The mass extinction at the end of the Triassic period occurred about 201 million years ago and was associated with large-scale volcanic eruptions that accompanied the breakup of Pangea. These eruptions released large amounts of carbon dioxide into the atmosphere, raising global temperatures by an estimated 5 to 10 degrees Celsius.
As the Earth heated, forests dominated by trees collapsed. The ferns quickly migrated into the affected landscape and spread across much of what is now northwestern Europe, creating vast savannah-like environments. A new study by an international team led by geologists from Utrecht University suggests that these fern-covered areas are highly vulnerable to fire. The ferns themselves may have provided much of the fuel for the flame’s continued expansion.
The survey results are natural earth science July 21, 2026.
Ancient wildfire reconstruction
To investigate wildfire activity in this distant era, the researchers studied highly preserved sediments from four drill cores. One of these was a 640 meter long core recently collected in the UK.
The researchers reconstructed ancient fire activity by measuring organic compounds known as polycyclic aromatic hydrocarbons (PAHs) produced from fossil coal and wildfire smoke.
These results, combined with fossil pollen and spore records, pointed to a sharp increase in wildfire activity during the main stages of the extinction. This fiery interval also coincided with a dramatic expansion of the fern.
However, both traditional indicators have limitations. Large pieces of charcoal will break down into many smaller pieces, which can make the amount of fire appear larger than it actually is. PAHs can also travel far from the fires that produced them, and some molecules may not be present in the geological record. Because of these issues, researchers developed another method to track fires deep in time.
“The novelty of this study arose from the analysis of color changes in organic microfossils,” explains Dr. Bas van de Shortbrugge from Utrecht University, senior author of the paper. “We used a simple and very low-cost technique, the so-called palynomorph darkness index, which quantifies the ‘darkness’ of fossil pollen and spores.”
Strange fossil-colored patterns
Organic microfossils usually have a dark color after burial, as the material gradually changes due to increased pressure and temperature. Sediments that sink deeper underground are exposed to more heat, and the organic matter within them becomes increasingly heated. In most cases, the deeper the depth, the darker the fossil.
“But here we found a completely different pattern,” Van de Schottbrugge says.
The oldest and deepest pollen and intranuclear spores remained light in color. However, fossils from the extinction period gradually became darker in color, eventually reaching an extremely dark brown color. Once the extinction period ended, the fossils returned to their pale yellow color.
“We were very perplexed by this phenomenon because it occurred in all four cores at exactly the same time. The four basins experienced very different geological histories, so we couldn’t think of a link to sediment burial,” Van de Schotbrugge explains.
Ancient Fire “Dark Zone”
Palynomorph Darkness Index measures color using the RGB spectrum. A camera connected to a light microscope records the fossil, and the color information is converted to an average grayscale value. This allows scientists to compare samples from different layers within the same core, or samples taken from cores at different locations.
The researchers completed 15,000 measurements of pollen and spores from plants that lived before, during, and after the extinction. They also compared tree pollen and fern spores to determine whether the blackheads could be caused by biological differences between the plant groups.
“All plant groups showed the same effect, which strongly suggests it is the result of an external force.”
When the researchers compared the color changes in the fossils to charcoal and PAH levels, the pattern became clear. An unusual “dark zone” appears to record prolonged periods of severe wildfire activity during fern surges.
“This darkening coincides precisely with the main extinction interval, the fern spike, which increases the abundance of charcoal and PAHs.”
Ferns spreading in a warming world
The rapid increase in ferns during the main extinction period is likely caused by several related factors, including deforestation, soil erosion, severe greenhouse warming, and frequent wildfires.
Van de Schootbrugge: “Ferns are truly remarkable plants that have endured many crises throughout Earth’s history, and some species are able to adapt to the most extreme environments. They can be considered true disaster species.”
Certain ferns can spread quickly throughout damaged ground, especially in areas where other vegetation has been destroyed. Fire can accelerate this process. The visible parts of the fern burn, but the plant can quickly regrow from its root system below the surface. This allows them to return faster and occupy more territory than many competing plants.
This ability may help explain why the fern proliferation lasted so long. Researchers estimate that this interval lasted at least 40,000 years, and perhaps as long as 300,000 years.
Ferns became fuel for repeated fires
“When the ferns dry out, the thick mats act as ideal fuel for starting large wildfires,” Van de Schottbrugge explains.
Rapidly spreading pioneer and weeding ferns formed vast fern savannahs. Some species may have acted as fire ladders, helping flames move through the landscape while crowding out and suffocating other plants.
“The ferns reacted by providing fuel that fanned the flames, triggering repeated large-scale wildfires. It’s a hell of a world.”
The result could have been a destructive feedback cycle. Due to climate warming and deforestation, landscapes populated by ferns have expanded. The ferns then provided abundant dry fuel for new fires, which then quickly grew and spread again.
“The lesson we can take from this is that the combination of climate change, deforestation and the prevalence of opportunistic species may provide all the ingredients for a perfect storm,” Van de Schottbrugge concluded.
