Giant, mysterious organisms discovered are impossible to categorize

For over 165 years, scientists have puzzled over a strange fossil called Prototaxites. Even its appearance was unusual. It stood up to 8 meters tall, and resembled a towering tree with no leaves.

These curious lifeforms appeared during the late Silurian period, around 400 million years ago. Found among early ferns and horsetails, they lacked branches and looked nothing like modern trees.

In 1859, Canadian geologist John William Dawson thought these fossils were decaying tree trunks. He named them “first conifer,” a name that stuck even as confusion deepened.

The mystery has only grown as modern science has applied new tools, yet the answers remained elusive.

Was Prototaxites a fungus?

For decades, the idea that Prototaxites was a fungus gained support.

In 2001, paleontologist Francis Hueber proposed it was a giant terrestrial fungus, based on anatomical features like the presence of tubular structures.

Later, in 2017, researchers identified what they believed were fruiting bodies in the fossil, linking it to Ascomycota, a modern fungal group.

But there was a problem – these supposed reproductive parts had never been shown to connect with the vegetative body. That raised concerns.

Could these fragments have belonged to an entirely different organism? Without direct evidence, the fungal identity of Prototaxites remained guesswork.

Despite ongoing debate, the fungus theory persisted. That is, until a team led by researchers from the University of Edinburgh took a closer look.

They re-examined Prototaxites taiti, a species preserved in Scotland’s Rhynie chert, which is one of the best fossil deposits from the early Devonian period. Their findings call everything we thought we knew into question.

Largest organism of the Devonian landscape

The team focused on an exceptionally well-preserved fossil labeled NSC.36. This fossil showed a cross-section of P. taiti and revealed medullary spots – dark, circular features – spread throughout its body.

The sample of rock that was used for analysis was roughly cylindrical, 5.6 cm (2.2 inches) wide and nearly 7 cm (2.8 inches) long, but incomplete. This suggests the original organism was much larger, likely a towering figure in its environment.

At the microscopic level, the researchers discovered three distinct types of tubes running lengthwise through the fossil.

Type 1 tubes were thin, septate, and branched. Type 2 tubes were wider, with smooth double walls and no internal cross-walls. Type 3 tubes were thickest, and featured ring-like thickenings that do not match any known fungal type. These structures appeared interwoven, forming a dense, interconnected network inside the fossil.

Interestingly, these anatomical features resembled some aspects of fungi – but not enough to confirm a match. The third tube type, with its annular thickenings, doesn’t appear in any known fungal group, which raised immediate doubts.

Strange medullary spots with no equal

Using confocal laser scanning microscopy and 3D imaging, the team examined the medullary spots in detail. These regions contained all three tube types and added even finer ones, some just 1 micron in diameter. The branching patterns inside were complex and lacked the clear order seen in today’s fungal systems.

In modern fungi, branching follows a hierarchy – generative hyphae give rise to other types in a predictable pattern. But in P. taiti, the branching had no clear direction or sequence. Tubes connected in all directions, forming a chaotic mesh.

“There are no structures analogous to medullary spots in extant fungi,” the researchers noted.

They also rejected the idea that these spots resembled fruiting bodies of ascomycetes. Even though some scientists once thought the spots hinted at spore production, no organic connection exists between these spots and any supposed reproductive structures.

Prototaxites lacked fungal or plant traits

Next, the team analyzed the fossil’s molecular makeup using FTIR spectroscopy.

They compared P. taiti with fungi, plants, animals, and bacteria from the same deposit. All these fossils had undergone the same fossilization process, which allowed for reliable side-by-side comparison.

The results were striking. The fossil lacked any trace of chitin or chitosan, which are essential compounds found in fungal cell walls. Even perylene – a chemical marker of ascomycete fungi – was absent in P. taiti, though present in the surrounding substratum.

Instead, P. taiti’s cells contained aromatic and phenolic compounds resembling fossilized lignin. Lignin is found in land plants, but this version differed chemically. It was something entirely distinct – resembling lignin, yet not plant-like.

Machine learning confirms its isolation

To test the molecular data further, the team used machine learning models. These models were trained to distinguish organisms based on their molecular fingerprints.

The results were clear. The models identified P. taiti as something different from fungi, animals, bacteria, or plants – with over 90% accuracy.

“We conclude that the morphology and molecular fingerprint of P. taiti is clearly distinct from that of the fungi and other organisms preserved alongside it in the [Devonian deposit], and we suggest that it is best considered a member of a previously undescribed, entirely extinct group of eukaryotes,” the authors note.

With no known taxonomic group sharing its combination of anatomy, chemistry, and lifestyle, Prototaxites stands apart. It was a eukaryote – meaning it had complex cells with nuclei – but that’s where the similarities end.

Not a plant, fungus, or animal

The researchers tested every possible affiliation. Could it be an alga? No, P. taiti lacked the necessary photosynthetic structures and was chemically dissimilar. Could it be an early land plant? Again, the absence of vascular tissues and other plant traits ruled that out.

How about lichen? Not possible without a photobiont. Oomycetes? The molecular composition didn’t match. Even animals were ruled out. Metazoans do not have tube-like structures with cell walls, and P. taiti clearly did.

In the end, the team’s classification table showed the same result for every group: rejected.

Prototaxites: Evolution’s lost experiment

Prototaxites might be the last known member of its kind. It formed massive vertical structures from underground networks. It likely absorbed nutrients from decaying matter, much like modern saprotrophs. Yet it was not a fungus, not a plant, and not an animal.

“Based on this investigation we are unable to assign Prototaxites to any extant lineage, reinforcing its uniqueness,” the authors concluded.

And that may be all we ever know. With no modern descendants and no living relatives, Prototaxites exists only as a fossil – part of a long-vanished branch of life. It reminds us that evolution is full of strange ideas, some of which thrived briefly before disappearing forever.

The ancient world was full of life unlike anything we see today. Prototaxites was one of its boldest experiments – and perhaps, one of its most fascinating failures.

The study is published in the journal bioRxiv.

Image Credit: CC BY-SA 4.0

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