View source for Ornithischia

{{Short description|Extinct clade of dinosaurs}}
{{Automatic taxobox
| fossil_range = <br />[[Early Jurassic]]&ndash;[[Late Cretaceous]], {{fossilrange|200.91|66|earliest=245}} (Possible Triassic record)
| image = 
{{multiple image
|total_width=330
|perrow=2/2/2
|image1=Triceratops Specimen at the Houston Museum of Natural Science v01.jpg
|image2=Heterodontosaurus_tucki_cast_-_University_of_California_Museum_of_Paleontology_-_Berkeley,_CA_-_DSC04696.JPG
|image3=Hypacrosaurus Royal Tyrrell.jpg
|image4=Berlin Naturkundemuseum Dino Eingangshalle.jpg
|image5=Europelta carbonensis Dinopolis Teruel 2.jpg
|image6=Royal Tyrrell Museum Stegoceras.jpg
|border=infobox
}}
| image_caption = A collection of ornithischian skeletons (clockwise from top left): ''[[Triceratops]]'' ([[Ceratopsia]]), ''[[Heterodontosaurus]]'' ([[Heterodontosauridae]]), ''[[Kentrosaurus]]'' ([[Stegosauria]]), ''[[Stegoceras]]'' ([[Pachycephalosauria]]), ''[[Europelta]]'' ([[Ankylosauria]]), and ''[[Hypacrosaurus]]'' ([[Ornithopoda]]).
| taxon = Ornithischia
| authority = [[Harry Govier Seeley|Seeley]], 1888<ref name="seeley88"/>
| subdivision_ranks = Subgroups<ref name="fonseca2024">{{cite journal|last1=Fonseca|first1=A.O.|last2=Reid|first2=I.J.|last3=Venner|first3=A.|last4=Duncan|first4=R.J.|last5=Garcia|first5=M.S.|last6=Müller|first6=R.T.|year=2024|title=A comprehensive phylogenetic analysis on early ornithischian evolution|journal=Journal of Systematic Palaeontology|volume=22|issue=1|article-number=2346577|doi=10.1080/14772019.2024.2346577|bibcode=2024JSPal..2246577F }}</ref>
| subdivision =
*{{extinct}}''[[Asilisaurus]]''?
*{{extinct}}''[[Gondwanax]]''?
*{{extinct}}''[[Laosaurus]]''
*{{extinct}}''[[Lewisuchus]]''?
*{{extinct}}''[[Soumyasaurus]]''?
*{{extinct}}''[[Syngonosaurus]]''
*{{extinct}}''[[Trimucrodon]]''
*{{extinct}}'''Parapredentata''' <small>Norman et al., 2022</small><ref name="normanea22"/>
**{{extinct}}''[[Alocodon]]''
**{{extinct}}''[[Amanasaurus]]''?
**{{extinct}}''[[Diodorus scytobrachion|Diodorus]]''?
**{{extinct}}''[[Eucoelophysis]]''?
**{{extinct}}''[[Gamatavus]]''?
**{{extinct}}''[[Gongbusaurus]]''?
**{{extinct}}''[[Ignotosaurus]]''?
**{{extinct}}''[[Kwanasaurus]]''?
**{{extinct}}''[[Lutungutali]]''?
**{{extinct}}''[[Pisanosaurus]]''
**{{extinct}}''[[Sacisaurus]]''?
**{{extinct}}''[[Silesaurus]]''?
**{{extinct}}''[[Technosaurus]]''?
**{{extinct}}'''Saphornithischia''' <small>Madzia et al., 2021</small><ref name="madziaea21"/>
***{{extinct}}''[[Archaeocursor]]''
***{{extinct}}''[[Eocursor]]''
***{{extinct}}''[[Fabrosaurus]]''
***{{extinct}}''[[Ferganocephale]]''
***{{extinct}}''[[Laquintasaura]]''
***{{extinct}}''[[Lesothosaurus]]''
***{{extinct}}[[Heterodontosauridae]]
***{{extinct}}'''Genasauria''' <small>Sereno, 1986</small><ref name="sereno86"/>
****{{extinct}}''[[Notoceratops]]''
****{{extinct}}''[[Serendipaceratops]]''
****{{extinct}}''[[Tichosteus]]''
****{{extinct}}[[Thyreophora]]
****{{extinct}}[[Neornithischia]]
| synonyms_ref =
| synonyms = 
* '''Predentata''' <small>[[Othniel Charles Marsh|Marsh]], 1894</small>
* '''Prionodontia''' <small>Owen, 1874</small><ref name="normanea22"/>
}}

'''Ornithischia''' ({{IPAc-en|ˌ|ɔːr|n|ə|ˈ|θ|ɪ|s|k|i|.|ə}}) is an [[extinct]] [[clade]] of mainly [[herbivorous]] [[dinosaur]]s characterized by a pelvic structure superficially similar to that of [[bird]]s.<ref name=":0">{{Cite book|title=Dinosaurs: A Concise Natural History|last1=Fastovsky|first1=David E.|last2=Weishampel|first2=David B.|publisher=Cambridge University Press|year=2012|isbn=978-1-107-27646-8|location=Cambridge}}</ref> The name ''Ornithischia'', or "bird-hipped", reflects this similarity and is derived from the [[Ancient Greek|Greek]] stem ''{{transliteration|el|ornith-}}'' ({{lang|el|ὀρνιθ-}}), meaning "bird", and ''{{transliteration|el|ischion}}'' ({{lang|el|ἴσχιον}}),{{efn|plural ''{{transliteration|el|ischia}}''}} meaning "hip".<ref>{{cite book |last1=Colbert |first1=Edwin H. (Edwin Harris) |last2=Knight |first2=Charles Robert |title=The dinosaur book: the ruling reptiles and their relatives |date=1951 |publisher=McGraw-Hill |location=New York |page=152 |url=https://archive.org/details/bookruli00colb/page/152/mode/2up}}</ref> However, as [[theropod]] dinosaurs, birds are only distantly related to this group.<ref name=":0" />

Ornithischians with well known anatomical adaptations include the [[ceratopsia]]ns or "horn-faced" dinosaurs (e.g. ''[[Triceratops]]''), the [[pachycephalosaurs]] or "thick-headed" dinosaurs, the armored dinosaurs ([[Thyreophora]]) such as [[stegosaurs]] and [[ankylosaurs]],  and the [[ornithopods]].<ref name=dkp/><ref name=":0" /> There is strong evidence that certain groups of ornithischians lived in herds,<ref name=":0" /><ref name=":1">{{Cite journal|last1=Qi|first1=Zhao|last2=Barrett|first2=Paul M.|last3=Eberth|first3=David A.|date=2007-09-01|title=Social Behaviour and Mass Mortality in the Basal Ceratopsian Dinosaur Psittacosaurus (early Cretaceous, People's Republic of China)|journal=Palaeontology|language=en|volume=50|issue=5|pages=1023–1029|doi=10.1111/j.1475-4983.2007.00709.x|bibcode=2007Palgy..50.1023Q |s2cid=128781816 |issn=1475-4983|url=http://doc.rero.ch/record/16127/files/PAL_E2504.pdf }}</ref> often segregated by age group, with juveniles forming their own flocks separate from adults.<ref name="Nest">{{Cite journal | last1 = Zhao | first1 = Q. | doi = 10.4202/app.2012.0128 | title = Juvenile-only clusters and behaviour of the Early Cretaceous dinosaur ''Psittacosaurus'' | journal = Acta Palaeontologica Polonica | year = 2013 | doi-access = free }}</ref> Some were at least partially covered in filamentous (hair- or feather- like) pelts, and there is much debate over whether these filaments found in specimens of ''[[Tianyulong]]'', ''[[Psittacosaurus]]'',<ref>{{Cite journal|last1=Mayr|first1=Gerald|last2=Peters|first2=Stefan D.|last3=Plodowski|first3=Gerhard|last4=Vogel|first4=Olaf|date=2002-08-01|title=Bristle-like integumentary structures at the tail of the horned dinosaur Psittacosaurus|journal=Naturwissenschaften|language=en|volume=89|issue=8|pages=361–365|doi=10.1007/s00114-002-0339-6|issn=0028-1042|pmid=12435037|bibcode=2002NW.....89..361M |s2cid=17781405}}</ref> and ''[[Kulindadromeus]]'' may have been primitive [[feather]]s.<ref name="Godefroit2014">{{cite journal | last1 = Godefroit | first1 = P. | last2 = Sinitsa | first2 = S.M. | last3 = Dhouailly | first3 = D. | last4 = Bolotsky | first4 = Y.L. | last5 = Sizov | first5 = A.V. | last6 = McNamara | first6 = M.E. | last7 = Benton | first7 = M.J. | last8 = Spagna | first8 = P. | year = 2014 | title = A Jurassic ornithischian dinosaur from Siberia with both feathers and scales | url = http://palaeo.gly.bris.ac.uk/Benton/reprints/2014Kulinda.pdf | journal = Science | volume = 345 | issue = 6195 | pages = 451–455 | doi = 10.1126/science.1253351 | pmid = 25061209 | bibcode = 2014Sci...345..451G | hdl = 1983/a7ae6dfb-55bf-4ca4-bd8b-a5ea5f323103 | s2cid = 206556907 | access-date = 2016-08-28 | archive-url = https://web.archive.org/web/20190209232112/http://palaeo.gly.bris.ac.uk/Benton/reprints/2014Kulinda.pdf | archive-date = 2019-02-09 | hdl-access = free }}</ref>

