The end-Triassic mass extinction about 201 million years ago has long been tied to vast volcanic outpourings during the breakup of Pangea. Those eruptions injected large volumes of CO2 into the atmosphere, raising global temperatures by an estimated 5 to 10 degrees Celsius and setting the stage for widespread Triassic wildfires.
As conditions warmed, tree-based forests collapsed. Ferns rapidly colonized the disturbed terrain, spreading across what is now Northwest Europe and forming broad, savannah-like expanses. An international team led by Utrecht University geologists reports that these fern-dominated landscapes were highly fire prone, and that the ferns themselves likely supplied much of the fuel that allowed flames to carry across the region.
The study was published in Nature Geoscience on July 21, 2026.
Reconstructing Triassic wildfires
To probe wildfire activity from this interval, the team examined exceptionally preserved sediments from four drill cores, including a recently recovered 640‑meter core from the United Kingdom.
Researchers reconstructed ancient burning by quantifying fossil charcoal and organic compounds produced in smoke, known as polycyclic aromatic hydrocarbons, or PAHs.
When combined with records of fossil pollen and spores, the results indicated a marked increase in burning during the main extinction phase. This intense fire interval coincided with a dramatic rise of ferns.
However, both indicators carry caveats. Large charcoal fragments can shatter into many smaller pieces, potentially exaggerating fire abundance. PAHs can be transported far from their source, and some compounds may not persist in the rock record. To address these issues, the team developed an additional proxy.
“The novelty of this study came from the analysis of color changes of organic microfossils,” said Dr. Bas van de Schootbrugge of Utrecht University, a senior author. “We used a simple and very low-cost technique that quantifies the darkness of fossil pollen and spores, a Palynomorph Darkness Index.”
A strange color signal emerges during the end-Triassic mass extinction
Organic microfossils typically darken with burial as pressure and temperature increase. Deeper sediments usually yield darker material.
“But here we found a very different pattern,” Van de Schootbrugge said.
The oldest and deepest pollen and spores in the cores remained light in color. Fossils from the extinction interval grew progressively darker, reaching an extremely dark brown, then returned to pale yellow after the interval ended.
“We were quite puzzled by this phenomenon as it occurs in all four 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 said.
The ancient fire “Dark Zone”
The Palynomorph Darkness Index relies on RGB color measurements captured by a microscope-mounted camera, which are converted to a gray scale value. This enables comparison within and between cores.
The team completed 15,000 measurements on pollen and spores from before, during, and after the extinction. They also compared tree pollen with fern spores to test for biological causes of the darkening.
“All plant groups show the same effect, which is a strong indication that it was the result of an outside force.”
When the color data were compared with charcoal and PAH records, a clear pattern emerged. The unusual “Dark Zone” aligned with an extended period of elevated wildfire activity during the fern expansion.
“The darkening overlaps exactly with the fern spike, the main extinction interval, and elevated abundance of charcoal and PAHs.”
Ferns spread across a warming world
The rapid rise of ferns during the peak extinction interval was likely driven by deforestation, soil erosion, intense greenhouse warming, and recurrent fires.
Van de Schootbrugge said, “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 ferns spread quickly over disturbed ground, especially where other plants had been eliminated. Fire can accelerate this process. Although fronds burn, many ferns can resprout from protected belowground parts, allowing them to return faster than competitors and expand further.
This resilience may explain the longevity of the fern spike. Researchers estimate it persisted for at least 40,000 years and perhaps up to 300,000 years.
Ferns became fuel for repeated Triassic wildfires
“When the ferns dry out, the thick mats act as the ideal fuel to trigger massive wildfires,” Van de Schootbrugge said.
Fast-spreading pioneer and weeding ferns created extensive fern savannahs. Some species may have formed fire ladders that helped flames climb through vegetation while suppressing other plants.
“Ferns responded to and delivered the fuel that fanned the flames, triggering repeated massive wildfires. A truly hellish world.”
The result was likely a destructive feedback loop. Warming and forest loss opened landscapes to fern colonization. The ferns then supplied abundant dry fuel for new burns, after which they rapidly regenerated and spread again.
“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 said.
The combination of rapid environmental change and opportunistic species has parallels in some modern ecosystems, where rising temperatures and shifting vegetation patterns are reshaping risks for both wildlife and people. Researchers are increasingly drawing connections between past planetary upheavals and present-day health and environmental threats, including how evolving risks can affect conditions such as colorectal cancer and other chronic diseases.
