Bashanosaurus primitivus is one of the earliest known stegosaurs and a strong candidate for representing an early form close to the ancestry of later plated dinosaurs like Stegosaurus. It lived roughly 168 million years ago in the Middle Jurassic, long before the classic North American Stegosaurus appeared. It was not necessarily the direct ancestor in a strict family-tree sense, but it sits close enough to the base of the stegosaur branch to work very well as your “proto-stegosaur” anchor.