== Description ==
Ornithischia is a very large and diverse group of dinosaurs, with members known from all continents, habitats, and a very large range of sizes. They are primarily herbivorous browsers or grazers, but some members may have also been opportunistic omnivores.<ref name="paul24"/> Ornithischians are united by multiple features of the skull, teeth, and skeleton, including especially the presence of a {{dinogloss|predentary}} and {{dinogloss|palpebral}}, an increased number of {{dinogloss|sacrum|sacral vertebrae}}, the absence of {{dinogloss|gastralia}}, and an {{dinogloss|pubis|opisthopubic pubis}}.<ref name="weishampel04"/> Early ornithischians ranged around {{convert|1|-|2|m|ft|abbr=on}} in length, with them increasing in size over time so that the largest armoured ornithischians were around {{convert|7.5|m|ft|abbr=on}} and {{convert|9|tonnes|ton|abbr=on}}, the largest horned ornithischians were around {{convert|8.5|m|ft|abbr=on}} and {{convert|11|tonne|ton|abbr=on}}, and the largest crested ornithischians were around {{convert|15|m|ft|abbr=on}} and {{convert|13.5|tonnes|ton|abbr=on}}.<ref name="paul24"/>
[[File:Lesothosaurus skull diagram.png|thumb|left|Skull of ''[[Lesothosaurus]]'', an early ornithischian]]
Much of the knowledge of early ornithischian anatomy comes from ''[[Lesothosaurus]]'', which is a taxon known from multiple skulls and skeletons from the [[Early Jurassic]] of [[Lesotho]]. The rear of its skull is box-like, while the snout tapers to a point. The {{dinogloss|external nares|nasal opening}} is small, the {{dinogloss|antorbital fossa}} that opens from the side of the skull into the palate is large, shallow and triangular, the {{dinogloss|orbit}} is large and round and has a palpebral creating a brow, and the lower jaw has a large {{dinogloss|mandibular fenestra}}.<ref name="normanea04a"/>

The skulls of ''[[Emausaurus]]'' and ''[[Scelidosaurus]]'', two early members of the armoured group [[Thyreophora]], show similarities in the box-like skull that tapers to the front. The antorbital fossa is smaller and forming an elongate oval in both taxa, and the palpebral which is elongate and slender in ''Lesothosaurus'' is widened in ''Emausaurus'' and completely incorporated into the skull as a flat bone in ''Scelidosaurus''.<ref name="normanea04a"/><ref name="normanea04b"/> Skulls in members of the thyreophoran group [[Stegosauria]] are much longer and lower, with the width at the back being greater than the height in ''[[Stegosaurus]]''. The snout and lower jaw are long and deep, and in some genera the {{dinogloss|premaxilla}} does not have any teeth. 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.<ref name="galtonupchurch04"/>

[[Ankylosaur]]s, the other group of armoured ornithischians, have very robust, immobile skulls, with three significant features that separate them from other groups. The antorbital fossa, {{dinogloss|supratemporal fenestra}} and mandibular fenestra are all closed, the sutures separating skull bones are almost completely obliterated by surface texturing, and there is bony armour above the orbits, and at the top and bottom corners of the back of the skull. Teeth are sometimes absent from the premaxilla, and both the upper and lower jaws have deeply inset teeth creating large cheeks. Ankylosaurs also have very extensive and complicated network of sinuses, formed by bone growth in the palate.<ref name="vickaryousea04"/>

The skulls are known from many early [[ornithopods]] and some [[heterodontosaurid]]s, showing similar general features. Skulls are relatively tall with shorter snouts, but the snout is elongated in some later taxa like ''[[Thescelosaurus]]''. 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]]. The premaxillary teeth and the first lower tooth in ''[[Heterodontosaurus]]'' are enlarged into sizeable canines.<ref name="normanea04c"/> In later ornithopods, the skulls are more elongate and sometimes fully rectangular, with a very large nasal opening, and a thin, elongate palpebral that can extend the entire way across the orbit. 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.<ref name="norman04"/> Members of the ornithopod family [[Hadrosauridae]] show further adaptations, including the formation of {{dinogloss|dental battery|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.<ref name="hornerea04"/>

[[Pachycephalosauria]], at one time thought to be close to ornithopods and now known to be related instead to [[ceratopsia]]ns, show a unique skull anatomy that is unlike any other ornithischian. The bones of the top of the skull are thickened and in many taxa expanded significantly to form round bony domes as the top of the head, as well as possessing small nodes or elongate spikes along the back edge of the skull. Many taxa are only known from these thick skull domes, which are fused from the {{dinogloss|frontal}} and {{dinogloss|parietal}} bones. As in many other ornithischians, the snout is short and tapering, the nasal opening is small, the antorbital fossa is sometimes absent, and there are premaxillary teeth, though only three. The two palpebrals are also incorporated into the skull roof as in thyreophorans, rather than free.<ref name="maryanskaea04"/>

Ceratopsians, the sister group to pachycephalosaurs, also display many cranial adaptations, most importantly the evolution of a bone called the {{dinogloss|rostral}} that forms the top beak opposite the predentary. The {{dinogloss|jugal}} bones flare to the sides to create a pentagonal skull seen from above, the nasal opening is closer to the top of the snout than the teeth, and while the snout tapers in some taxa, it is very deep and short in ''[[Psittacosaurus]]''. The ceratopsian palpebral is generally triangular, and the back edge of the skull roof forms a flat frill that is enlarged in more derived ceratopsians.<ref name="youdodson04"/> The ceratopsian family [[Ceratopsidae]] progresses on these features with the addition of horns above each orbit and on the top of the snout, as well as substantial elongation of the frill and in many genera the development of two large {{dinogloss|parietal fenestrae}} forming holes in the frill. The skull and frill elongation makes the skulls of ''[[Torosaurus]]'' and ''[[Pentaceratops]]'' the largest of any known terrestrial vertebrate, at over {{convert|2|m|ft|abbr=on}} long.<ref name="dodsonea04"/>

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. This suggests that early ornithischians had relatively huge eyes that faced laterally. The forelimbs of early ornithischians are considerably shorter than their hindlimbs. A small forelimb such as those present in early ornithischians would not have been useful for locomotion, and it is evident that early ornithischians were bipedal dinosaurs. The entire skeleton was lightly built, with a largely fenestrated skull and a very stout neck and trunk. The tail is nearly half of the dinosaurs' overall length. The long tail presumably acted as a counterbalance and as a compensating mechanism for shifts in the creature's center of gravity. The hindlimbs of early ornithischians show that the tibia is considerably longer than the femur, a feature that suggests that early ornithischians were adapted for bipedality, and were fast runners.<ref>[[Edwin H. Colbert|Colbert, E. H.]] (1981). A primitive ornithischian dinosaur from the [[Kayenta Formation]] of [[Arizona]]. ''Museum Northern Arizona Bull''. '''53''', 1-61</ref>

=== "Bird-hip" ===
The ornithischian pelvis was "opisthopubic", meaning that the pubis pointed down and backwards ([[Anterior|posterior]]), parallel with the ischium, while in the [[saurischia]]n pelvis, the pelvis jutted forwards as in lizards.<ref name=":0" /><ref name=dkp>{{cite book|title=The Ultimate Visual Family Dictionary|location=New Delhi|publisher=[[DK (publisher)|DK Pub]]|year=2012|chapter=Prehistoric Earth|page=82-83|language=en|isbn=978-0-1434-1954-9}}</ref> This resulted in a four-pronged pelvic structure. In contrast to this, the saurischian pelvis was "propubic", meaning the pubis pointed toward the head ([[anterior]]), as in ancestral reptiles (Figure 1b).<ref name=":0"/>

The opisthopubic pelvis independently evolved at least three times in dinosaurs (in ornithischians, birds and [[therizinosaur]]oids).<ref name=":3">{{cite book |url=https://books.google.com/books?id=7t9M5TsmjOUC |title=Encyclopedia of Dinosaurs |last1=Currie |first1=Philip J. |last2=Padian |first2=Kevin |date=1997-10-06 |publisher=Academic Press |isbn=978-0-08-049474-6 |pages=537–538 |language=en}}</ref> Some argue that the opisthopubic pelvis evolved a fourth time, in the clade [[Dromaeosauridae]], but this is controversial, as other authors argue that dromaeosaurids are mesopubic.<ref name=":3"/> It has also been argued that the opisthopubic condition is basal to maniraptorans (including among others birds, therizinosauroids and dromaeosaurids), with some clades having later experienced a reversal to the propubic condition.<ref name="HoltzOsmolska.04">Holtz, T.R. and Osmólska, H. (2004). "Saurischia." In Weishampel, Dodson and Osmólska (eds.), ''The Dinosauria'', second edition. Berkeley: University of California Press.</ref>

