanding of early terrestrial life.
13:00
Prototaxites was among the most important
13:02
organisms in the earliest terrestrial ecosystems.
13:05
At its size, it likely played a major ecological role in
13:08
early terrestrial ecosystems, providing a habitat and
13:12
a source of nutrients for all sorts of other organisms.
13:15
And for another thing, Prototaxites wasn’t around very long!
13:18
Around 370 million years ago, these giants vanished,
13:22
meaning it was only around for about 50 million years.
13:25
That’s pretty short in geologic terms!
13:27
If we can learn more about how it functioned, then we’ll better
13:30
understand the intricacies of ancient ecosystems as they adapted to
13:34
life on land, and maybe it’d also help us understand why it went extinct.
13:38
Land ecosystems as we know them might never have evolved without
13:41
that early Prototaxites-world. So we owe a lot to Prototaxites and its
13:46
role on ancient Earth, even if we don’t know what it is… yet!
13:56
[♪OUTRO]
