Ornithischia

Ornithischia (/ɔːrnɪˈθɪskiə/) is an extinct clade of mainly herbivorous dinosaurs characterized by a pelvic structure similar to that of birds.[2] The name Ornithischia, or "bird-hipped", reflects this similarity and is derived from the Greek stem ornith- (ὀρνιθ-), meaning "of a bird", and ischion (ἴσχιον), plural ischia, meaning "hip joint". However, birds are only distantly related to this group as birds are theropod dinosaurs.[2]

Ornithischians with well known anatomical adaptations include the ceratopsians or "horn-faced" dinosaurs (e.g. Triceratops), armored dinosaurs (Thyreophora) such as stegosaurs and ankylosaurs, pachycephalosaurids and the ornithopods.[2] There is strong evidence that certain groups of ornithischians lived in herds,[2][3] often segregated by age group, with juveniles forming their own flocks separate from adults.[4] 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,[5] Psittacosaurus,[6] and Kulindadromeus may have been primitive feathers.[7]

Ornithischians
Temporal range:
Early JurassicLate Cretaceous, 200–66 Ma
(Possible Late Triassic record)
Ornithischia Diversity
A collection of ornithischian fossil skeletons. Clockwise from upper left: Heterodontosaurus tucki (a heterodontosaurid), Stegosaurus stenops (a plated stegosaur), Scolosaurus thronus (an armored ankylosaur), Edmontosaurus annectens (a duck-billed hadrosaur), Stegoceras validum (a thick-headed pachycephalosaur), and Triceratops horridus (a horned ceratopsian).
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Clade: Dinosauria
Order: Ornithischia
Seeley, 1888
Subgroups
Synonyms

Description

In 1887, Harry Seeley divided Dinosauria into two clades: Ornithischia and Saurischia. Ornithischia is a strongly supported clade with an abundance of diagnostic characters (common traits).[2] The two most notable traits are a "bird-like" hip and beak-like predentary structure, though they shared other features as well.[2]

"Bird-hip"

The ornithischian pelvis was "opisthopubic", meaning that the pubis pointed down and backwards (posterior), parallel with the ischium (Figure 1a).[2] Additionally, the ilium had a forward-pointing process (the preacetabular process) to support the abdomen.[2] 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).[2]

The opisthopubic pelvis independently evolved at least three times in dinosaurs (in ornithischians, birds and therizinosauroids).[8] 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.[8]

Predentary

Ornithischians shared a unique bone called the predentary (Figure 2).[2] This unpaired bone was situated at the front of the lower jaw, where it extended the dentary (the main lower jaw bone). The predentary coincided with the premaxilla in the upper jaw. Together, they formed a beak-like apparatus used to clip off plant material. In ceratopsian dinosaurs, it opposed the rostral bone.

In 2017 Baron & Barrett suggested that Chilesaurus may represent an early diverging ornithischian that had not yet acquired the predentary of all other ornithischians.[9]

Ornithischia pelvis structure

Figure 1a - Ornithischian opisthopubic pelvic structure (left side)

Saurischia pelvis structure

Figure 1b - Saurischian propubic pelvic structure (left side)

Heterodontosaurus skull reconstruction sereno 2012 edited

Figure 2 - Heterodontosaurus skull with palpebral bone (red), antorbital fenestra (yellow) and predentary (green) colored.

Other characteristics

  • Ornithischians had paired premaxillary bones that were toothless and roughened at the tip of the snout (presumably due to the attachment of a keratinous beak).[2]
  • Ornithischians developed a narrow "eyebrow", or palpebral bone, across the outside of the eye socket.[2]
  • Ornithischians had reduced, or even closed-off, antorbital fenestrae (the fenestra in front of the eye socket).[2]
  • Ornithischian jaw joints were lowered below the level of the teeth, bringing the teeth into simultaneous occlusion.[2]
  • Ornithischians had "leaf-shaped" cheek teeth.[2]
  • Ornithischian backbones were stiffened near the pelvis by the ossification of tendons above the sacrum. Additionally, ornithischians had at least five sacral vertebrae attaching to the pelvis.[2]

Classification

Ornithischia is a branch-based taxon defined as all dinosaurs more closely related to Triceratops horridus Marsh, 1889 than to either Passer domesticus (Linnaeus, 1758) or Saltasaurus loricatus Bonaparte & Powell, 1980.[10] Genasauria comprises the clades Thyreophora and Neornithischia. Thyreophora includes Stegosauria (like the armored Stegosaurus) and Ankylosauria (like Ankylosaurus). Neornithischia comprises several basal taxa, Marginocephalia (Ceratopsia and Pachycephalosauria), and Ornithopoda (including duck-bills (hadrosaurs), such as Edmontosaurus). Cerapoda is a relatively recent concept (Sereno, 1986).