==Classification==

===History===
{{multiple image
|align = left
|total_width = 350
|image1 = Ornithischia pelvis structure.svg
|alt1 = 
|image2 = Saurischia pelvis structure.svg
|alt2 = 
|footer = The general pelvis of Ornithischia (left) and Saurischia (right) illustrating the differences highlighted by Seeley (1888) as justifying their independent evolution
}}
The first recognition of an herbivorous group of dinosaurs was named Orthopoda in [[1866 in paleontology|1866]] by [[Edward Drinker Cope]],<ref name="cope66"/> a name that is now recognized as a synonym of Ornithischia.<ref name="kuhn46"/> Discussions on the taxonomy of dinosaurs by [[Othniel Charles Marsh]] identified two major groups of herbivorous dinosaurs, [[Ornithopoda]] and [[Stegosauria]],<ref name="marsh81"/> containing genera from a broad geographic and stratigraphic distribution.<ref name="weishampel04"/> While often these groups were placed within Dinosauria, [[Harry Govier Seeley]] suggested instead in [[1888 in paleontology|1888]] that ornithopods and stegosaurs, which shared many features in the skull, limbs, and hip, were unrelated to other dinosaurs, and so he proposed that Dinosauria was an unnatural grouping of two independently-evolved [[suborder]]s, [[Saurischia]] and Ornithischia. It is from the anatomy of the hip that Seeley chose the name Ornithischia, referencing the bird-like anatomy of the [[ischium]] bone.<ref name="seeley88"/> Many researchers did not follow the division of Seeley at first, with Marsh naming the group Predentata to unite ornithopods, stegosaurs, and [[Ceratopsia]] within Dinosauria,<ref name="marsh94"/> but with additional work and new discoveries the unnatural nature of Dinosauria came to be accepted, and the names Seeley proposed found common use.<ref name="romer66"/> After further decades, in [[1974 in paleontology|1974]] [[Robert T. Bakker]] and [[Peter M. Galton]] provided new evidence in support of the grouping of ornithischians and saurischians together within a natural Dinosauria,<ref name="bakkergalton74"/> which has been supported since.<ref name="weishampel04"/>

The first cladistic studies on Ornithischia were published simultaneously in [[1984 in paleontology|1984]] by [[David B. Norman]], Andrew R. Milner, and [[Paul C. Sereno]]. These studies differed somewhat in their results, but found that ''[[Iguanodon]]'' was closer to [[Hadrosauridae|hadrosaurs]] than other ornithopods, followed by ''[[Dryosaurus]]'', ''[[Hypsilophodon]]'' and then ''[[Lesothosaurus]]'' and its relatives.<ref name="milnernorman84"/><ref name="norman84a"/><ref name="norman84b"/><ref name="sereno84"/><ref name="weishampel04"/> While the study of Norman placed ceratopsians between ''Hypsilophodon'' and more derived ornithopods,<ref name="norman84a"/> the study of Sereno placed ceratopsians with ankylosaurs and stegosaurs.<ref name="sereno84"/> It has since been recognized by that ceratopsians are closer to ornithopods than the armoured ankylosaurs and stegosaurs,<ref name="sereno86"/> but the relationships of some groups are still in states of change, with some more consistent results than others.<ref name="butlerea08"/><ref name="madziaea21"/> An early study that looked at the relationships within Ornithischia with greater detail was that of Sereno in [[1986 in paleontology|1986]], who provided features that supported the evolution of all ornithischian groups and shared similarities with earlier studies. Sereno found that ''Lesothosaurus'' was the most primitive ornithischian, with all other ornithischians united within the clade Genasauria, which has two subgroups. The first subgroup, [[Thyreophora]], unites ankylosaurs and stegosaurs along with more primitive taxa like ''[[Scelidosaurus]]'', while the second subgroup, [[Cerapoda]], contained ornithopods, ceratopsians, pachycephalosaurs, and small primitive forms.<ref name="sereno86"/> One group of the small primitive forms considered to be cerapodans by Sereno, [[Heterodontosauridae]], has since been found to be a group of very early ornithischians of similar evolutionary status as ''Lesothosaurus'',<ref name="butlerea08"/> although this result is not definitive.<ref name="dieudonneea21"/>

The first large-scale numerical analysis of the phylogenetics of Ornithischia was published in [[2008 in paleontology|2008]] by [[Richard J. Butler]] and colleagues, including many primitive ornithischians and members from all of the major subgroups, to test some of the hypotheses given previously about ornithischian evolution and the relationships of the groups. Thyreophora was found to be a supported group, as well as the clade of pachycephalosaurs and ceratopsians that Sereno named [[Marginocephalia]] in 1986. 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. The early Argentinian taxon ''[[Pisanosaurus]]'' was found to be the most primitive ornithischian, but while overall results agreed with earlier studies and showed some stability, areas of the evolutionary tree were found to be problematic, and with potential for later change.<ref name="butlerea08"/> In [[2021 in paleontology|2021]], a new phylogenetic study was published authored by Paul-Emile Dieudonné and colleagues that instead found Heterodontosauridae to nest alongside Pachycephalosauria within Marginocephalia, changing the early evolution of ornithopods considerably, and showing that the evolution of ornithischians was far from definitive.<ref name="dieudonneea21"/> Below are the cladograms of Sereno, Butler and colleagues, and Dieudonné and colleagues, restricted to the major clades of Ornithischia, Heterodontosauridae, ''Lesothosaurus'' and ''Pisanosaurus''.

'''Sereno, 1986'''<ref name="sereno86"/>
{{clade| style=font-size:85%; line-height:85%
|label1='''Ornithischia'''
|1={{clade
 |1=''[[Lesothosaurus]]''
 |label2='''Genasauria'''
 |2={{clade
  |label1=[[Thyreophora]]
  |1={{clade
   |1=[[Stegosauria]] [[File:Stegosaurus stenops.png|80 px]]
   |2=[[Ankylosauria]] [[File:Animantarx ramaljonesi.png|80 px]]}}
  |label2=[[Cerapoda]]
  |2={{clade
   |label1=[[Marginocephalia]]
   |1={{clade
    |1=[[Pachycephalosauria]] [[File:Platytholus clemensi.png|80 px]]
    |2=[[Ceratopsia]] [[File:Triceratops horridus.png|80 px]]}}
   |label2=[[Euornithopoda]]
   |2={{clade
    |1=[[Heterodontosauria]]
    |2=[[Ornithopoda]] [[File:Life reconstruction of Parasaurolophus walkeri.png|80 px]]}} }} }} }} }}

'''Butler et al., 2008'''<ref name="butlerea08"/>
{{clade| style=font-size:85%; line-height:85%
|label1='''Ornithischia'''
|1={{clade
 |1=''[[Pisanosaurus]]''
 |2={{clade
  |1=[[Heterodontosauridae]]
  |label2='''Genasauria'''
  |2={{clade
   |label1=[[Thyreophora]] 
   |1={{clade
    |1=''[[Lesothosaurus]]''
    |2={{clade
     |1=[[Stegosauria]] [[File:Stegosaurus stenops.png|80 px]]
     |2=[[Ankylosauria]] [[File:Animantarx ramaljonesi.png|80 px]]}} }}
   |label2=[[Neornithischia]]
   |2={{clade
    |1={{clade
    |label1=[[Cerapoda]]
    |1={{clade
     |1=[[Ornithopoda]] [[File:Life reconstruction of Parasaurolophus walkeri.png|80 px]]
     |label2=[[Marginocephalia]]
     |2={{clade
      |1=[[Pachycephalosauria]] [[File:Platytholus clemensi.png|80 px]]
      |2=[[Ceratopsia]] [[File:Triceratops horridus.png|80 px]]
}} }} }} }} }} }} }} }}


'''Dieudonné et al., 2021'''<ref name="dieudonneea21"/>
{{clade| style=font-size:85%; line-height:85%
|label1='''Ornithischia'''
|1={{clade
 |1=''[[Lesothosaurus]]''
 |2={{clade
  |1={{clade
  |label1=[[Thyreophora]]
  |1={{clade
   |1=[[Stegosauria]] [[File:Stegosaurus stenops.png|80 px]]
   |2=[[Ankylosauria]] [[File:Animantarx ramaljonesi.png|80 px]]}} }}
  |2={{clade
   |label1=[[Cerapoda]]
   |1={{clade
    |1=[[Ornithopoda]] [[File:Life reconstruction of Parasaurolophus walkeri.png|80 px]]
    |label2=[[Marginocephalia]]
    |2={{clade
     |1=[[Ceratopsia]] [[File:Triceratops horridus.png|80 px]]
     |2=[[Pachycephalosauria]] <small>(incl. [[heterodontosaurid]]s)</small> [[File:Platytholus clemensi.png|80 px]]}} }} }} }} }} }}

