If you strolled along a  shoreline 420 million years ago, you would tower over just about  every living thing in sight.

You’d be stepping over tiny plants and bugs, most of which wouldn’t even  come up to your ankles. That is, until you stumbled across  one of the world’s first giants: an enormous tree-trunk-like organism so big that you couldn’t wrap your arms around it.

This is Prototaxites, and I  can tell you it’s not a tree. But I also can’t tell you what it is. That’s because scientists  have spent more than a century and a half trying to classify  these ancient giants, and they still haven’t settled on an answer!

Over the decades it’s been called  “plant”, “algae”, and “fungus”, but one hypothesis is gaining traction. Prototaxites might be  something completely different— an entirely unknown lineage of life that could deserve a Kingdom all its own. [♪INTRO] In 1843, a Canadian geologist  named William E. Logan collected a bunch of plant fossils from a  place called Seal Cove in Quebec.

They were around 400 million years old, making them some of the oldest  land plants in the fossil record. A decade later, Logan passed  some of these fossils along to another Canadian geologist,  John William Dawson, who was particularly interested in what appeared to be a chunk of fossilized wood  from some sort of small tree. After close examination, Dawson decided that it was indeed the wood of an ancient tree.

More than likely a partially rotted ancient log, judging by the presence of what  seemed to be fossilized fungi. He identified the tree as a  conifer related to modern yews, which belong to the scientific genus Taxus. He gave this “proto-yew” a  fitting name: Prototaxites.

Dawson was pretty proud of his  discovery, but it wasn’t long before another scientist came along to  rain on his Prototaxites parade. In the 1870s, a British botanist  named William Carruthers took a look at the fossil and proclaimed that  the microscopic structure was all wrong for wood. Instead, he identified  it as some sort of ancient algae.

Being a pair of prominent  scientists in the late 1800s, the two men naturally treated  their academic disagreement with the appropriate level  of politeness and civility. In 1871, Dawson called Carruthers’  identification an “entire mistake,” a “blunder” that could “only be excused by defective observations or imperfect specimens.” In 1872, Carruthers countered, “If Dr. Dawson knew anything whatever about a vegetable cell, he would not have written such nonsense.” In the 1880s, more excavations  in Quebec uncovered very tree-trunk-shaped, full cylindrical  fragments of Prototaxites, which Dawson considered excellent support for his conifer identification.

Meanwhile, Carruthers tried to  officially change the name of Prototaxites to distance it  from Dawson’s coniferous theory. Both men were pretty rude, and— in a satisfying bit of scientific karma— they were both wrong. We’re now pretty certain that  Prototaxites is neither tree nor algae, but we still don’t know what it actually is!

To this day, scientists continue to  debate the identity of these very confusing fossils, but they mostly  have better manners about it. You see, for most of Earth’s history, the continents were pretty much empty of life. It wasn’t until a little more  than 400 million years ago that enough species emerged from the sea to establish complex ecosystems on land.

We think Prototaxites was a purely land-lubbing creature because it’s found  almost exclusively in sites along ancient shorelines and  other terrestrial habitats. The only exceptions are the occasional specimens found among ocean fossils, which probably represent individuals that got washed out to sea. So in those early ecosystems, Prototaxites really stood  above the rest… literally!

Some of the fossils are massive chunks of trunk more than a meter across  and over eight meters long! These were by far the largest  living things on land in their time. They were the Earth’s first terrestrial giants, towering above their centimeter-scale neighbors.

They also would have served as  concentrated nutritional hubs. Some fossils have bore-holes made  by hungry ancient arthropods, and it’s likely that Prototaxites was an all-you-can-eat buffet for a  whole variety of little critters. So at a glance, Prototaxites  might sound a lot like a tree, but the similarities end with the trunk.

For one thing, they don’t appear  to have had branches like trees do. And under the microscope, Prototaxites’ tissues do not look like wood—there are none of wood’s characteristic structures, such as  spiraling or closely-bundled fibers. Instead Prototaxites’ tissues are  woven layers of tube-like filaments.

