Why would they have left no traces? Dinosaurs lived ~200 M years ago and left enough traces for us to discover them. Wouldn't an industrial civilization 50 mya have left some kind of refined metallic artifacts? Even if _most_ traces are eroded away with time it seems difficult to imagine every trace of a global industrial civilization would disappear.

There also don't appear to have been any spikes in atmospheric carbon dioxide during the relevant period: https://en.wikipedia.org/wiki/Carbon_dioxide_in_Earth%27s_at...

Though maybe you were looking at a data source with better resolution?