According to Official Guinness Records,
The earliest wildfire smouldered approximately 419 million years ago during the Silurian period, when oxygen levels may have been higher than today's. A team of scientists from Cardiff University’s School of Earth and Ocean Sciences (UK) found evidence of a low-intensity burn, probably started by a lightning strike, while studying three-dimensional charred fossils of small plants found in rocks near Ludlow, in the Welsh Borderland, UK, in April 2004.
For a complete list of Unknown date records, please visit Unknown date Guinness Records in United Kingdom.