[12] Ornithischians are united by multiple features of the skull, teeth, and skeleton, including especially the presence of a predentary and palpebral, an increased number of sacral vertebrae, the absence of gastralia, and an opisthopubic pubis.

As in Scelidosaurus, the palpebral forms the top border of the orbit as a flat brow bone, but the antorbital fossa is reduced to the point of absence in some genera.

The orbit and antorbital fossa are large, but the nasal opening is small, and while teeth are present in the premaxilla, there is a toothless front tip that likely formed a keratinous beak.

Teeth are almost always absent from the premaxilla, the antorbital fossa is reduced and round to slit-like, the tip of the snout is sometimes flared to form a broad beak.

[19] Members of the ornithopod family Hadrosauridae show further adaptations, including the formation of dental batteries where teeth are continuously replaced, and in many genera the development of prominent cranial crests formed by multiple different bones of the skull.

[20] Pachycephalosauria, at one time thought to be close to ornithopods and now known to be related instead to ceratopsians, show a unique skull anatomy that is unlike any other ornithischian.

[21] Ceratopsians, the sister group to pachycephalosaurs, also display many cranial adaptations, most importantly the evolution of a bone called the rostral that forms the top beak opposite the predentary.

[23] Early ornithischians were relatively small dinosaurs, averaging about 1–2 meters in body length, with a triangular skull that had large circular orbits on the sides.

Some taxa considered earlier to be ornithopods, like heterodontosaurids, Agilisaurus, Hexinlusaurus and Othnielia, were instead found to be outside of both Ornithopoda and Ceratopsia, but still closer to those two groups than thyreophorans.

[22][23] Ornithopods, which range from the Early Jurassic in some studies until the end of the Cretaceous with continuous diversity, are generally bipedal and unarmoured, though some later groups like Hadrosauridae evolved complex dental anatomy in the form of batteries of teeth.

[17] The fifth recognized major subgroup of ornithischians is Pachycephalosauria,[13] which was first named in 1974 after being confused for a long time with the theropod Troodon on account of their similarly omnivorous and unique teeth.

[39][21] Pachycephalosaurians are unique for their tall, thickened skulls and small, bipedal bauplan, suggesting that their domes were for sexual display or combat in the form of head-butting or flank-butting.

[21] Some taxa, particularly those at one point groupt together in the ornithopod family Hypsilophodontidae, are now recognized to not fall within any of the major ornithischian groups, and either be outside Genasauria, or on the basal stem of Neornithischia outside Cerapoda.

[3] Heterodontosaurus tucki (Heterodontosauridae) Huayangosaurus taibaii (Huayangosauridae) Stegosaurus stenops (Stegosauridae) Mymoorapelta maysi Polacanthus foxii (Polacanthinae) Nodosaurus textilis Panoplosaurus mirus (Panoplosaurini) Struthiosaurus austriacus (Struthiosaurini) Hylaeosaurus armatus Gobisaurus domoculus Shamosaurus scutatus Pinacosaurus grangeri Saichania chulsanensis Ankylosaurus magniventris (Ankylosaurini) Jeholosaurus shangyuanensis (Jeholosauridae) Orodromeus makelai (Orodrominae) Thescelosaurus neglectus (Thescelosaurinae) Hypsilophodon foxii (Hypsilophodontidae) Stegoceras validum Sphaerotholus goodwini Pachycephalosaurus wyomingensis (Pachycephalosaurini) Psittacosaurus mongoliensis Chaoyangsaurus youngi (Chaoyangsauridae) Leptoceratops gracilis (Leptoceratopsidae) Protoceratops andrewsi (Protoceratopsidae) Ceratops montanus Chasmosaurus belli Arrhinoceratops brachyops Triceratops horridus (Triceratopsini) Nasutoceratops titusi (Nasutoceratopsini) Achelousaurus horneri Pachyrhinosaurus canadensis Gasparinisaura cincosaltensis Macrogryphosaurus gondwanicus Talenkauen santacrucensis Rhabdodon priscus Zalmoxes robustus Tenontosaurus tilletti Dryosaurus altus (Dryosauridae) Camptosaurus dispar (Camptosauridae) Hypselospinus fittoni Iguanodon bernissartensis (Iguanodontidae) Probactrosaurus gobiensis Hadrosaurus foulkii (Hadrosaurinae) Saurolophus osborni (Saurolophinae) Lambeosaurus lambei (Lambeosaurinae) Multiple taxa within Ornithischia fall around the origin of the group, or cannot be classified definitively.

[2][42] For a long time, the only understanding of the origins of Ornithischia came from Lesothosaurus and Pisanosaurus, which together represented the best-known Early Jurassic and Triassic ornithischians respectively.

Many suggestions of taxa and specimens that could be referred to Ornithischia from the Triassic were based on teeth and jaw bones, as they showed similar adaptations for herbivory.

The genera Revueltosaurus, Galtonia, Pekinosaurus, Tecovasaurus, Lucianosaurus, Protecovasaurus, Crosbysaurus, and Azendohsaurus were all at one time considered to be Triassic ornithischians with only their teeth known, but are now recognized to be completely unrelated.

This hypothesis has found support in multiple different phylogenetic analyses,[47][48] but the results are not yet accepted as definitive enough to contradict other possible evolutionary strategies of dinosaurs.

[2] The 2017 phylogenetic study of Matthew G. Baron and colleagues suggested that instead of a Saurischia-Ornithischia split, ornithischians were instead closest to theropods in the clade Ornithoscelida, with sauropodomorphs being outside the grouping.

While it was originally named as a derived theropod with unique anatomy, it was found in studies based on Baron and colleagues results to instead be either the basalmost ornithischian, or a sauropodomorph.

The problematic nature of Chilesaurus requires further revisiting of its anatomy, but the details of vertebral air pockets, pelvis shape, and hand support it as a theropod.

[53] The phylogenetic analysis of Norman and colleagues in 2022 recovered the members of Silesauridae as forming an ancestral grade within Ornithischia even with the inclusion of Chilesaurus, supporting the earlier results of Müller and Garcia and their evolutionary trends for early ornithischian anatomy.

[48] Herrerasauridae Daemonosaurus Chindesaurus Tawa hallae Eodromaeus Sauropodomorpha Theropoda Lewisuchus Soumyasaurus Asilisaurus Diodorus scytobrachion Technosaurus Ignotosaurus Silesaurus Sacisaurus Lutungutali Kwanasaurus Eucoelophysis Pisanosaurus Laquintasaura Thyreophora Neornithischia Ornithischians shifted from bipedal to quadrupedal posture at least three times in their evolutionary history and it has been shown primitive members may have been capable of both forms of movement.

[57] At least one species of ankylosaurian, Liaoningosaurus paradoxus, appears to have been at least partially carnivorous, with hooked claws, fork-like teeth, and stomach contents suggesting that it may have fed on fish.

[59] Genasaurians most often had their head at the level of one meter, which suggests they were feeding primarily on “ground-level plants such as ferns, cycads, and other herbaceous gymnosperms.

[6][8] This evidence consists of multiple bone beds where large numbers of individuals of the same species and of different age groups died simultaneously.