Pre-human civilizations on Earth

[thrash]

Originally Posted by Flyndaran

It's not just a single species that would have to hide. It would have to be at least an entire order which is unlikely. Not technically impossible for the pre KT event, but exceedingly unlikely.

On the other hand, the Elder Things were colonists, as were their enemies. For a game setting, it might be sufficient to know when an offworld-engendered civilization could plausibly have been active.