There’s a book series by julian may that deals with a time travel device being invented to 6 million years ago where to everyone’s surprise an alien civilizatio

There’s a book series by julian may that deals with a time travel device being invented to 6 million years ago where to everyone’s surprise an alien civilization is ruling the planet. https://en.m.wikipedia.org/wiki/Saga_of_Pliocene_Exile https://en.m.wikipedia.org/wiki/Saga_of_Pliocene_Exile