I'm having difficulty with your assumptions here
Assumption 1 is fine. Vague, but fine. There have been many.
Assumption 3 is workable. Far-fetched, but workable. As has been pointed out already, it's not quite comprehensively disprovable and so it's valid for a hypothesis.
Asssumption 2 is a big nono. Like BIG. I can't give you exact figures here, but I think we're talking about several million years of separation between the earliest known human remains and the latest known dinosaur remains.
Unless you can give us reliable sources on these 'earlier civilisations' that place them concurrently with the dinosaurs, I really can't see this idea working.
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"No one was behaving from very Buddhist motives. Then, thought Pigsy, he was hardly a Buddha, nor was he a monkey. Presently, he was a pig spirit changed into a little girl pretending to be a little boy to be offered to a water monster. It was all very simple to a pig spirit."
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