Chunks of 360-million-year-old fossil charcoal found in sediments in north-central Pennsylvania provide the earliest evidence yet that wildfires swept ancient landscapes.
Road construction exposed a 1-meter-thick layer of siltstone containing several layers of charcoal chunks, most 1.5 centimeters or less in length. Although the chunks had been entombed millions of years ago, they are still brittle and have a silky sheen. The charcoal leaves black marks when rubbed on paper, says Walter L. Cressler III, a paleontologist at West Chester University in Pennsylvania.
Besides revealing early wildfire activity, Cressler says, the charcoal provides details about the ancient environment. For instance, the presence of charcoal indicates that Earths atmosphere was at least 13 percent oxygen, the lower limit needed for combustion. Also, nearby rocky sediments show signs that the climate included both wet and dry seasons, Cressler says. Those rocks contain fine material that had become trapped within cracks in the ancient clay-rich soils as they shrank and swelled with changes in moisture.
Cressler says that these and other clues suggest that the thick, charcoal-bearing siltstone was originally deposited in thin layers at the bottom of a lake that had formed from the meander of an ancient river. He also notes that in the Late Devonian period, before tectonic motions carried this region north, the area was located in the tropics. Cressler reports his find in the April/May issue of the bimonthly Palaios.
Fossils found in the charcoal-bearing siltstone layers, as well as in sediments above and below, include fish and unburned branches and leaves. Much of the unburned foliage comes from Archaeopteris, the first known tree to live en masse in forests, but none of the charcoal has been linked to this plant. Instead, the charcoal that has been identified came from branches of Rhacophyton, a shallow-rooted fern that probably coated the forest floor.
Archaeopteris shed its foliage often, possibly in response to the same stressful environmental conditions that led to the wildfires, says Mary L. Trivett, a paleobotanist at Ohio University in Athens. In the tropics, that stress is usually associated with seasonal decreases in rainfall rather than temperature extremes. The changes in rainfall also show up in the trees growth rings, she notes.
After a dry season, desiccated ferns would have been susceptible to wildfires sparked when lightning struck Archaeopteris trees, which towered up to 30 m. Charcoal and unburned vegetation that littered the floodplain surrounding the lake would have been swept into its calm waters and become entombed there during the subsequent rainy season.
The presence of unburned branches of Archaeopteris throughout the Pennsylvania siltstone suggests that these first forest trees had already developed means to flourish despite fires. The deep roots of the trees–and possibly a wide spacing between them in the forest–may have helped limit fires to the tinder-dry forest floor, Cressler says.
Peter A. Bernhardt, an evolutionary botanist at Saint Louis University, says the Late Devonian charcoal is an exciting find that suggests that terrestrial plants evolved an adaptation to cyclical fires much earlier than anyone assumed.