===Subgroups===
When Ornithischia was first named, Seeley united the orders Ornithopoda and Stegosauria of Marsh's taxonomy within the new group.<ref name="seeley88"/> [[Ceratopsia]] was then recognized as a unique group related to ornithopods and stegosaurs by Marsh by 1894, with each of the three suborders still being recognized as distinct groups today.<ref name="weishampel04"/><ref name="marsh94"/> Ceratopsians are recognized as group that grew in diversity later in the [[Cretaceous]] after evolving in the [[Late Jurassic]], encompassing a diverse array of bodyforms from the small, bipedal ''[[Psittacosaurus]]'' up to the very large, quadrupedal, horned and frilled [[ceratopsid]]s like ''[[Torosaurus]]'', which has the longest skull of any terrestrial vertebrate.<ref name="youdodson04"/><ref name="dodsonea04"/> 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.<ref name="normanea04c"/><ref name="hornerea04"/> Stegosaurs are comparatively limited, restricted to a primarily Jurassic group of moderate to large, quadrupedal herbivores with two rows of vertical plates ornamenting their spine, which possibly did not go extinct until the Late Cretaceous, though at the time of Marsh Stegosauria was used for all armored and quadrupedal taxa, many of which are now separated into [[Ankylosauria]].<ref name="galtonupchurch04"/><ref name="vickaryousea04"/> Ankylosaurs were only recognized as a distinct group from stegosaurs in the 1920s despite many members being known for decades before, with the group now encompassing a broad array of heavy, quadrupedal ornithischians with extensive armour covering their body and skull.<ref name="vickaryousea04"/> The fifth recognized major subgroup of ornithischians is [[Pachycephalosauria]],<ref name="weishampel04"/> which was first named in [[1974 in paleontology|1974]] after being confused for a long time with the theropod ''[[Troodon]]'' on account of their similarly omnivorous and unique teeth.<ref name="sternberg45"/><ref name="maryanskaea04"/> 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.<ref name="maryanskaea04"/> Some taxa, particularly those at one point grouped 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.<ref name="butlerea08"/>

Following the publication of the [[PhyloCode]] to provide rules and regulations on the use of taxonomic names for groups, the internal classification of Ornithischia was revised by Daniel Madzia and colleagues in 2021 to provide a framework of definitions and taxa for other studies to follow and modify from. They names the new clade Saphornithischia to unite heterodontosaurids with more derived ornithischians to encompass the concept of the well-supported clear ornithischians, as the origins of the group and the relationships of primitive taxa like ''[[Pisanosaurus]]'' and members of [[Silesauridae]] may sometimes be found to be ornithischians outside this core grouping. Madzia and colleagues also provided a composite cladogram of Ornithischia to illustrate the consensus of internal divisions, which can be seen below. Ornithischia has been defined as all taxa closer to ''Iguanodon'' than ''[[Allosaurus]]'' or ''[[Camarasaurus]]''. Genasauria has been defined as the smallest clade containing ''[[Ankylosaurus]]'', ''[[Iguanodon]]'', ''[[Stegosaurus]]'', and ''[[Triceratops]]''.<ref name="madziaea21"/>

{{clade| style=font-size:85%; line-height:85%
|label1='''Ornithischia'''
|1={{clade
 |1=''[[Heterodontosaurus tucki]]'' <small>([[Heterodontosauridae]])</small> [[File:Heterodontosaurus restoration.jpg|50px]]
 |label2='''Genasauria'''
 |2={{clade
  |label1=[[Thyreophora]]
  |1={{clade
   |label1=[[Stegosauria]]
   |1={{clade
    |1=''[[Huayangosaurus taibaii]]'' <small>([[Huayangosauridae]])</small>
    |2=''[[Stegosaurus stenops]]'' <small>([[Stegosauridae]])</small> [[File:Stegosaurus stenops sophie wiki martyniuk.png|50px]] }}
   |label2=[[Ankylosauria]]
   |2={{clade
    |1=''[[Mymoorapelta maysi]]''
    |2={{clade
     |label1=[[Nodosauridae]]
     |1={{clade
      |1=''[[Polacanthus foxii]]'' <small>([[Polacanthinae]])</small> [[File:Polacanthus foxii.jpg|50px]]
      |label2=[[Nodosaurinae]]
      |2={{clade
       |1=''[[Nodosaurus textilis]]''
       |2={{clade
        |1=''[[Panoplosaurus mirus]]'' <small>([[Panoplosaurini]])</small>
        |2=''[[Struthiosaurus austriacus]]'' <small>([[Struthiosaurini]])</small> }} }} }}
     |label2=[[Ankylosauridae]]
     |2={{clade
      |1=''[[Hylaeosaurus armatus]]''
      |2={{clade
       |label1=[[Shamosaurinae]]
       |1={{clade
        |1=''[[Gobisaurus domoculus]]''
        |2=''[[Shamosaurus scutatus]]'' }}
       |label2=[[Ankylosaurinae]]
       |2={{clade
        |1=''[[Pinacosaurus grangeri]]''
        |2={{clade
         |1=''[[Saichania chulsanensis]]''
         |2=''[[Ankylosaurus magniventris]]'' <small>([[Ankylosaurini]])</small> [[File:Ankylosaurus magniventris reconstruction.png|50px]] }} }} }} }} }} }} }}
  |label2=[[Neornithischia]]
  |2={{clade
   |1=''[[Jeholosaurus shangyuanensis]]'' <small>([[Jeholosauridae]])</small>
   |2={{clade
    |1={{clade
    |label1=[[Thescelosauridae]]
    |1={{clade
     |1=''[[Orodromeus makelai]]'' <small>([[Orodrominae]])</small>
     |2=''[[Thescelosaurus neglectus]]'' <small>([[Thescelosaurinae]])</small> }} }}
    |2={{clade
     |1=''[[Hypsilophodon foxii]]'' <small>([[Hypsilophodontidae]])</small> [[File:Hypsilophodon.jpg|50px]]
     |label2=[[Cerapoda]]
     |2={{clade
      |label1=[[Marginocephalia]]
      |1={{clade
       |1={{clade
       |label1=[[Pachycephalosauria]]
       |1={{clade
        |1=''[[Stegoceras validum]]'' [[File:Stegoceras validum.jpg|50px]]
        |2={{clade
        |label1=[[Pachycephalosaurinae]]
        |1={{clade
         |1=''[[Sphaerotholus goodwini]]''
         |2=''[[Pachycephalosaurus wyomingensis]]'' <small>([[Pachycephalosaurini]])</small> }} }} }} }}
       |2={{clade
       |label1=[[Ceratopsia]]
       |1={{clade
        |1=''[[Psittacosaurus mongoliensis]]''
        |2=''[[Chaoyangsaurus youngi]]'' <small>([[Chaoyangsauridae]])</small>
        |label3=[[Neoceratopsia]]
        |3={{clade
         |1=''[[Leptoceratops gracilis]]'' <small>([[Leptoceratopsidae]])</small>
         |label2=[[Coronosauria]]
         |2={{clade
          |1=''[[Protoceratops andrewsi]]'' <small>([[Protoceratopsidae]])</small> [[File:Protoceratops reconstruction.png|50px]]
          |label2=[[Ceratopsidae]]
          |2={{clade
           |1=''[[Ceratops montanus]]''
           |label2=[[Chasmosaurinae]]
           |2={{clade
            |1=''[[Chasmosaurus belli]]''
            |2={{clade
             |1=''[[Anchiceratops ornatus]]'' <div style="{{mirrorH}}">[[File:Anchiceratops dinosaur.png|50px]]</div>
             |2={{clade
              |1=''[[Arrhinoceratops brachyops]]''
              |2=''[[Triceratops horridus]]'' <small>([[Triceratopsini]])</small> }} }} }}
           |label3=[[Centrosaurinae]]
           |3={{clade
            |1=''[[Nasutoceratops titusi]]'' <small>([[Nasutoceratopsini]])</small>
            |label2=[[Eucentrosaura]]
            |2={{clade
             |1=''[[Centrosaurus apertus]]'' <small>([[Centrosaurini]])</small> <div style="{{mirrorH}}">[[File:Centrosaurus.png|50px]]</div>
             |label2=[[Pachyrhinosaurini]]
             |2={{clade
              |1=''[[Achelousaurus horneri]]''
              |2=''[[Pachyrhinosaurus canadensis]]'' }} }} }} }} }} }} }} }} }}
      |label2=[[Ornithopoda]]
      |2={{clade
       |label1=[[Elasmaria]]
       |1={{clade
        |1=''[[Gasparinisaura cincosaltensis]]'' [[File:Gasparinisaura BW.jpg|50px]]
        |2=''[[Macrogryphosaurus gondwanicus]]''
        |3=''[[Talenkauen santacrucensis]]'' }}
       |label2=[[Iguanodontia]]
       |2={{clade
        |1={{clade
         |label1=[[Rhabdodontidae]]
         |1={{clade
          |1=''[[Rhabdodon priscus]]''
          |2=''[[Zalmoxes robustus]]'' }} }}
        |2={{clade
         |1=''[[Tenontosaurus tilletti]]'' [[File:Tenontosaurus BW.jpg|50px]]
         |label2=[[Dryomorpha]]
         |2={{clade
          |1=''[[Dryosaurus altus]]'' <small>([[Dryosauridae]])</small>
          |label2=[[Ankylopollexia]]
          |2={{clade
           |1=''[[Camptosaurus dispar]]'' <small>([[Camptosauridae]])</small> [[File:Camptosaurus.jpg|50px]]
           |label2=[[Styracosterna]]
           |2={{clade
            |1=''[[Hypselospinus fittoni]]''
            |label2=[[Hadrosauriformes]]
            |2={{clade
             |1=''[[Iguanodon bernissartensis]]'' <small>([[Iguanodontidae]])</small> [[File:Iguanodon NT.jpg|50px]]
             |label2=[[Hadrosauroidea]]
             |2={{clade
              |1=''[[Probactrosaurus gobiensis]]''
              |label2=[[Hadrosauridae]]
              |2={{clade
               |1=''[[Hadrosaurus foulkii]]'' <small>([[Hadrosaurinae]])</small>
               |2=''[[Saurolophus osborni]]'' <small>([[Saurolophinae]])</small> [[File:Saurolophus scalation.png|50px]]
               |3=''[[Lambeosaurus lambei]]'' <small>([[Lambeosaurinae]])</small> }} }} }} }} }} }} }} }} }} }} }} }} }} }} }} }}

