The large shields which cover the heads of Shieldon have been used all around the world by humans and pokemon alike as a way to defend themselves in battle. This custom is as old as recorded history and predates our modern understanding of fossils by millennia, and in parts of southern Africa where gunpowder was slow to penetrate it continued well into the 19th century. That fossilization took so long to discover should not be a surprise, for Shieldon heads are preserved perfectly and are remarkably common anywhere they once lived, but their bodies are subject to the same fossilization processes as other pokemon and rarely leave DNA behind.
The fact that Shieldon were not naturally occurring minerals had long been suspected, but most people over the years had attributed their creation to the remains of past warriors or long-lost civilizations: none had imagined them to be the remains of pokemon from the middle cretaceous! It was only when archaeologists, looking to find the age of a lost city, ran molecular tests on their shields that they discovered that the shields themselves were 100 million years old and contained recoverable DNA.
Word of the find reached Oreburgh, whose gym leader at the time, Byron, decided to resolve endless debates about the newly discovered fossil's characteristics by reviving one himself. The shield he used was a centuries old family heirloom, and he had personally expected it to be either the arm of a colossal beast or the shell of an ancient turtle. It was the metallic head of a yellow and remarkably sturdy dinosaur, which still bore the marks of rust and battle and the elaborate decorations of warriors from ages past. Only when they began breeding did humanity learn that Shieldon heads were gray.