Ancient ferns may have helped keep Triassic Europe burning by rapidly regrowing after fires and supplying fresh fuel for the next blaze.
Around 201 million years ago, Earth was rocked by the end-Triassic mass extinction. The crisis has been linked to enormous volcanic activity associated with the breakup of Pangea. Those eruptions released huge quantities of CO2 into the atmosphere and drove global temperatures up by an estimated 5 to 10 degrees Celsius.
As the climate rapidly warmed, forests dominated by trees collapsed. Ferns then spread across the damaged landscapes, forming vast savannahs over much of what is now Northwest Europe. New research from an international team led by geologists at Utrecht University suggests those fern-covered regions were especially vulnerable to fire. In many cases, the ferns themselves provided the fuel that allowed enormous wildfires to spread.
The findings were published in Nature Geoscience.
Tracing Wildfires 201 Million Years Ago
To reconstruct this ancient period of fire, researchers examined unusually well-preserved sediments from four drill cores. One of them was a recently collected core from the United Kingdom that extended 640 meters underground.
The scientists built records of ancient fire activity by measuring fossil charcoal as well as organic compounds produced in wildfire smoke called polycyclic aromatic hydrocarbons (PAHs).
When the researchers compared those records with fossil pollen and spores, they found evidence for an intense period of wildfire activity during the main extinction interval. That same period also corresponded with a dramatic expansion of ferns.
Traditional fire indicators, however, have important limitations. Large pieces of charcoal can break into many smaller fragments, potentially making fire activity appear more intense than it really was. Smoke-related molecules can also travel far from the fires that produced them, and they are not always preserved in sediments.
Because of these problems, the team developed another method for identifying ancient wildfire activity.
“The novelty of this study came from the analysis of color changes of organic microfossils,” explains Dr. Bas van de Schootbrugge from Utrecht University, a senior author on the paper. “We used a simple and very low-cost technique that quantifies the ‘darkness’ of fossil pollen and spores, a so-called Palynomorph Darkness Index.”
Fossil Pollen Takes an Unexpected Turn
Organic microfossils commonly become darker after they are buried. As sediments accumulate, deeper layers experience greater temperatures and pressure, gradually altering the organic material they contain. In general, fossils buried deeper should therefore look darker.
But the drill cores showed something very different.
“But here we found a very different pattern,” Van de Schootbrugge says.
The oldest and deepest samples contained relatively light colored pollen and spores. Fossils from the extinction interval, however, became progressively darker until they reached an extremely dark brown. Once the extinction interval ended, the fossils returned to a much lighter yellow color.
“We were quite puzzled by this phenomenon as it occurs in all 4 cores at exactly the same time, so it could not have been related to burial of the sediments as the four basins experienced very different geological histories,” Van de Schootbrugge explains.
A Mysterious Dark Zone
The researchers used the Palynomorph Darkness Index to quantify those color differences. A camera attached to a light microscope measured the RGB color spectrum of each fossil, which was then converted into an average grey scale value. This allowed samples from different depths and different locations to be directly compared.
Altogether, the scientists made 15,000 measurements of fossil pollen and spores from plants that lived before, during, and after the extinction. They also compared pollen from trees with spores from ferns, allowing them to rule out the possibility that the effect was limited to particular types of plants.
“All plant groups show the same effect, which is a strong indication that it was the result of an outside force.”
When the team compared this unusual darkening with other indicators of ancient fires, a pattern emerged. The researchers concluded that the so-called ‘Dark Zone’ recorded a prolonged period of intense wildfire activity during the surge in fern abundance.
“The darkening overlaps exactly with the fern spike, the main extinction interval, and elevated abundance of charcoal and PAHs.”
Ferns Thrive in a Triassic Inferno
Several forces appear to have combined to produce the enormous spread of ferns during the extinction interval. Forest loss, erosion, powerful greenhouse warming, and widespread fires all transformed the environment in ways that favored plants capable of rapidly colonizing disturbed ground.
Van de Schootbrugge: “Ferns are truly remarkable plants that have withstood many crises throughout Earth history, and some species can adapt to some of the most extreme environments. They can be considered to be true disaster species.”
Certain fern species can quickly spread across damaged landscapes. Fire can make that expansion even easier. Although flames destroy the parts of the plants growing above ground, ferns can rapidly return from underground root systems. This gives them an advantage over other plants and allows them to colonize newly burned terrain.
That process may help explain why the fern-dominated interval lasted so long. Estimates suggest it persisted for at least 40,000 years and possibly as long as 300,000 years.
Ferns Became Fuel for the Next Fire
Once these widespread fern savannahs dried out, they could become highly flammable.
“When the ferns dry out, the thick mats act as the ideal fuel to trigger massive wildfires,” Van de Schootbrugge explains.
Fast-spreading pioneer ferns formed extensive savannahs across the landscape. Some species could act as fire ladders, helping flames spread while also crowding out other vegetation.
The result may have been a powerful ecological cycle. Fires created disturbed terrain where ferns could rapidly spread. Those ferns then produced large amounts of combustible material, setting the stage for more fires.
“Ferns responded to and delivered the fuel that fanned the flames, triggering repeated massive wildfires. A truly hellish world.”
The researchers say this ancient episode offers a broader warning about what can happen when several environmental pressures reinforce one another.
“The lesson we can learn from this is that the combination of climate change, deforestation, and the spread of opportunistic species can provide all the ingredients for a perfect storm,” Van de Schootbrugge concludes.
Reference: “Continental-scale fern savannah wildfires during end-Triassic greenhouse warming” by T. P. Hollaar, M. S. Kent, B. H. Lomax, W. Meredith, S. L. Lindström, R. Bos, C. V. Looy, J. P. Benca, I. A. P. Duijnstee, S. P. Hesselbo, S. Richoz, H. M. Viðarsdóttir, T. R. A. Vandenbroucke, J. Vermeer, N. Kuhlmann, I. M. Waajen-Labee, F. Peterse, K. G. J. Nierop and B. van de Schootbrugge, 21 July 2026, Nature Geoscience.
DOI: 10.1038/s41561-026-02048-4
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