First known fire scar on a fossil tree trunk provides direct evidence of wildfire in Late Triassic forests
Fire scars are well known to fire ecologists and dendrochronologists worldwide, and are used in dating fires and reconstructing the fire histories of modern forests. Evidence of fires in ancient forests, such as fossil charcoal, is well known to paleontologists and has been reported in geologic formations dating back to the Late Devonian. No fire scar resembling those in modern trees has previously been described in petrified wood as far as we can determine. We described the external features of a specimen of a fossil log from the Late Triassic Chinle Formation of southeastern Utah with what appears to be a fire scar, and analyzed the well-preserved wood structure associated with the scarring event in a cut and polished cross-section of the specimen. We compared these macro- and microscopic features with the characteristics of scars on modern trees resulting from fire and other causes. We later conducted a rapid reconnaissance in five major exposures of fossil logs in the Petrified Forest National Park, Arizona, to try to identify more potential examples of fire scars in the fossil record.
Changes in wood structure in the fossil specimen prove this to be an ancient scar, and the external features match those of modern fire scars better than those of scars created by other kinds of wounding events. The specimen also exhibits a number of changes in wood anatomy similar to those reported in modern fire-scarred trees, including: a band of very small tracheids that indicate growth suppression immediately associated with the scarring event; an area with a tangential row of probable traumatic resin ducts; and a significant increase in tracheid size following the scarring event that indicates a growth release. In the Petrified Forest, we found thirteen fossil log segments with external features similar to those of modern fire scars among the hundreds we examined. Potential ancient fire scars may not have been previously reported because paleobotanists did not have a search image for fire scars, but they may not be rare. The identification of the first known fire scar in a fossil tree confirms the occurrence of wildfires in Late Triassic forests and shows that at least some trees survived those fires, indicating that fire could have been an ecological and evolutionary force more than 200 million years ago.