Fossils Reveal Earth's Oldest-Known Wildfire

Scientists reported evidence of Earth's oldest-known wildfire in the May issue of the journal Geology.

Tiny charred plant fossils found in the rocks from the Welsh borders of England were dated to the Silurian Period, about 419 million years ago, according to the article.

Paleobotanist Ian Glasspool of the Cardiff University School of Earth Ocean and Planetary Science said that in this time — before there were many plants and even fewer land animals — a lightning strike would have started the fire.

Unlike many other such ancient fossils, these fossils are three-dimensional — about 5/32 of an inch tall (4.5 millimeters) and 1/64 of an inch wide (0.5 millimeter).

It's not clear exactly how the minuscule rhyniophytoid plants were preserved so well, but scientist believe a low-temperature smoldering fire caused at least some of the plants to char. The carbonization made the plants resistant to decay and able to withstand eons of being buried without being squashed flat, said Glasspool.

Similar charred land plant fossils, even of flowers, have been discovered from much later periods in Earth's history, according to Geology, but nothing from so far back as the Silurian Period.

A second interesting aspect of the discovery is what it indicates about oxygen levels in the atmosphere. The air had to be at least 13% oxygen for the plants to burn at all, but much more would have caused a hotter fire and reduced the plants to ashes, leaving no fossils.

Previously researchers had put oxygen levels during the Silurian Period at about 18% (compared to today's 21%).

Geology is published by the Geological Society of America. For more information, see www.gsajournals.org.