Stegosaurs range across the northern Hemisphere, and are known from the Jurassic into the Cretaceous, but weren't very successful compared to their ankylosaurian relatives. And stegosaurs seem fairly significant looking what with all of their tall plates and huge tail spikes. They are tall and narrow; ankylosaurs are low and fat. But some of the earlier stegosaurs are not so ... stegosaurian. Some looked very ankylosaur-like, with large forelimbs, longish necks, and spiny armaments down the back rather than just giant plates and a thagomizer at one end.
The first fully-recognized stegosaur is Dacentrurus armatus, named from the Kimmeridge Clay of England, from the Late Jurassic. It seems vaguely similar to Kentrosaurus from Africa and roughly the same age. But few specimens of Dacentrurus armatus are known. Some specimens referred to it have actually come across the Jurassic/Cretaceous boundary, from the Weald and from continental Europe. Originally named Omosaurus, the name belonged to a crocodilians; yet by the time this was recognized, half a dozen species had been named on the basis of stegosaur-like bones or armor. Many of these were split off, but some remain. A complete specimen is not known, and many scattered remains are difficult to verify without understanding range of variation. It is conventional to assume many European Jurassic/Cretaceous spanning stegosaurs are or are very similar to Dacentrurus armatus, and it is these specimens which provide the basis of the reconstruction. A shoulder spike (which can also be a hip spike) is placed there in keeping with similarities to similar spikes found in other stegosaurs.
Prepared for an upcoming book on the dinosaurs of the British Isles.