In Stephen Baxter's fairly recent novel, "Evolution" (highly recommended, BTW), one chapter entitled "The Hunters of Pangaea" posits a stone-age civilization of humanoid dinosaurs, but they don't have enough of an impact on the environment to be noticed by humans, 145 million years later. Quoting from the book:
The whole of the orniths’ rise and fall was contained in a few thousand years, a thin slice of time compared to the eighty million years the dinosaur empire would yet persist. They made tools only of perishable materials — wood, vegetable fiber, leather. They never discovered metals, or learned how to shape stone. They didn’t even build fires, which might have left hearths. Their stay had been too brief; the thin strata would not preserve their inflated skulls. When they were gone the orniths would leave no trace for human archaeologists to ponder, none but the puzzle of the great sauropods’ abrupt extinction.
When asked whether some of the strange species he invented for the book actually existed, he answered essentially, "Of course not".
Fun to read about, though!
Ooooooo! I gotta read that one!