Multiple taxa within Ornithischia fall around the origin of the group, or cannot be classified definitively. ''Lesothosaurus'' and ''[[Laquintasaura]]'' have been found as basal thyreophorans or basal ornithischians, ''[[Chilesaurus]]'' is either a theropod or a basal ornithischian, ''Pisanosaurus'' has been found as a basal ornithischian or a non-ornithischian silesaurid, ''[[Eocursor]]'' has been a basal ornithischian or a basal member of Neornithischia, ''[[Serendipaceratops]]'' cannot be classified beyond Ornithischia as it is either an ankylosaur or a ceratopsian, and ''[[Alocodon]]'', ''[[Fabrosaurus]]'', ''[[Ferganocephale]]'', ''[[Gongbusaurus]]'', ''[[Taveirosaurus]]'', ''[[Trimucrodon]]'' and ''[[Xiaosaurus]]'' are dubious ornithischians of uncertain basal classification.<ref name="madziaea21"/><ref name="normanea22"/><ref name="poropatea23"/><ref name="normanea04a"/><ref name="sullivan06"/> Depending on the phylogenetic results, [[Silesauridae]] could either be a clade within Ornithischia, its members could form an evolutionary gradient, or some members found form a clade while others are part of a gradient.<ref name="normanea22"/><ref name="mullergarcia23"/>

===Evolution===
[[File:Revueltosaurus-Skull.jpg|thumb|Skull of ''[[Revueltosaurus]]'', a genus originally considered an early ornithischian based on its teeth<ref name="parkerea05"/>]]
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 gibbidens|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.<ref name="irmisea07"/> The only early ornithischians that were considered to be diagnostic in a 2004 review by Norman and colleagues were ''Lesothosaurus'', ''Pisanosaurus'' and ''[[Technosaurus]]'', limiting the early ornithischian record to only two Triassic genera from [[Argentina]] and the United States and one Early Jurassic genus from [[South Africa]], with all the tooth taxa being considered undiagnostic.<ref name="normanea04a"/> Referrals of isolated teeth to Ornithischia based on herbivorous features began to be extensively questioned by William G. Parker and colleagues in [[2005 in paleontology|2005]] after the discovery of skull and skeleton material clearly from ''Revueltosaurus'' showing that the "ornithischian-like" teeth were from an animal more closely related to [[crocodile]]s than [[bird]]s, and there were multiple occurrences of herbivory throughout Triassic reptiles.<ref name="irmisea07"/><ref name="parkerea05"/> Removing the list of Triassic tooth taxa from Ornithischia, the early diversity of the group was substantially reduced, especially in comparison to the known Triassic diversity of theropods and sauropodomorphs. If ''Pisanosaurus'' represented the earliest ornithischian, there would be at least a 20 million year gap in the evolution of Ornithischia until ''Lesothosaurus'' and heterodontosaurids. It is possible that the limited early record of ornithischians is due to them inhabiting environments that were less conducive to fossilization, or that the phylogenetics of the group were incorrect and that early ornithischians were already known but identified as members of other groups.<ref name="irmisea07"/>
{{cladogram |align=left |title=Possible&nbsp;evolutionary&nbsp;relationships&nbsp;of&nbsp;Ornithischia<ref name="normanea22"/>
|cladogram=
{{clade| style=font-size:85%; line-height:85%
|style1=background-color:#eeeeee
|1={{clade
 |1=[[Silesauridae]]
 |label2=[[Dinosauria]]
 |2={{clade
  |2='''Ornithischia'''
  |label1=[[Saurischia]]
  |1={{clade
   |1=[[Sauropodomorpha]]&nbsp;
   |2=[[Theropoda]] }} }} }}
|grouplabel1={{clade
|style1=background-color:#eeeeee
|1={{clade
 |1=[[Silesauridae]]
 |label2=[[Dinosauria]]
 |2={{clade
  |1=[[Theropoda]]
  |label2=[[Phytodinosauria]]
  |2={{clade
   |1=[[Sauropodomorpha]]&nbsp;
   |2='''Ornithischia''' }} }} }} }}
}}
|cladogram2=
{{clade| style=font-size:85%; line-height:85%
|style1=background-color:#eeeeee
|1={{clade
 |1=[[Silesauridae]]
 |label2=[[Dinosauria]]
 |2={{clade
  |1=[[Sauropodomorpha]]&nbsp;
  |label2=[[Ornithoscelida]]
  |2={{clade
   |2='''Ornithischia'''
   |1=[[Theropoda]] }} }} }}
|grouplabel1=
{{clade
|style1=background-color:#eeeeee
|label1=[[Dinosauria]]
|1={{clade
 |label2='''Ornithischia'''
 |2={{clade
  |1=[[silesaur]]s
  |2="traditional" ornithischians }}
 |label1=[[Saurischia]]
 |1={{clade
  |1=[[Sauropodomorpha]]&nbsp;
  |2=[[Theropoda]] }} }} }}
}}
}}
First noted in the [[2003 in paleontology|2003]] naming of the early taxon ''[[Silesaurus]]'', some taxa generally considered non-dinosaurs show similarities to ornithischians in the teeth and jaw anatomy.<ref name="dzik03"/><ref name="ferigololanger07"/> These basal taxa, which were then grouped within [[Silesauridae]] and commonly as the sister group to Dinosauria, may instead be the earliest ornithischians. They show adaptations for the evolution of herbivory, and can fill in the gap in early evolution of ornithischians that were otherwise only clearly known since the beginning of the [[Jurassic]]. This hypothesis has found support in multiple different phylogenetic analyses,<ref name="cabreiraea16"/><ref name="mullergarcia20"/> but the results are not yet accepted as definitive enough to contradict other possible evolutionary strategies of dinosaurs. Alternatively, and more in line with earlier studies on dinosaur evolution, silesaurids may be the sister taxa to the Saurischia-Ornithischia split, or even other arrangements of the three main dinosaur groups Ornithischia, [[Sauropodomorpha]], and [[Theropoda]].<ref name="normanea22"/> The [[2017 in paleontology|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. Under this case, the omnivory in the earliest sauropodomorphs and ornithischians would be the ancestral condition for dinosaurs, along with the grasping abilities seen in the earliest ornithischians and theropods.<ref name="baronea17a"/> While Ornithoscelida is a possible hypothesis for the evolution of dinosaurs and the close relationships of Ornithischia, follow-up studies have not found it statistically more likely than the traditional dichotomy of Ornithischia and Saurischia, or the third alternative, [[Phytodinosauria]], where ornithischians and sauropodomorphs are closer to each other than theropods.<ref name="langerea17"/><ref name="baronea17b"/><ref name="parryea17"/>

Along with ''Pisanosaurus'', which was supported as the earliest ornithischian for a time before being considered just as likely to be a silesaur rather than an ornithischian, an additional problematic taxon is ''[[Chilesaurus]]'' from the Late Jurassic of [[Chile]]. 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. As the earliest ornithischian, ''Chilesaurus'' tied multiple details of ornithischian and theropod anatomy together supporting their union in Ornithoscelida, though when it is not the basalmost ornithischian, a traditional Saurischia is recovered. 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.<ref name="normanea22"/> ''Daemonosaurus'', typically a theropod or close relative of [[herrerasaur]]s, has also been found as the basalmost ornithischian at times when Ornithoscelida is recovered, but it does not share any unique features with ornithischians and redescribing its anatomy found it fairly confidently to be a basal dinosaur not related closely to Ornithischia.<ref name="nesbittsues21"/>