The cladogram below follows a 2009 analysis by Zheng and colleagues. All tested members of Heterodontosauridae form a polytomy.[11]

Ornithischia

Pisanosaurus Pisanosaurus.jpg

Heterodontosauridae Fruitadens.jpg

Genasauria
Thyreophora

Lesothosaurus

Scutellosaurus Scutellosaurus.jpg

Emausaurus

Scelidosaurus Scelidosaurus harrisonii.png

Stegosauria Stegosaurus BW.jpg

Ankylosauria Edmontonia dinosaur.png

Neornithischia

Stormbergia

Agilisaurus Agilisaurus2.jpg

Hexinlusaurus

Cerapoda

Othnielia

Hypsilophodon Hypsilophodon.jpg

Jeholosaurus

Yandusaurus

Orodromeus Orodromeus (pencil 2013).png

Zephyrosaurus

Ornithopoda Parasaurolophuspic steveoc.jpg

Marginocephalia

Pachycephalosauria Pachycephalosauria jmallon.jpg

Ceratopsia Psittacosaurus mongoliensis whole BW.jpgTriceratops BW.jpg

Cladogram after Butler et al., 2011. Ornithopoda includes Hypsilophodon, Jeholosaurus and others.[5]

Ornithischia

Pisanosaurus Pisanosaurus.jpg

Heterodontosauridae Fruitadens.jpg

Eocursor

Genasauria

Lesothosaurus

Thyreophora

Scutellosaurus Scutellosaurus.jpg

Emausaurus

Scelidosaurus Scelidosaurus harrisonii.png

Stegosauria Stegosaurus BW.jpg

Ankylosauria Edmontonia dinosaur.png

Neornithischia

Stormbergia

Agilisaurus Agilisaurus2.jpg

Hexinlusaurus

Othnielosaurus

Cerapoda

Ornithopoda Parasaurolophuspic steveoc.jpg

Marginocephalia

Pachycephalosauria Pachycephalosauria jmallon.jpg

Ceratopsia Psittacosaurus mongoliensis whole BW.jpgTriceratops BW.jpg


Currently, the exact placement of Ornithischia within the dinosaur lineage is a contentious issue.[12] Traditionally, Ornithischia is considered the sister group of Saurischia (which contains Theropoda and Sauropodomorpha).[13] However, in the alternative hypothesis of dinosaur relationships that was proposed by Baron, Norman & Barrett in the journal Nature in 2017, Ornithischia was recovered as the sister group to the Theropoda, which grouped together in the clade Ornithoscelida.[14][15] This hypothesis was recently challenged by an international consortium of early dinosaur experts led by Max Langer. However, the data that supported the more traditional placement of Ornithischia, as sister taxon of Saurischia, was found not to be statistically significant from the evidence that supported the Ornithoscelida hypothesis, in both the study by Langer et al. and the reply to the study by Baron et al.[16][17] A further 2017 study found some support for the previously abandoned Phytodinosauria model, which classifies ornithischians together with sauropodomorphs.[18]

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.[19]

Most ornithischians were herbivorous.[2] In fact, most of the unifying characters of Ornithischia are thought to be related to this herbivory.[2] 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 digest plant matter better.[2] The smallest known ornithischian is Fruitadens haagarorum.[20] The largest Fruitadens individuals reached just 65–75 cm. Previously, only carnivorous, saurischian theropods were known to reach such small sizes.[20] At the other end of the spectrum, the largest known ornithischians reach about 15 meters (smaller than the largest saurischians).[21]

However, not all ornithischians were strictly herbivorous. Some groups, like the heterodontosaurids, were likely omnivores.[22] 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.[23]

There is strong evidence that some ornithischians lived in herds.[2][3] This evidence consists of multiple bone beds where large numbers of individuals of the same species and of different age groups died simultaneously.[2][3]

