Dinosaurs are the first kind of creature to develop intelligence on a world, I think--the dragons were from a line of dinosaurs but were seriously more advanced, sentience-wise. They're not native to earth though, and circumstances prevented them from ever settling this world, which is why you can't find any dragon fossils. The few that may have died here didn't leave any traces that could survive time. They had more brittle bones like birds and there were never enough of them here to leave enough pieces to reconstruct one like you can a dinosaur.