Ancient History
We may think we have a good idea about earth‘s past, but the truth is that fossilisation is actually a really rare event because it requires a lot of special conditions.
It‘s estimated that 99% of all species that ever lived on earth are already extinct and of those only about 1% had the chance to be preserved as fossils (and of those only a tiny percentage has actually been discovered by scientists).
Add to that various gaps in the fossil record, like Romer’s Gap and the fact that the fossil records of two entire continents, Antarctica and Zealandia, are largely unknown to us and it becomes apparent that the majority of lifeforms that ever lived on earth will forever be unknown to us.
Now, an industrialised civilization like us today would leave rather obvious global signs in the geologic record, so it seems unlikely that something like that existed in prehistory without us noticing, but there‘s still somewhat of a possibility that some species,
Perhaps from the Mesozoic or Cenozoic, were able to reach their versions of paleolithic, neolithic or maybe even medieval-level technology without us ever noticing. Only time and research will tell.
Story Credit: Reddit/Romboteryx