The phylogenetic analysis of Norman and colleagues in [[2022 in paleontology|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. Norman and colleagues used Prionodontia over both Saphornithischia and Genasauria, since all were recovered as encompassing the same node.<ref name="normanea22"/> The earliest ornithischians under this reconstruction were faunivorous, as seen by ''[[Lewisuchus]]'', which has typical teeth like theropods. Serrations on teeth become larger for taxa more derived than ''[[Asilisaurus]]'', the development of a [[cingulum (tooth)|cingulum]] in teeth is seen in ''Technosaurus'' and later ornithischians, the lower jaw becomes more elongate in taxa above ''Silesaurus'', and core ornithischians are united by the [[pubic bone]] angling backwards, and the modification of the ankle joint.<ref name="mullergarcia20"/>
[[File:Silesaurid left dentaries.png|thumb|right|Lower jaws and teeth of (top to bottom) ''[[Kwanasaurus]]'', ''[[Asilisaurus]]'', ''[[Eucoelophysis]]'', ''[[Technosaurus]]'', ''[[Sacisaurus]]'', ''[[Silesaurus]]'', ''[[Diodorus scytobrachion]]'' and ''[[Soumyasaurus]]'']]
{{clade| style=font-size:85%; line-height:85%
|label1='''[[Dinosauria]]'''
|1={{clade
 |label1='''[[Saurischia]]'''
 |grouplabel1={{Clade labels|label1=Faunivorous|color=pink}}
 |1={{clade
  |1=[[Herrerasauridae]]|barbegin1=pink
  |2={{clade
   |1=''[[Daemonosaurus]]''|bar1=pink
   |2={{clade
    |1={{clade
     |1=''[[Chindesaurus]]''|bar1=pink
     |2=''[[Tawa hallae]]''|bar2=pink }}
    |2={{clade
     |1=''[[Eodromaeus]]''|bar1=pink
     |2={{clade
      |1='''[[Sauropodomorpha]]'''|bar1=pink
      |2='''[[Theropoda]]'''|bar2=pink }} }} }} }} }}
 |label2='''Ornithischia'''
 |grouplabel2={{Clade labels|label1=Herbivorous|color=lightgreen}}
 |2={{clade
  |1=''[[Lewisuchus]]''|barend1=pink
  |2={{clade
   |1=''[[Soumyasaurus]]''|barbegin1=lightgreen
   |2={{clade
    |1=''[[Asilisaurus]]''|bar1=lightgreen
    |2={{clade
    |label1=Sulcimentisauria
    |1={{clade
     |1=''[[Diodorus scytobrachion]]''|bar1=lightgreen
     |2={{clade
      |1=''[[Technosaurus]]''|bar1=lightgreen
      |2={{clade
      |label1=Parapredentata
      |1={{clade
       |1={{clade
        |1=''[[Ignotosaurus]]''|bar1=lightgreen
        |2=''[[Silesaurus]]''|bar2=lightgreen }}
       |2={{clade
        |1={{clade
         |1=''[[Sacisaurus]]''|bar1=lightgreen
         |2={{clade
          |1=''[[Lutungutali]]''|bar1=lightgreen
          |2=''[[Kwanasaurus]]''|bar2=lightgreen }} }}
        |2={{clade
         |1=''[[Eucoelophysis]]''|bar1=lightgreen
         |2={{clade
          |1=''[[Pisanosaurus]]''|bar1=lightgreen
          |2={{clade
           |1=''[[Laquintasaura]]''|bar1=lightgreen
           |label2=Prionodontia
           |2={{clade
            |1='''[[Thyreophora]]'''|bar1=lightgreen
            |2='''[[Neornithischia]]'''|barend2=lightgreen }} }} }} }} }} }} }} }} }} }} }} }} }} }} }}

== Palaeoecology ==
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.<ref>{{cite journal|title=Dynamic Locomotor Capabilities Revealed by Early Dinosaur Trackmakers from Southern Africa|author=Jeffrey A. Wilson|author2=Claudia A. Marsicano|author3=Roger M. H. Smith|date=6 October 2009|doi=10.1371/journal.pone.0007331|pmid=19806213|volume=4|issue = 10|journal=PLOS ONE|article-number=e7331|pmc=2752196|bibcode=2009PLoSO...4.7331W |doi-access=free}}</ref>

Most ornithischians were herbivorous.<ref name=":0" /> In fact, most of the unifying characters of Ornithischia are thought to be related to this herbivory.<ref name=":0" /> For example, the shift to an opisthopubic pelvis is thought to be related to the development of a large stomach or stomachs and gut which would allow ornithischians to more effectively digest plant matter.<ref name=":0" /> The smallest known ornithischian is ''[[Fruitadens|Fruitadens haagarorum]]''.<ref name=":4">{{Cite journal|last1=Butler|first1=Richard J.|last2=Galton|first2=Peter M.|last3=Porro|first3=Laura B.|last4=Chiappe|first4=Luis M.|last5=Henderson|first5=Donald M.|last6=Erickson|first6=Gregory M.|date=2010-02-07|title=Lower limits of ornithischian dinosaur body size inferred from a new Upper Jurassic heterodontosaurid from North America|journal=Proceedings of the Royal Society of London B: Biological Sciences|language=en|volume=277|issue=1680|pages=375–381|doi=10.1098/rspb.2009.1494|issn=0962-8452|pmc=2842649|pmid=19846460}}</ref> The largest ''Fruitadens'' individuals reached just 65–75&nbsp;cm. Previously, only carnivorous, saurischian theropods were known to reach such small sizes.<ref name=":4" /> At the other end of the spectrum, the largest known ornithischians reach about 15 meters (smaller than the largest saurischians).<ref>{{Cite journal|last1=Yannan|first1=Ji|last2=Xuri|first2=Wang|last3=Yongqing|first3=Liu|last4=Qiang|first4=Ji|date=2011-02-01|title=Systematics, Behavior and Living Environment of Shantungosaurus Giganteus (Dinosauria: Hadrosauridae)|journal=Acta Geologica Sinica - English Edition|language=en|volume=85|issue=1|pages=58–65|doi=10.1111/j.1755-6724.2011.00378.x|bibcode=2011AcGlS..85...58J |s2cid=85351874 |issn=1755-6724}}</ref>

However, not all ornithischians were strictly herbivorous. Some groups, like the [[heterodontosauridae|heterodontosaurids]], were likely [[omnivore]]s.<ref>{{cite journal | last1 = Barrett | first1 = P. M. | last2 = Rayfield | first2 = E. J. | year = 2006 | title = Ecological and evolutionary implications of dinosaur feeding behaviour | journal = Trends in Ecology & Evolution | volume = 21 | issue = 4| pages = 217–224 | doi=10.1016/j.tree.2006.01.002| pmid = 16701088 | url = http://doc.rero.ch/record/16151/files/PAL_E2427.pdf }}</ref> At least one species of [[ankylosauria]]n, ''[[Liaoningosaurus paradoxus]]'', appears to have been at least partially [[carnivore|carnivorous]], with hooked claws, fork-like teeth, and stomach contents suggesting that it may have fed on fish.<ref name=liaoningosaurus_fish>{{cite journal | last1 = Ji | first1 = Q. | last2 = Wu | first2 = X. | last3 = Cheng | first3 = Y. | last4 = Ten | first4 = F. | last5 = Wang | first5 = X. | last6 = Ji | first6 = Y. | year = 2016 | title = Fish-hunting ankylosaurs (Dinosauria, Ornithischia) from the Cretaceous of China | journal = Journal of Geology | volume = 40 | page = 2 }}</ref> The members of Genasauria were primarily herbivores.<ref>Barrett, P. M., & Rayfield, E. J. (2006). Ecological and evolutionary implications of dinosaur feeding behaviour. Trends in Ecology & Evolution, 21(4), 217–224.</ref> 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 [[gymnosperm]]s."<ref name="fastovsky">Fastovsky, D. E., & Weishampel, D. B. (2012). Dinosaurs: a concise natural history (2nd ed). Cambridge; New York: Cambridge University Press.</ref>

There is strong evidence that some ornithischians lived in herds.<ref name=":0" /><ref name=":1" /> This evidence consists of multiple [[bone bed]]s where large numbers of individuals of the same species and of different age groups died simultaneously.<ref name=":0" /><ref name=":1" />

