Scientists have argued for more than a century about these mysterious giants, whose fossils suggest a kind of life that no longer exists. New research is now pushing the idea that they were neither plant nor fungus, but representatives of a completely lost branch of life.
Strange towers on a treeless Earth
Travel back roughly 400 million years, to the Devonian period, and the Earth would look shockingly empty to us. No forests, no towering conifers, just low mats of mosses and small plants creeping across damp ground. Yet, in this sparse scenery, enormous vertical columns rose more than 7.5 metres high.
These structures, found as fossils since the 1840s, were so tree-like that early scientists assumed they must be primitive conifers. They called them Prototaxites, meaning “early yew”. That name stuck, but the interpretation did not.
What looked like the first trees on land turned out not to be trees at all – and not clearly anything we recognise today.
The fossils show massive, log-shaped forms, up to a metre wide, lying in ancient rock layers. In life, they would have appeared as solitary pillars, poking up from largely empty plains, looking almost like the chimneys of some alien industrial world.
A fossil that refuses to fit the rules
Early on, researchers realised Prototaxites lacked the features of vascular plants. No growth rings, no clear wood structure, and no obvious leaves or branches. That ruled out the tree hypothesis.
Instead, two main ideas took shape:
- a gigantic fungus, like an overgrown mushroom or puffball
- a completely distinct life form, neither plant nor fungus, with no living relatives
New work published in Science Advances has given fresh weight to the second option. By comparing thin slices of Prototaxites fossils with fossil fungi from the same rocks, researchers spotted several striking differences.
Why the fungus explanation is in trouble
Under the microscope, Prototaxites is made of networks of tubes, which at first glance resemble fungal filaments, known as hyphae. That similarity led many scientists to support the fungus idea.
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Yet, when the internal architecture is studied in detail, the pattern looks wrong for fungi. The tubes branch in messy, irregular ways. Fungal networks, even ancient ones, usually show a more organised structure, with clear patterns of growth and branching.
There is another problem: chemistry. Fungi build their cell walls partly from chitin, a tough molecule also found in insect shells. Chitin can survive fossilisation and has been detected in other fossil fungi from the same sites.
In Prototaxites, researchers find no trace of chitin at all, though it appears clearly in neighbouring fossil mushrooms.
This absence does not absolutely prove it was not a fungus, but it adds a serious complication. To keep the fungal interpretation, scientists would need to argue for a strange, chitin-free fungus that leaves almost no chemical signature we recognise.
A lost branch of life?
Some palaeobiologists are increasingly open to a more radical scenario: Prototaxites as a member of a completely extinct lineage, part of a “failed experiment” in multicellular life.
Earth’s history is full of such experiments. Many groups flourished for millions of years and then vanished, leaving only puzzling traces in rock. Prototaxites may be one of those casualties, neither plant, animal, fungus, nor algae as we currently define them.
Other researchers stay cautious, suggesting it could still be a highly unusual fungus that simply pushed the limits of what fungi could do during the Devonian. This tension between bold reinterpretation and scientific caution is driving new investigations.
A giant in a knee-high ecosystem
Whatever its identity, one thing stands out: size. In a world where most land plants barely reached ankle height, Prototaxites soared like a tower. That raises hard questions about how such an organism survived.
Previous studies indicate it probably fed like a modern fungus or decomposer, breaking down dead organic material and recycling nutrients. Stable isotope analyses hint that it did not photosynthesise like a plant, but relied on external carbon sources.
A decomposer that tall is hard to explain: where did it find enough decaying matter in such a sparse landscape to grow to several metres?
One possibility is that it grew extremely slowly, adding tissue over decades or even centuries as thin layers of organic material accumulated. Another is that whole networks of low-lying organisms, such as early plants and microbial mats, provided a constant trickle of food, which the giant columns tapped into.
Reconstructing a vanished ecosystem
Artists have tried to picture ancient sites like the Rhynie Chert in Scotland, where Prototaxites is preserved. The reconstructions show a damp, steamy setting, with small primitive plants, shallow pools and scattered giant columns rising above it all.
These scenes are necessarily speculative. Fossils rarely tell the full story of texture, colour or growth habit. Some scientists imagine Prototaxites as a rigid tower. Others think it might have been more spongy or flexible, perhaps hollow or filled with softer material.
| Feature | Typical plant | Typical fungus | Prototaxites |
|---|---|---|---|
| Main food source | Sunlight (photosynthesis) | Decaying organic matter | Likely decaying organic matter |
| Cell wall material | Cellulose | Chitin | No clear chitin detected |
| Internal structure | Ordered vascular tissues | Organised hyphal networks | Chaotic tube networks |
| Typical size (Devonian) | Few centimetres high | Small mushrooms, mats | Over 7.5 metres high |
What “kingdoms of life” really mean
Stories like this highlight how our tidy textbooks can hide messy realities. We often learn that life is split into clear kingdoms: animals, plants, fungi, and a few others. Fossils such as Prototaxites show those rules are human conveniences, not strict natural laws.
Ancient organisms may sit in the gaps between our categories or form their own separate branches. As new techniques appear – from high-resolution imaging to advanced chemical analysis – they can reveal weird combinations of traits that no current kingdom fully captures.
For readers, two terms often cause confusion:
- Multicellular: made of many cooperating cells, often specialised for different roles, like support, feeding or reproduction.
- Decomposer: an organism that feeds on dead material, breaking it down and recycling nutrients into soil and water.
Prototaxites seems to have been both: a large, complex body built from many cells, and probably a decomposer at landscape scale.
What this mystery means for life on other planets
The idea of a 7.5-metre-tall, tube-built column that is neither tree nor classic fungus raises wider questions about how life might appear elsewhere. If Earth’s own history includes such oddities, alien life may follow paths even harder to recognise.
Astrobiologists sometimes use cases like Prototaxites to remind mission planners that unfamiliar chemistry and structure should not be dismissed too quickly. A towering, non-photosynthetic organism on an exoplanet might resemble rock pillars or vents at first glance, while actually being alive.
Simulations of early ecosystems also suggest that strange giants can play major roles in shaping soils and nutrient cycles. On a young planet, large decomposers could stabilise sediments, lock away carbon or even influence the growth of later, more familiar forms such as trees.
For students and amateur fossil hunters, this mystery carries a practical lesson: when a specimen refuses to fit the usual labels, that difficulty can be a clue rather than a problem. The next “impossible” fossil might point, once again, to a lost experiment in life that briefly dominated Earth’s surface, and then vanished without leaving a clear descendant.