together in the family Ceratopsidae and the order Ceratopsia. This prompted Cope to reexamine his own specimens and to realize that Triceratops, Monoclonius, and Agathaumas all represented a single group of similar dinosaurs, which he named Agathaumidae in 1891. Cope redescribed Monoclonius as a horned dinosaur, with a large nasal horn and two smaller horns over the eyes, and a large frill. Classification Ceratopsia was coined by Othniel Charles Marsh in 1890 to include dinosaurs possessing certain characteristic features, including horns, a rostral bone, teeth with two roots, fused neck vertebrae, and a forward-oriented pubis. Marsh considered the group distinct enough to warrant its own suborder within Ornithischia. The name is derived from the Greek κέρας/kéras meaning 'horn' and ὄψῐς/ópsis meaning 'appearance, view' and by extension 'face'. As early as the 1960s, it was noted that the name Ceratopsia is actually incorrect linguistically and that it should be Ceratopia. However, this spelling, while technically correct, has been used only rarely in the scientific literature, and the vast majority of paleontologists continue to use Ceratopsia. As the ICZN does not govern taxa above the level of superfamily, this is unlikely to change. Following Marsh, Ceratopsia has usually been classified as a suborder within the order Ornithischia. While ranked taxonomy has largely fallen out of favor among dinosaur paleontologists, some researchers have continued to employ such a classification, though sources have differed on what its rank should be. Most who still employ the use of ranks have retained its traditional ranking of suborder, though some have reduced to the level of infraorder. Phylogeny In clade-based phylogenetic taxonomy, Ceratopsia is often defined to include all marginocephalians more closely related to Triceratops than to Pachycephalosaurus. Under this definition, the most basal known ceratopsians are Yinlong, from the Late Jurassic Period, along with Chaoyangsaurus and the family Psittacosauridae, from the Early Cretaceous Period, all of which were discovered in northern China or Mongolia. The rostral bone and flared jugals are already present in all of these forms, indicating that even earlier ceratopsians remain to be discovered. The clade Neoceratopsia includes all ceratopsians more derived than psittacosaurids. Another subset of neoceratopsians is called Coronosauria, which either includes all ceratopsians more derived than Auroraceratops, or more derived than Leptoceratopsidae. Coronosaurs show the first development of the neck frill and the fusion of the first several neck vertebrae to support the increasingly heavy head. Within Coronosauria, three groups