== See also ==
* {{Portal-inline|Dinosaurs}}

==Notes==
{{Notelist}}

==References==
<references>
<ref name="weishampel04">{{cite book |last1=Weishampel |first1=D.B. |editor1-last=Weishampel |editor1-first=D.B. |editor2-last=Osmólska |editor2-first=H. |editor3-last=Dodson |editor3-first=P. |title=The Dinosauria |date=2004 |publisher=University of California Press |isbn=0-520-24209-2 |pages=323–324 |edition=2nd |chapter=Ornithischia}}</ref>
<ref name="normanea04a">{{cite book |last1=Norman |first1=D.B. |last2=Witmer |first2=L.M. |last3=Weishampel |first3=D.B. |editor1-last=Weishampel |editor1-first=D.B. |editor2-last=Osmólska |editor2-first=H. |editor3-last=Dodson |editor3-first=P. |title=The Dinosauria |date=2004 |publisher=University of California Press |isbn=0-520-24209-2 |pages=325–334 |edition=2nd |chapter=Basal Ornithischia}}</ref>
<ref name="madziaea21">{{cite journal |last1=Madzia |first1=D. |last2=Arbour |first2=V.M. |last3=Boyd |first3=C.A. |last4=Farke |first4=A.A. |last5=Cruzado-Caballero |first5=P. |last6=Evans |first6=D.C. |title=The phylogenetic nomenclature of ornithischian dinosaurs |journal=PeerJ |date=2021 |volume=9 |article-number=e12362 |doi=10.7717/peerj.12362|doi-access=free |pmid=34966571 |pmc=8667728 }}</ref>
<ref name="seeley88">{{cite journal |last1=Seeley |first1=H.G. |title=On the classification of the fossil animals commonly named Dinosauria |journal=Proceedings of the Royal Society of London |date=1888 |volume=43 |issue=258–265 |pages=165–171 |doi=10.1098/rspl.1887.0117}}</ref>
<ref name="sereno86">{{cite journal |last1=Sereno |first1=P.C. |title=Phylogeny of the Bird-Hipped Dinosaurs (Ornithischia) |journal=National Geographic Research |date=1986 |volume=2 |issue=2 |pages=234–256}}</ref>
<ref name="butlerea08">{{cite journal |last1=Butler |first1=R.J. |last2=Upchurch |first2=P. |last3=Norman |first3=D.B. |title=The phylogeny of the ornithischian dinosaurs |journal=Journal of Systematic Palaeontology |date=2008 |volume=6 |issue=1 |pages=1–40 |doi=10.1017/S1477201907002271|bibcode=2008JSPal...6....1B }}</ref>
<ref name="normanea22">{{cite journal |last1=Norman |first1=D.B. |last2=Baron |first2=M.G. |last3=Garcia |first3=M.S. |last4=Müller |first4=R.T. |title=Taxonomic, palaeobiological and evolutionary implications of a phylogenetic hypothesis for Ornithischia (Archosauria: Dinosauria) |journal=Zoological Journal of the Linnean Society |date=2022 |volume=196 |issue=4 |pages=1273–1309 |doi=10.1093/zoolinnean/zlac062}}</ref>
<ref name="cope66">{{cite journal |last1=Cope |first1=E.D. |title=The anomalous relations existing between the tibia and fibula in certain of the Dinosauria |journal=Proceedings of the Academy of Natural Sciences of Philadelphia |date=1866 |volume=18 |pages=316–317}}</ref>
<ref name="marsh94">{{cite journal |last1=Marsh |first1=O.C. |title=The typical Ornithopoda of the American Jurassic |journal=American Journal of Science |date=1894 |volume=48 |issue=283 |pages=85–90|doi=10.2475/ajs.s3-48.283.85 |bibcode=1894AmJS...48...85M |url=https://zenodo.org/record/2161731 }}</ref>
<ref name="marsh81">{{cite journal |last1=Marsh |first1=O.C. |title=Principal characters of the American Jurassic Dinosaurs |journal=American Journal of Science |date=1881 |volume=21 |issue=3 |pages=417–423}}</ref>
<ref name="romer66">{{cite book |last1=Romer |first1=A.S. |title=Vertebrate Paleontology |date=1966 |publisher=University of Chicago Press |isbn=0-7167-1822-7 |pages=148–163 |edition=3rd |chapter=Dinosaurs}}</ref>
<ref name="kuhn46">{{cite journal |last1=Kuhn |first1=O. |title=Das System der fossilen und rezenten Amphibien und Reptilien |journal=Bericht der Naturforschenden Gesellschaft in Bamberg |date=1946 |volume=29 |pages=49–67}}</ref>
<ref name="bakkergalton74">{{cite journal |last1=Bakker |first1=R.T. |last2=Galton |first2=P.M. |title=Dinosaur Monophyly and a New Class of Vertebrates |journal=Nature |date=1974 |volume=248 |issue=5444 |pages=168–172 |doi=10.1038/248168a0|bibcode=1974Natur.248..168B }}</ref>
<ref name="milnernorman84">{{cite book |last1=Milner |first1=A.R. |last2=Norman |first2=D.B. |editor1-last=Reif |editor1-first=W.E. |editor2-last=Westphal |editor2-first=F. |title=Third Symposium on Mesozoic Terrestrial Ecosystems, Short Papers |date=1984 |publisher=ATTEMPTO Verlag |pages=145–150 |chapter=The biogeography of advanced ornithopod dinosaurs (Archosauria: Ornithischia) - a cladistic-vicariance model}}</ref>
<ref name="norman84a">{{cite book |last1=Norman |first1=D.B. |editor1-last=Reif |editor1-first=W.E. |editor2-last=Westphal |editor2-first=F. |title=Third Symposium on Mesozoic Terrestrial Ecosystems, Short Papers |date=1984 |publisher=ATTEMPTO Verlag |pages=157–162 |chapter=A systematic reappraisal of the reptile order Ornithischia}}</ref>
<ref name="norman84b">{{cite book |last1=Norman |first1=D.B. |editor1-last=Ferguson |editor1-first=M.W.J. |title=The Structure, Development, and Evolution of Reptiles |date=1984 |publisher=Symposia of the Zoological Society of London |volume=52 |pages=521–547 |chapter=On the Cranial Morphology and Evolution of Ornithopod Dinosaurs}}</ref>
<ref name="sereno84">{{cite book |last1=Sereno |first1=P.C. |editor1-last=Reif |editor1-first=W.E. |editor2-last=Westphal |editor2-first=F. |title=Third Symposium on Mesozoic Terrestrial Ecosystems, Short Papers |date=1984 |publisher=ATTEMPTO Verlag |pages=219–226 |chapter=Thy phylogeny of the Ornithischia : a reappraisal}}</ref>
<ref name="dieudonneea21">{{cite journal |last1=Dieudonné |first1=P.-E. |last2=Cruzado-Caballero |first2=P. |last3=Godefroit |first3=P. |last4=Tortosa |first4=T. |title=A new phylogeny of cerapodan dinosaurs |journal=Historical Biology |date=2021 |volume=33 |issue=10 |pages=2335–2355 |doi=10.1080/08912963.2020.1793979|bibcode=2021HBio...33.2335D |doi-access=free }}</ref>
<ref name="dzik03">{{cite journal |last1=Dzik |first1=J. |title=A beaked herbivorous archosaur with dinosaur affinities from the early Late Triassic of Poland |journal=Journal of Vertebrate Paleontology |date=2003 |volume=23 |issue=3 |pages=556–574|doi=10.1671/A1097 |bibcode=2003JVPal..23..556D }}</ref>
<ref name="ferigololanger07">{{cite journal |last1=Ferigolo |first1=J. |last2=Langer |first2=M.C. |title=A Late Triassic dinosauriform from south Brazil and the origin of the ornithischian predentary bone |journal=Historical Biology |date=2007 |volume=19 |issue=1 |pages=23–33 |doi=10.1080/08912960600845767|bibcode=2007HBio...19...23F }}</ref>
<ref name="cabreiraea16">{{cite journal |last1=Cabreira |first1=S.F. |last2=Kellner |first2=A.W.A. |last3=Dias-da-Silva |first3=S. |last4=da Silva |first4=L.R. |last5=Bronzati |first5=M. |last6=Marsola |first6=J.C.A. |last7=Müller |first7=R.T. |last8=Bittencourt |first8=J.S. |last9=Batista |first9=B.J.A. |last10=Raugust |first10=T. |last11=Carrilho |first11=R. |last12=Brodt |first12=A. |last13=Langer |first13=M.C. |title=A Unique Late Triassic Dinosauromorph Assemblage Reveals Dinosaur Ancestral Anatomy and Diet |journal=Current Biology |date=2016 |volume=26 |issue=22 |pages=3090–3095 |doi=10.1016/j.cub.2016.09.040|pmid=27839975 |bibcode=2016CBio...26.3090C |doi-access=free }}</ref>
<ref name="mullergarcia20">{{cite journal |last1=Müller |first1=R.T. |last2=Garcia |first2=M.S. |title=A paraphyletic 'Silesauridae' as an alternative hypothesis for the initial radiation of ornithischian dinosaurs |journal=Biology Letters |date=2020 |volume=16 |issue=8 |pages=1–5 |doi=10.1098/rsbl.2020.0417|pmid=32842895 |pmc=7480155 }}</ref>
<ref name="baronea17a">{{cite journal |last1=Baron |first1=M.G. |last2=Norman |first2=D.B. |last3=Barrett |first3=P.M. |title=A new hypothesis of dinosaur relationships and early dinosaur evolution |journal=Nature |date=2017 |volume=543 |issue=7646 |pages=501–506 |doi=10.1038/nature21700|pmid=28332513 |bibcode=2017Natur.543..501B }}</ref>