See also

  • Tyrannoskull.jpg Dinosaurs portal

References

  1. ^ Ferigolo, J.; Langer, M. C. (2007). "A Late Triassic dinosauriform from south Brazil and the origin of the ornithischian predentary bone". Historical Biology. 19: 23–33. doi:10.1080/08912960600845767.
  2. ^ a b c d e f g h i j k l m n o p q r s t u Fastovsky, David E.; Weishampel, David B. (2012). Dinosaurs: A Concise Natural History. Cambridge: Cambridge University Press. ISBN 978-1107276468.
  3. ^ a b c Qi, Zhao; Barrett, Paul M.; Eberth, David A. (2007-09-01). "Social Behaviour and Mass Mortality in the Basal Ceratopsian Dinosaur Psittacosaurus (early Cretaceous, People's Republic of China)". Palaeontology. 50 (5): 1023–1029. doi:10.1111/j.1475-4983.2007.00709.x. ISSN 1475-4983.
  4. ^ Zhao, Q. (2013). "Juvenile-only clusters and behaviour of the Early Cretaceous dinosaur Psittacosaurus". Acta Palaeontologica Polonica. doi:10.4202/app.2012.0128.
  5. ^ a b Richard J. Butler, Jin Liyong, Chen Jun, Pascal Godefroit (May 2011). "The postcranial osteology and phylogenetic position of the small ornithischian dinosaur Changchunsaurus parvus from the Quantou Formation (Cretaceous: Aptian–Cenomanian) of Jilin Province, north-eastern China". Palaeontology. 54 (3): 667–683. doi:10.1111/j.1475-4983.2011.01046.x.CS1 maint: Multiple names: authors list (link)
  6. ^ Mayr, Gerald; Peters, Stefan D.; Plodowski, Gerhard; Vogel, Olaf (2002-08-01). "Bristle-like integumentary structures at the tail of the horned dinosaur Psittacosaurus". Naturwissenschaften. 89 (8): 361–365. doi:10.1007/s00114-002-0339-6. ISSN 0028-1042. PMID 12435037.
  7. ^ Godefroit, P.; Sinitsa, S.M.; Dhouailly, D.; Bolotsky, Y.L.; Sizov, A.V.; McNamara, M.E.; Benton, M.J.; Spagna, P. (2014). "A Jurassic ornithischian dinosaur from Siberia with both feathers and scales" (PDF). Science. 345 (6195): 451–455. doi:10.1126/science.1253351. hdl:1983/a7ae6dfb-55bf-4ca4-bd8b-a5ea5f323103. PMID 25061209.
  8. ^ a b Currie, Philip J.; Padian, Kevin (1997-10-06). Encyclopedia of Dinosaurs. Academic Press. pp. 537–538. ISBN 9780080494746.
  9. ^ Baron, Matthew G.; Barrett, Paul M. (2017). "A dinosaur missing-link? Chilesaurus and the early evolution of ornithischian dinosaurs". Biology Letters. 13 (8): 20170220. doi:10.1098/rsbl.2017.0220. PMC 5582101. PMID 28814574.
  10. ^ Butler, Richard; Upchurch, Paul; Norman, David (2008). "The phylogeny of ornithischian dinosaurs". Journal of Systematic Palaeontology. 6 (1): 1–40. doi:10.1017/S1477201907002271.
  11. ^ Zheng, Xiao-Ting; You, Hai-Lu; Xu, Xing; Dong, Zhi-Ming (19 March 2009). "An Early Cretaceous heterodontosaurid dinosaur with filamentous integumentary structures". Nature. 458 (7236): 333–336. doi:10.1038/nature07856. PMID 19295609.
  12. ^ Matthew G. Baron (2018). "Pisanosaurus mertii and the Triassic ornithischian crisis: could phylogeny offer a solution?". Historical Biology: An International Journal of Paleobiology. in press: 1–15. doi:10.1080/08912963.2017.1410705.
  13. ^ Seeley, H.G. (1888). "On the classification of the fossil animals commonly named Dinosauria". Proceedings of the Royal Society of London. 43 (258–265): 165–171. doi:10.1098/rspl.1887.0117.
  14. ^ Baron, M.G.; Norman, D.B.; Barrett, P.M. (2017). "A new hypothesis of dinosaur relationships and early dinosaur evolution" (PDF). Nature. 543 (7646): 501–506. doi:10.1038/nature21700. PMID 28332513.
  15. ^ "New study shakes the roots of the dinosaur family tree". 2017-03-22.
  16. ^ Max C. Langer; Martín D. Ezcurra; Oliver W. M. Rauhut; Michael J. Benton; Fabien Knoll; Blair W. McPhee; Fernando E. Novas; Diego Pol; Stephen L. Brusatte (2017). "Untangling the dinosaur family tree". Nature. 551 (7678): E1–E3. doi:10.1038/nature24011. hdl:1983/d088dae2-c7fa-4d41-9fa2-aeebbfcd2fa3. PMID 29094688.
  17. ^ Matthew G. Baron; David B. Norman; Paul M. Barrett (2017). "Baron et al. reply". Nature. 551 (7678): E4–E5. doi:10.1038/nature24012. PMID 29094705.
  18. ^ Luke A. Parry; Matthew G. Baron; Jakob Vinther (2017). "Multiple optimality criteria support Ornithoscelida". Royal Society Open Science. 4 (10): 170833. doi:10.1098/rsos.170833. PMC 5666269. PMID 29134086.
  19. ^ Jeffrey A. Wilson; Claudia A. Marsicano; Roger M. H. Smith (6 October 2009). "Dynamic Locomotor Capabilities Revealed by Early Dinosaur Trackmakers from Southern Africa". PLoS ONE. 4 (10): e7331. doi:10.1371/journal.pone.0007331. PMC 2752196.
  20. ^ a b Butler, Richard J.; Galton, Peter M.; Porro, Laura B.; Chiappe, Luis M.; Henderson, Donald M.; Erickson, Gregory M. (2010-02-07). "Lower limits of ornithischian dinosaur body size inferred from a new Upper Jurassic heterodontosaurid from North America". Proceedings of the Royal Society of London B: Biological Sciences. 277 (1680): 375–381. doi:10.1098/rspb.2009.1494. ISSN 0962-8452. PMC 2842649. PMID 19846460.
  21. ^ Yannan, Ji; Xuri, Wang; Yongqing, Liu; Qiang, Ji (2011-02-01). "Systematics, Behavior and Living Environment of Shantungosaurus Giganteus (Dinosauria: Hadrosauridae)". Acta Geologica Sinica - English Edition. 85 (1): 58–65. doi:10.1111/j.1755-6724.2011.00378.x. ISSN 1755-6724.
  22. ^ Barrett, P. M.; Rayfield, E. J. (2006). "Ecological and evolutionary implications of dinosaur feeding behaviour". Trends in Ecology & Evolution. 21 (4): 217–224. doi:10.1016/j.tree.2006.01.002. PMID 16701088.
  23. ^ Ji, Q.; Wu, X.; Cheng, Y.; Ten, F.; Wang, X.; Ji, Y. (2016). "Fish-hunting ankylosaurs (Dinosauria, Ornithischia) from the Cretaceous of China". Journal of Geology. 40: 2.
  • Butler, R.J. (2005). "The 'fabrosaurid' ornithischian dinosaurs of the Upper Elliot Formation (Lower Jurassic) of South Africa and Lesotho". Zoological Journal of the Linnean Society. 145 (2): 175–218. doi:10.1111/j.1096-3642.2005.00182.x.
  • Sereno, P.C. (1986). "Phylogeny of the bird-hipped dinosaurs (order Ornithischia)". National Geographic Research. 2 (2): 234–256.