On the other hand, these filaments form concentric layers of the trunk, again,  reminiscent of tree rings. This confusing combination of  traits makes it challenging to determine how Prototaxites lived its life… or even which direction it grew! Most artwork depicts Prototaxites  standing upright like a tree, and it’s often described as  reaching high into the sky, towering over the rest of the ecosystem.

Like what we did at the beginning of this episode. But some scientists point out that there isn’t much evidence to support this. Prototaxites trunks have a pretty  consistent width throughout, unlike tree trunks which taper to help  stabilize it against wind and gravity.

And while there are cases of fossil  tree stumps preserved in their original upright position, no such  fossils are known for Prototaxites. So it’s possible that Prototaxites  grew horizontally along the ground, or perhaps even below ground  like the roots of a tree. Or some scientists have compared  Prototaxites to fungal structures called rhizomorphs, root-like  tubes of bundled fungus tissue.

And all this mystery adds to the  confusion over what Prototaxites is. The debate has outlived Dawson and Carruthers. Over the years, scientists have tried classifying Prototaxites as just about everything: plant, seaweed, green algae, red algae, fungus, or even some sort of symbiotic  relationship like lichens.

Gradually, we’ve been ruling them out. We’ll tell you everything Prototaxites isn’t right after this quick ad break. This SciShow video is supported by DeleteMe, the hands-free subscription service that removes your personal information, like  addresses and phone numbers, from hundreds of data broker  and people search sites.

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Let’s start with what we  know for sure: Prototaxites is definitely eukaryotic. Its cells are complex. And they have walls, which  we can see thanks to some incredible fossils that preserve  its microscopic tissue structure.

So it can’t be a prokaryotic  organism like bacteria. Having cell walls also  means it can’t be an animal, though that’s never been a  particularly popular suggestion anyway. Plants are eukaryotes, but  Prototaxites isn’t a plant.

Not only is its microscopic structure all wrong, as Carruthers so delicately pointed out, but it also doesn’t appear to  have been photosynthesizing. Paleontologists can get clues  about how an ancient organism got its energy by counting  carbon isotopes in the fossils. See, if you get your food by eating things, the carbon you’re taking in  depends on what you’re eating.

But photosynthesizers … just photosynthesize. So their carbon acquisition  system is very consistent, and they’ll end up with very  consistent carbon signals, even in their fossils. But when researchers examine  the carbon isotopes of Prototaxites across different fossil  sites, they see a lot of variation, suggesting Prototaxites was a  consumer, not a photosynthesizer.

This is a pretty strong strike  against plants and algae. So, like I said before, Dawson  and Carruthers were both wrong. It also casts doubt on another popular suggestion: that Prototaxites was a lichen.

Lichens are very unique  structures, made of fungal tubes with algae inside—an inseparable  symbiotic relationship. The fungi feed on decomposing  matter, the algae get energy from the sun, and they  share the bountiful harvest. So if Prototaxites was some sort  of giant, lichen-like symbiotic structure, a variable carbon  isotope might be expected.

BUT no one has ever found fossil  evidence of symbiotic organisms living among Prototaxites  tissues, even in those extremely well-preserved fossils that retain  microscopic cellular structures. And besides that, Prototaxites just  isn’t shaped like a photosynthesizer. A good photosynthesizer has a lot  of surface area per volume, to absorb as much sunlight as possible  without feeding too much tissue.

This is why leaves are  leaf-shaped, because being flat and thin means they can do lots of light-capturing without being very energetically expensive. But Prototaxites is just a big thick trunk. There’s no evidence of branches  or leaf-like structures to spread out and capture the sunlight.

Even if it did stand up tall, which  is a great strategy for catching rays, it would have way too much  bulk and not enough surface. So, if we’ve ruled out plants,  animals, algae, and all manner of prokaryotes, that really only  leaves us with one option: fungus. And Prototaxites does seem to  check off a lot of fungal features.

Fungi are eukaryotes, closely related to animals. Like animals, they have complex cells, but unlike animals, fungi have cell walls. Check!

