The first trace of the plated, spiky stegosaurian dinosaurs was found in Early Cretaceous rock near Grahamstown, South Africa. Uncovered by W. G. Atherstone and A. G. Bain in 1845, the dinosaur was represented by a partial skull and several limb bones. The naturalists felt unqualified to study them and sent the fossils to Richard Owen in England. When Owen eventually got around to describing them, he confused parts of the stegosaur with bones from armored reptiles called pariesaurs which came from South African rock of much older age. Things only got worse from there.
In 1890 the naturalist Richard Lydekker realized that Owen had erred, and he instead attributed all the material to a pariesaur, but Lydekker was wrong, too. When the paleontologist Robert Broom looked at the same material in 1910, he saw that some parts definitely belonged to a dinosaur, which he thought was an ankylosaur. Franz Nopcsa disagreed, casting the fossils as belonging to a stegosaur in his own 1929 study, but it was not until 1981 that paleontologists P.M. Galton and W.P. Coombs straightened things out. The dinosaur was indeed a stegosaur, and is called Paranthodon africanus today.
Paranthodon was not the only cryptic stegosaur with a tortured history. As reviewed by Susannah Maidment in her new paper on the history of stegosaur discoveries, in 1874, just three years prior to the description of the famous Stegosaurus, the scrappy remains of another stegosaur were found in Bedfordshire, England. Described as a partial skull by H.G. Seeley—though actually part of a vertebra—Craterosaurus pottonensis was so incomplete that it was not recognized for what it was until the 1980s.
Another stegosaur, called Omosaurus armatus by Richard Owen (and known as Dacentrurus today), was found the same year in Swindon, England, though its discovery, too, was plagued by confusion over whether its armored plates belonged to the dinosaur or were the head plates of a giant fish. We can look back at them today as the first stegosaurs to be described, although the fossils that initially set the image of what this group was like were the specimens found by O.C. Marsh and E.D. Cope during the "Bone Wars" of the late 19th century.
We have come a long way since Craterosaurus, "Omosaurus," and Stegosaurus were initially described. Since that time stegosaurs have been found in the Middle Jurassic to Early Cretaceous rock of North America, Europe, Africa and Asia, although the most familiar one is perhaps one of the strangest. Stegosaurus lacked the large shoulder spikes seen among other dinosaurs of its kind, and it had an alternating pattern of plates on its back rather than a combination of back spikes and plates arranged in straight double rows. Even compared to other groups of dinosaurs, though, the stegosaurs were among the most unusual groups of dinosaurs to have ever lived, and new discoveries—such as species with extra-long necks—continue to underscore how bizarre they were.
Maidment, S. (2010). Stegosauria: a historical review of the body fossil record and phylogenetic relationships Swiss Journal of Geosciences DOI: 10.1007/s00015-010-0023-3