We figured out giant bones were of lizard-like creatures and not giant humans around the same time we invented the telegraph, made the first trains and were working on the first general purpose computer.
And dinosaurs were huge. Imagine if the human civilization dies, and another one emerges in a million years, they'll probably be oblivious of our existence by the time they start their space program, and would figure out we had a civilization of our own right about... now.
I remember watching the VOY episode, Distant Origins, which had in it a background story that said that a dinosaurs species achieved sentience and became intelligent enough to travel in space.
I know it is just fiction, but is there any chance that a species that lived millions of years before us could have achieved sentience. I am not only talking about dinosaurs but other ancient species as well. Animals have been around for 600 millions years at least.
Why not? We will never knw of course, but sentience is not a magical thing that "BOOM" appears suddenly after x numbers of evolutonary steps.
It's all gradual and some degree of sentience is very possible I think.
Most animals would score somewhere on the sentience scale.
I would put dolphins pretty high on that scale... how long have they been around?