Fungi eat stuff, mainly feeding by breaking down organic material and absorbing  it into their cells. Check! Fungi build their bodies of  filamentous tubes called hyphae, which sometimes weave together into dense, complex structures like  mushrooms and rhizomorphs.

Check! And we already know fungi were  part of ancient ecosystems, since more recognizable fungus fossils  are found alongside Prototaxites. For a long time, the only major mark against fungi seemed to be Prototaxites’ incredible size.

It would be Earth history’s most  humongous fungus by a wide margin. Nevertheless, the most popular Prototaxites hypothesis in recent decades is that it was some kind of giant fungal fruiting  body or root-like structure. But this theory is far from airtight… Every time scientists try to place Prototaxites within a known fungal  family, it doesn’t quite fit.

What’s more, researchers are  discovering little details about Prototaxites’ anatomy  and chemistry that just don’t line up with any known  fungi, modern or ancient. So there’s one final possibility.  Maybe Prototaxites isn’t a fungus, or a plant, or an algae. Maybe it doesn’t  belong to any known lineage of life.

Maybe it’s a totally separate,  extinct type of eukaryotic life-form. At least that’s the case a 2026 study makes, after documenting a whole bunch of  these non-fungus features. Which, in sum, make it seem more likely than ever that Prototaxites  isn’t fungal after all.

In that study, researchers took  a very close look at chunks of Prototaxites from a site called  the Rhynie Chert in Scotland. At around 407 million years  old, it’s one of the oldest known terrestrial communities  in the fossil record, and it’s loaded with lots  of plant and fungus fossils. Although Prototaxites is constructed  of woven tubes, a lot like fungi, the scientists found that the  tubes make complex, branching intersections unlike anything  seen in known fungal species.

Researchers suspect these  structures were sites of gas and nutrient exchange, and might have  helped Prototaxites efficiently manage resources so it could  grow to such prodigious sizes. This is a really neat insight into the physiology of these ancient species, while also nudging them another small step farther away from fungi. And in a giant leap, when  the researchers examined the fossil’s chemical composition,  they didn’t find any of the molecular building blocks  you’d expect a fungus to have.

Specifically, their analysis  didn’t turn up any of the evidence you’d expect if Prototaxites’  tissues had beta-glucan or chitin, both of which are universal  fungal cell components. Now, you might argue that those  molecules could have gotten warped by the fossilization  process, rendering them unrecognizable to chemical analysis. But the researchers thought of  that.

They did the same analysis on fossil fungi from the same site,  and did find the expected molecules. So, unless something strange and unique happened to Prototaxites during fossilization— something that didn’t happen to anyone else— it looks like it’s missing some of  the fundamental fungal ingredients. In light of these results, some  experts suggest that the evidence simply doesn’t support identifying  Prototaxites as a fungus.

At this point, the most  logical answer might be that Prototaxites was completely  different, part of an entirely unknown and extinct family of  complex, multicellular life. Of course, this one study won’t end a debate that’s been raging for 160 years. It’s still possible that Prototaxites  is a fungus, just a very odd one.

But as time goes on, the idea of a fungal Prototaxites seems less and less likely. So for now, Prototaxites’  classification remains a mystery, and leaves a gaping hole in our  understanding of early terrestrial life. Prototaxites was among the most important organisms in the earliest terrestrial ecosystems.

At its size, it likely played  a major ecological role in early terrestrial ecosystems,  providing a habitat and a source of nutrients for  all sorts of other organisms. And for another thing, Prototaxites  wasn’t around very long! Around 370 million years  ago, these giants vanished, meaning it was only around  for about 50 million years.

That’s pretty short in geologic terms! If we can learn more about how  it functioned, then we’ll better understand the intricacies of  ancient ecosystems as they adapted to life on land, and maybe it’d also help  us understand why it went extinct. Land ecosystems as we know them  might never have evolved without that early Prototaxites-world.

So  we owe a lot to Prototaxites and its role on ancient Earth, even if  we don’t know what it is… yet! [♪OUTRO]