By Paul Rincon
BBC News Online science staff
Scientists have discovered evidence for the earliest known wildfire in Earth's history, the journal Geology reports.
The partially charred fossils may indicate lower atmospheric oxygen
It comes in the form of small plant fossils preserved as charcoal, which were unearthed by researchers near the town of Ludlow on the Welsh borders.
The plant remains date to the Silurian Period, about 443 to 417 million years ago, say a Cardiff University team.
Previously, charcoal was known only from the later Devonian Period, which saw an explosion in plant diversity.
But in the Silurian, plants were generally quite small, which would have restricted the fuel source for wildfires.
"The plants were only a few centimetres in height.
"It's amazing that there should be an accumulation of vegetation, either as litter or as living vegetation drying out - to provide fuel for a fire," co-author Professor Dianne Edwards of Cardiff University told BBC News Online.
"And even more amazing that a lightning strike set it off."
Feeling the heat
The team used a technique known as reflected-light microscopy on a collection of well-preserved plants recovered from a dig at Ludford Lane.
The fossils are preserved in three-dimensions, rather than being compressed. This is unique amongst Silurian plant remains.
Charcoal has a high optical reflectance due to chemical changes that occur when organic material burns. The results confirmed that this was the case for the Silurian plants.
The researchers also found other evidence that the fossils were charred such as shrinkage of the layer of cells, or epidermis, which covers the surface of the plant.
In Geology, the University of Cardiff team proposes that the exceptional three-dimensional preservation of the plants is due to their charring in a wildfire.
However, comparisons with the reflectance values of experimentally charred plants show most of the Ludford Lane specimens were only partially turned to charcoal.
This indicates that they were burnt by either a short-lived low-temperature fire or a smouldering fire that was only intense enough to partially char them.
This agrees well with proposed compositions for the Silurian atmosphere.
Oxygen levels are supposed to have been lower in Silurian times; about 18% compared with present-day levels of roughly 21%.
Lower levels of oxygen in the atmosphere, combined with a restricted fuel source, may have produced less intense wildfires and therefore less reflective charcoal.
The Ludford Lane fossils have high optical reflectance, characteristic of charcoal
Professor Edwards said that the fire might have been similar to a modern heathland fire.
"The climate of the time would have had some extremely dry seasons as well as wet seasons. There would have been aridity and presumably plants would have dried out," she explained.
"Or there could have been accumulations of plants that had been in floods and that could have formed a fuel."
The researchers also found a charred coprolite - fossilised faeces - amongst the Ludford Lane specimens which probably came from a millipede.