Article: The families of the ornithischian dinosaur order Ankylosauria
All reasonably complete specimens of the ornithischian suborder Ankylosauria can be allocated to one of two families, the Ankylosauridae and the Nodosauridae. which differ in many anatomical features. Evolution within these two families was very conservative, with little morphological diversity. There is no truly 'primitive' ankylosaur that is morphologically suitable as a common ancestor for both families. Scelidostntrus, sometimes suggested as an ancestral ankylosaur, has no features diagnostic of the Ankylosauria. The origin of ankylosaurs is unknown, but the group may have an undocumented history extending back to the earliest beginnings of the Ornithischia.