Around 400 million years ago, Earth’s landscapes looked nothing like the wooded scenes we know. Mosses and small plants hugged the soil, animals were only beginning to move onto land, and yet vast vertical structures punctured the horizon. These mysterious giants, now known from scattered fossils, challenge biologists because they fit almost nowhere in the tree of life.
Ancient trunks in a treeless world
The story begins in the mid-19th century, when strange cylindrical fossils were unearthed in rocks from the Devonian period, roughly 420 to 360 million years ago. These fossils looked uncannily like tree trunks, some more than 7.5 metres high, standing in an otherwise low, scrubby ecosystem.
In 1859, they received a name: Prototaxites, literally “primitive yew”. The label reflected what Victorian scientists thought they were seeing — the remains of some very early kind of tree. That idea did not last.
The towering fossils of Prototaxites stood upright in a world where most plants barely reached human ankles.
Detailed study soon showed that these vertical structures lacked the basic features of true trees. There were no growth rings like those of wood, no leaves, and no familiar pattern of roots. Instead, the internal tissue formed a mottled, spotted network of tubes, unlike any tree anatomy known today.
Not quite plant, not quite fungus
Once the “primitive tree” explanation crumbled, a new favourite emerged: giant fungi. To many researchers, Prototaxites looked like an enormous mushroom stalk frozen in stone. That debate dragged on for decades.
A recent study, published in Science Advances, has now thrown cold water on the classic fungal interpretation. Scientists compared Prototaxites fossils with other ancient fungi preserved in the same rock layers. At first glance, both showed networks of fine tubes, reminiscent of fungal filaments called hyphae. On closer inspection, the similarities fell apart.
In fungi, those filaments usually form organised, branching patterns. In Prototaxites, the tubes veer and overlap in an almost chaotic way, creating a tangled interior that does not follow the standard fungal blueprint.
Chemical tests have not detected chitin in Prototaxites, even though this key fungal component is present in nearby fossil fungi.
Chitin is a tough molecule that forms the walls of fungal cells and the shells of insects. In the same rock formations that hold Prototaxites, scientists clearly see chitin in other, unmistakable fossil fungi. That makes its absence in Prototaxites hard to ignore. If this giant were a fungus, it was unlike any fungus we know, past or present.
A lost branch of life?
These inconsistencies have pushed some researchers toward a bolder idea: Prototaxites might belong to an entirely separate branch of life, one that has left no living descendants.
Under this view, the Devonian period hosted an experimental form of multicellular life that never made it to the modern day. It would sit outside the major kingdoms we learn about in school — animals, plants, fungi — representing a vanished lineage with its own rules and chemistry.
Other scientists urge caution. They suggest Prototaxites could still be an extremely unusual fungus, part of a side branch that later died out. Without DNA to test, and with only fragmentary fossils to work from, the argument remains open.
What we think Prototaxites looked like
Recreations place Prototaxites as a towering, column-like structure, rising from the ground like a solitary totem. Picture a pale, trunk-sized pole several metres tall, surrounded by knee-high greenery and shallow streams.
- Height: up to at least 7.5 metres
- Shape: cylindrical, tree-trunk-like, often with a tapered top
- Environment: early Devonian landscapes with low plants and wet soils
- Internal structure: tangled tubes, giving a speckled, mottled appearance
Artists’ reconstructions of the famous Rhynie ecosystem in Scotland often feature Prototaxites as the skyline element, looming over primitive plants and early land arthropods. These images remain best guesses, built from partial fossils and modern ecological reasoning.
How did such giants feed?
Even if scientists cannot fully agree on what Prototaxites was, many suspect it filled a role similar to that of decomposers today. Earlier studies of its chemistry hinted that it might have fed on dead organic matter — bits of early plants and microbial mats — rather than using sunlight like a plant.
If that is right, Prototaxites would have acted as a massive recycling tower, drawing nutrients from the ground and turning ancient biomass back into the soil system. Yet a puzzle remains: in a world with limited plant cover, where did a seven-metre decomposer find enough food?
The energy budget of Prototaxites is a major unresolved problem: its size does not neatly match the sparse vegetation of its time.
Some researchers suggest that early land surfaces were coated in thick microbial films and mats rich in organic material, providing more food than the few visible plants would suggest. Others argue that we may be underestimating the productivity of Devonian wetlands, where organic sludge could accumulate in abundance.
Why Prototaxites matters for understanding life on land
The existence of such towering organisms in the Devonian forces scientists to rethink how quickly life became complex on land. Before true forests appeared, multicellular life had already reached large sizes and intricate structures.
This tells us that the step from microscopic mats to towering bodies might not be as slow or linear as once thought. Environmental pressures — such as competition for light, access to airborne spores, or the need to escape flooding — could push organisms to grow upwards surprisingly early.
It also shapes how we read the fossil record. Early terrestrial rocks do not only record miniature, simple forms. They also capture bold experiments in body design that did not make it to the present day.
Key terms that help make sense of the debate
A few scientific concepts sit at the heart of the Prototaxites mystery:
- Chitin: A tough carbohydrate that forms the cell walls of fungi and the exoskeletons of insects and crustaceans. Its absence in Prototaxites is a major argument against a fungal identity.
- Hyphae: The thread-like filaments that build fungal bodies. Prototaxites has tube-like structures, but their messy layout differs from typical hyphal networks.
- Multicellularity: The condition of being made from many cooperating cells. Prototaxites shows that complex multicellular organisation existed on land well before modern trees.
How scientists test strange fossils without DNA
Working with fossils this old means giving up on direct genetic information. Instead, researchers blend several approaches to piece together the story.
| Method | What it reveals |
|---|---|
| Thin-section microscopy | Shows the internal arrangement of cells and tubes, helping compare with plants and fungi. |
| Geochemical analysis | Looks for specific molecules, such as chitin or plant pigments, preserved in the rock. |
| Stable isotope ratios | Gives clues about diet and how the organism processed carbon. |
| Ecological modelling | Tests whether the size and number of individuals fit the likely energy available in the ancient environment. |
By comparing these lines of evidence, scientists can reject some hypotheses and keep others alive. In the case of Prototaxites, this process has steadily chipped away at simple plant or fungus explanations.
What Prototaxites can teach us beyond paleontology
The idea of a lost kingdom of life raises questions that stretch beyond ancient Earth. If our own planet hosted large organisms that left no direct descendants, then exoplanets could hold similarly strange lineages that never evolve into familiar animals or trees.
Astrobiologists pay attention to such cases because they widen the range of possible life signatures. A world populated by Prototaxites-like giants might leave chemical traces very different from a forest or coral reef, yet still signal active biology. Understanding these ancient enigmas sharpens the way we search for life beyond Earth.
There are also lessons for today’s ecosystems. Decomposers such as fungi, bacteria and soil organisms sit at the heart of nutrient cycles. Prototaxites hints that these recycling roles have deep evolutionary roots, stretching back to the dawn of land life. When modern soils are degraded or stripped of microbial diversity, a long heritage of ecological function is disrupted.
For anyone curious about Earth’s past, Prototaxites offers a reminder that evolution is not a straight ladder but a tangled history of experiments. Some branches, like trees and mammals, flourished. Others, like these Devonian giants, ruled for a time and then vanished, leaving just enough stone behind to raise new questions.
