If you entertain the notion that 55M years ago the PETM was caused by the Silurian civilization then geologically you have to realize that it was only 40M years ago that the Indian plate slammed into the Eurasian plate and caused the uplift of the Himalayas. Mt Everest just didn't exist at all back then. And shield glaciers have carved the continents on a 100k year cycle for at least the past 1M years which would have destroyed all of the surface mining evidence at northern latitudes. A lot changes geologically over those kinds of time periods.

Of course most of our oil reserves come from the Mesozoic age (66-252 million years ago) so either the Silurians quickly transitioned away from fossil fuels, or blew themselves up and went extinct before they could extract all of it.