Later members, including ceratopsids like Centrosaurus and Triceratops, became very large quadrupeds and developed elaborate facial horns and frills extending over the neck.

This triangular appearance is accentuated in later ceratopsians by the rearwards extension of the parietal and squamosal bones of the skull roof, to form the neck frill.

[13] During another of Hayden's expeditions in 1872, Fielding Bradford Meek found several giant bones protruding from a hillside in southwestern Wyoming.

Monoclonius was notable for the number of disassociated remains found, including the first evidence of ceratopsid horns and frills.

Several Monoclonius fossils were found by Cope, assisted by Charles Hazelius Sternberg, in summer 1876 near the Judith River in Chouteau County, Montana.

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.

[17] 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.

[19] In clade-based phylogenetic taxonomy, Ceratopsia is officially defined in the PhyloCode as "the largest clade containing Ceratops montanus and Triceratops horridus, but not Pachycephalosaurus wyomingensis.

[20] Under this definition, the most basal known ceratopsians are the family Chaoyangsauridae and the well known genus Psittacosaurus, from the Early Cretaceous Period, all of which were discovered in northern China or Mongolia.

A slightly less inclusive group is Euceratopsia, named and defined by Daniel Madzia and colleagues in 2021 as "the smallest clade containing Leptoceratops gracilis, Protoceratops andrewsi, and Triceratops horridus".

[citation needed] All previously published neoceratopsian phylogenetic analyses were incorporated into the analysis of Eric M. Morschhauser and colleagues in 2019, along with all previously published diagnostic species excluding the incomplete juvenile Archaeoceratops yujingziensis and the problematic genera Bainoceratops, Lamaceratops, Platyceratops and Gobiceratops that are very closely related to and potentially synonymous with Bagaceratops.

[21] Psittacosaurus sinensis Psittacosaurus mongoliensis Yinlong downsi Chaoyangsaurus youngi Hualianceratops wucaiwanensis Xuanhuaceratops niei Stenopelix valdensis Liaoceratops yanzigouensis Archaeoceratops oshimai Aquilops americanus Auroraceratops rugosus Graciliceratops mongoliensis Asiaceratops salsopaludalis Mosaiceratops azumai Yamaceratops dorngobiensis Helioceratops brachygnathus Leptoceratops gracilis Ischioceratops zhuchengensis Prenoceratops pieganensis Udanoceratops tchizhovi Zhuchengceratops inexpectus Koreaceratops hwaseongensis Montanoceratops cerorhynchus Cerasinops hodgskissi Gryphoceratops morrisoni Unescoceratops kopelhusae Bagaceratops rozhdestvenskyi Magnirostris dodsoni Protoceratops hellenikorhinus Protoceratops andrewsi Ajkaceratops kozmai Zuniceratops christopheri Turanoceratops tardabilis Diabloceratops eatoni Ceratopsidae Unlike almost all other dinosaur groups, skulls are the most commonly preserved elements of ceratopsian skeletons and many species are known only from skulls.

Complete growth series from embryo to adult are known for Psittacosaurus and Protoceratops, allowing the study of ontogenetic variation in these species.

But Paul and Christiansen (2000) argued that at least the later ceratopsians had upright forelimbs and the larger species may have been as fast as rhinos, which can run at up to 56 km or 35 miles per hour.

[25] However, comparisons between the scleral rings of Protoceratops and Psittacosaurus and modern birds and reptiles indicate that they may have been cathemeral, active throughout the day at short intervals.

Fragmentary remains, including teeth, which appear to be neoceratopsian, are found in North America from the Albian stage (112 to 100 million years ago), indicating that the group had dispersed across what is now the Bering Strait by the middle of the Cretaceous Period.

[7][28] The traditional view that ceratopsoids originated in North America was called into question by the 2009 discovery of better specimens of the dubious Asian form Turanoceratops, which may it as a ceratopsid.

[29] Possible ceratopsians from the Southern Hemisphere include the Australian Serendipaceratops, known from an ulna, and Notoceratops from Argentina is known from a single toothless jaw (which has been lost).