Ornithischians are a group of herbivorous dinosaurs that are characterized by a bird-like hip structure. There are several synonyms for this paleontological term, including bird-hipped dinosaurs, ornithopods, and thyreophorans. Ornithopods are the most diverse and widespread group of ornithischians, and they include bipedal dinosaurs such as Iguanodon and the duck-billed Hadrosaurs. Thyreophorans, on the other hand, are a group of armored dinosaurs, which include the Stegosaurus and Ankylosaurus. Both of these groups share a common ancestry within the larger clade of ornithischians and have many similarities in their skull structures, tooth morphology, and limb anatomy.