<ref name="langerea17">{{cite journal |last1=Langer |first1=M.C. |last2=Ezcurra |first2=M.D. |last3=Rauhut |first3=O.W.M. |last4=Benton |first4=M.J. |last5=Knoll |first5=F. |last6=McPhee |first6=B.W. |last7=Novas |first7=F.E. |last8=Pol |first8=D. |last9=Brusatte |first9=S.L. |title=Untangling the dinosaur family tree |journal=Nature |date=2017 |volume=551 |issue=7678 |pages=E1–E3 |doi=10.1038/nature24011|pmid=29094688 |bibcode=2017Natur.551E...1L |hdl=1983/d088dae2-c7fa-4d41-9fa2-aeebbfcd2fa3 |hdl-access=free }}</ref>
<ref name="baronea17b">{{cite journal |last1=Baron |first1=M.G. |last2=Norman |first2=D.B. |last3=Barrett |first3=P.M. |title=Baron et al. reply |journal=Nature |date=2017 |volume=551 |issue=7678 |pages=E4–E5 |doi=10.1038/nature24012|pmid=29094705 |bibcode=2017Natur.551E...4B }}</ref>
<ref name="parryea17">{{cite journal |last1=Parry |first1=L.A. |last2=Baron |first2=M.G. |last3=Vinther |first3=J. |title=Multiple optimality criteria support Ornithoscelida |journal=Royal Society Open Science |date=2017 |volume=4 |issue=10 |article-number=170833 |doi=10.1098/rsos.170833|pmid=29134086 |pmc=5666269 |bibcode=2017RSOS....470833P }}</ref>
<ref name="irmisea07">{{cite journal |last1=Irmis |first1=R.B. |last2=Parker |first2=W.G. |last3=Nesbitt |first3=S.J. |last4=Liu |first4=J. |title=Early ornithischian dinosaurs: the Triassic record |journal=Historical Biology |date=2007 |volume=19 |issue=1 |pages=3–22 |doi=10.1080/08912960600719988|bibcode=2007HBio...19....3I }}</ref>
<ref name="parkerea05">{{cite journal |last1=Parker |first1=W.G. |last2=Irmis |first2=R.B. |last3=Nesbitt |first3=S.J. |last4=Martz |first4=J.W. |last5=Browne |first5=L.S. |title=The Late Triassic pseudosuchian ''Revueltosaurus callenderi'' and its implications for the diversity of early ornithischian dinosaurs |journal=Proceedings of the Royal Society B |date=2005 |volume=272 |issue=1566 |pages=963–969 |doi=10.1098/rspb.2004.3047|pmid=16024353 |pmc=1564089 }}</ref>
<ref name="youdodson04">{{cite book |last1=You |first1=H. |last2=Dodson |first2=P. |last3=Weishampel |first3=D.B. |editor1-last=Weishampel |editor1-first=D.B. |editor2-last=Osmólska |editor2-first=H. |editor3-last=Dodson |editor3-first=P. |title=The Dinosauria |date=2004 |publisher=University of California Press |isbn=0-520-24209-2 |pages=478–494 |edition=2nd |chapter=Basal Ceratopsia}}</ref>
<ref name="dodsonea04">{{cite book |last1=Dodson |first1=P. |last2=Forster |first2=C.A. |last3=Sampson |first3=S.D. |editor1-last=Weishampel |editor1-first=D.B. |editor2-last=Osmólska |editor2-first=H. |editor3-last=Dodson |editor3-first=P. |title=The Dinosauria |date=2004 |publisher=University of California Press |isbn=0-520-24209-2 |pages=494–513 |edition=2nd |chapter=Ceratopsidae}}</ref>
<ref name="normanea04c">{{cite book |last1=Norman |first1=D.B. |last2=Sues |first2=H.-D. |last3=Witmer |first3=L.M. |last4=Coria |first4=R.A. |editor1-last=Weishampel |editor1-first=D.B. |editor2-last=Osmólska |editor2-first=H. |editor3-last=Dodson |editor3-first=P. |title=The Dinosauria |date=2004 |publisher=University of California Press |isbn=0-520-24209-2 |pages=393–412 |edition=2nd |chapter=Basal Ornithopoda}}</ref>
<ref name="hornerea04">{{cite book |last1=Horner |first1=J.R. |last2=Weishampel |first2=D.B. |last3=Forster |first3=C.A. |editor1-last=Weishampel |editor1-first=D.B. |editor2-last=Osmólska |editor2-first=H. |editor3-last=Dodson |editor3-first=P. |title=The Dinosauria |date=2004 |publisher=University of California Press |isbn=0-520-24209-2 |pages=438–463 |edition=2nd |chapter=Hadrosauridae}}</ref>
<ref name="galtonupchurch04">{{cite book |last1=Galton |first1=P.M. |last2=Upchurch |first2=P.C. |editor1-last=Weishampel |editor1-first=D.B. |editor2-last=Osmólska |editor2-first=H. |editor3-last=Dodson |editor3-first=P. |title=The Dinosauria |date=2004 |publisher=University of California Press |isbn=0-520-24209-2 |pages=343–362 |edition=2nd |chapter=Stegosauria}}</ref>
<ref name="vickaryousea04">{{cite book |last1=Vickaryous |first1=M.K. |last2=Maryańska |first2=T. |last3=Weishampel |first3=D.B. |editor1-last=Weishampel |editor1-first=D.B. |editor2-last=Osmólska |editor2-first=H. |editor3-last=Dodson |editor3-first=P. |title=The Dinosauria |date=2004 |publisher=University of California Press |isbn=0-520-24209-2 |pages=363–392 |edition=2nd |chapter=Ankylosauria}}</ref>
<ref name="maryanskaea04">{{cite book |last1=Maryańska |first1=T. |last2=Chapman |first2=R.E. |last3=Weishampel |first3=D.B. |editor1-last=Weishampel |editor1-first=D.B. |editor2-last=Osmólska |editor2-first=H. |editor3-last=Dodson |editor3-first=P. |title=The Dinosauria |date=2004 |publisher=University of California Press |isbn=0-520-24209-2 |pages=464–477 |edition=2nd |chapter=Pachycephalosauria}}</ref>
<ref name="sternberg45">{{cite journal |last1=Sternberg |first1=C.M. |title=Pachycephalosauridae Proposed for Dome-Headed Dinosaurs, ''Stegoceras lambei'', n. sp., Described |journal=Journal of Paleontology |date=1945 |volume=19 |issue=5 |pages=534–538}}</ref>
<ref name="nesbittsues21">{{cite journal |last1=Nesbitt |first1=S.J. |last2=Sues |first2=H.-D. |title=The osteology of the early-diverging dinosaur ''Daemonosaurus chauliodus'' (Archosauria: Dinosauria) from the ''Coelophysis'' Quarry (Triassic: Rhaetian) of New Mexico and its relationships to other early dinosaurs |journal=Zoological Journal of the Linnean Society |date=2021 |volume=191 |issue=1 |pages=150–179 |doi=10.1093/zoolinnean/zlaa080}}</ref>
<ref name="paul24">{{cite book |last1=Paul |first1=G.S. |title=The Princeton Field Guide to Dinosaurs |date=2024 |publisher=Princeton University Press |isbn=978-0-691-23157-0 |pages=261–373 |edition=3rd}}</ref>
<ref name="normanea04b">{{cite book |last1=Norman |first1=D.B. |last2=Witmer |first2=L.M. |last3=Weishampel |first3=D.B. |editor1-last=Weishampel |editor1-first=D.B. |editor2-last=Osmólska |editor2-first=H. |editor3-last=Dodson |editor3-first=P. |title=The Dinosauria |date=2004 |publisher=University of California Press |isbn=0-520-24209-2 |pages=335–342 |edition=2nd |chapter=Basal Thyreophora}}</ref>
<ref name="norman04">{{cite book |last1=Norman |first1=D.B. |editor1-last=Weishampel |editor1-first=D.B. |editor2-last=Osmólska |editor2-first=H. |editor3-last=Dodson |editor3-first=P. |title=The Dinosauria |date=2004 |publisher=University of California Press |isbn=0-520-24209-2 |pages=413–437 |edition=2nd |chapter=Basal Iguanodontia}}</ref>
<ref name="poropatea23">{{cite journal |last1=Poropat |first1=S.F. |last2=Bell |first2=P.R. |last3=Hart |first3=L.J. |last4=Salisbury |first4=S.W. |last5=Kear |first5=B.P. |title=An annotated checklist of Australian Mesozoic tetrapods |journal=Alcheringa: An Australasian Journal of Palaeontology |date=2023 |volume=47 |issue=2 |pages=129–205 |doi=10.1080/03115518.2023.2228367|bibcode=2023Alch...47..129P |hdl=20.500.11937/96166 |hdl-access=free }}</ref>
<ref name="sullivan06">{{cite journal |last1=Sullivan |first1=R.M. |title=A taxonomic review of the Pachycephalosauridae (Dinosauria: Ornithischia) |journal=New Mexico Museum of Natural History and Science Bulletin |date=2006 |volume=35 |pages=347–366}}</ref>
<ref name="mullergarcia23">{{cite journal |last1=Müller |first1=R.T. |last2=Garcia |first2=M.S. |title=A new silesaurid from Carnian beds of Brazil flls a gap in the radiation of avian line archosaurs |journal=Scientific Reports |date=2023 |volume=13 |issue=1 |page=4981 |doi=10.1038/s41598-023-32057-x|pmid=37041170 |pmc=10090097 |bibcode=2023NatSR..13.4981M }}</ref>
</references>

==External links==
{{Wikispecies|Ornithischia}}
* [https://web.archive.org/web/20081201083750/http://www.palaeos.com/Vertebrates/Units/320Ornithischia/100.html#Ornithischia Ornithischia], from Palæos. (cladogram, characteristics)

{{Avemetatarsalia|state=autocollapse}}
{{Ornithischia|state=autocollapse}}
{{Taxonbar|from1=Q191488|from2=Q1390908}}
{{Authority control}}

[[Category:Ornithischia| ]]
[[Category:Dinosaur clades]]