The end-Triassic mass extinction, 201 million years ago, is linked to volcanism that resulted from the break-up of Pangea and which pumped out vast amounts of CO2 into the atmosphere causing extreme global warming of 5 to 10 degrees Celsius. As a result, tree-dominated vegetation collapsed and the highly disturbed landscapes were colonized by ferns that formed savannahs across large swaths of what is now Northwest Europe. New research by an international team led by geologists from Utrecht University and published in Nature Geoscience on 21 July 2026 has found that these fern savannahs were prone to large-scale burning, whereby the ferns themselves acted as the fuel that fanned the flames.
The research team had access to exceptional sedimentary material from 4 drill cores, including a recently drilled 640-meter-long core from the United Kingdom. The team generated paleo-fire records based on the abundance of fossil charcoal and organic molecules that form in the smoke emitted from wildfires, so-called polycyclic aromatic hydrocarbons (PAHs).
Combined with pollen and spore records, these proxy records show a period of intense wildfire activity during the main extinction interval coincident with the proliferation of ferns. However, charcoal records can be misinterpreted when larger pieces of charcoal break up in smaller fragments in the sample. Similarly, the PAHs emitted during wildfires do not necessarily land close to the wildfires, and the molecules may not be preserved at all. The team therefore came up with a novel way to track wildfire activity in deep time.
"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."
Normally, the color of organic microfossils can change when they get buried in sediments, due to pressure and temperature changes. As sediments get buried deeper over time, temperature increases with depth, causing the enclosed organic matter to become progressively more cooked. The deeper, the darker.
"But here we found a very different pattern," Van de Schootbrugge says. In the oldest, deepest samples in the cores, the pollen and spores are light colored, but pollen and spores from the extinction interval itself become progressively extremely dark brown. After the extinction, colors return to a light yellow again. "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.
The Palynomorph Darkness Index uses the RGB color spectrum measured with a camera attached to a light microscope and converts it into an average grey scale value that can be compared between samples within cores but also between cores from different locations. The team generated 15,000 measurements from both pollen and spores from plants that occur before, during, and after the extinction. And by comparing tree pollen and fern spores it was possible to exclude any biological effects specific for plant groups.
"All plant groups show the same effect, which is a strong indication that it was the result of an outside force." By comparing the color changes in the microfossils to the other fire indicators, the team realized that this 'Dark Zone' was reflecting prolonged and intense wildfires in the fern spike interval. "The darkening overlaps exactly with the fern spike, the main extinction interval, and elevated abundance of charcoal and PAHs."
The explosive growth of ferns within the main extinction interval was the result of a combination of factors including deforestation, soil erosion, strong greenhouse warming, and wildfire activity. 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."
Some ferns can rapidly spread across disturbed landscapes, and wildfires can stimulate the ferns to spread even further and faster. While ferns would burn aboveground, they can rapidly grow back from underground root systems, outcompeting other plants in the process. This effect likely played an important role in the duration of the fern spike interval that is estimated to have lasted from 40,000 years to perhaps as long as 300,000 years.
"When the ferns dry out, the thick mats act as the ideal fuel to trigger massive wildfires," Van de Schootbrugge explains. Weeding and pioneering ferns formed widespread fern savannahs, with some species functioning as fire ladders while smothering other vegetation. "Ferns responded to and delivered the fuel that fanned the flames, triggering repeated massive wildfires. A truly hellish world."
"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.