External links

Ankylosauria

Ankylosauria is a group of mainly herbivorous dinosaurs of the order Ornithischia. It includes the great majority of dinosaurs with armor in the form of bony osteoderms. Ankylosaurs were bulky quadrupeds, with short, powerful limbs. They are known to have first appeared in the early Jurassic Period, and persisted until the end of the Cretaceous Period. They have been found on every continent. The first dinosaur discovered in Antarctica was the ankylosaurian Antarctopelta, fossils of which were recovered from Ross Island in 1986.

Ankylosauria was first named by Henry Fairfield Osborn in 1923. In the Linnaean classification system, the group is usually considered either a suborder or an infraorder. It is contained within the group Thyreophora, which also includes the stegosaurs, armored dinosaurs known for their combination of plates and spikes.

Cerapoda

Cerapoda ("ceratopsians and ornithopods") is a clade of the dinosaur order Ornithischia.

Changchunsaurus

Changchunsaurus (meaning "Changchun lizard") is an extinct genus of small herbivorous parksosaurid dinosaur from Early Cretaceous deposits of Gongzhuling, Jilin, China. It is the first named dinosaur genus from Jilin.

Como Bluff

Como Bluff is a long ridge extending east-west, located between the towns of Rock River and Medicine Bow, Wyoming. The ridge is an anticline, formed as a result of compressional geological folding. Three geological formations, the Sundance, the Morrison, and the Cloverly Formations, containing fossil remains from the Late Jurassic of the Mesozoic Era are exposed. Nineteenth century paleontologists discovered many well-preserved specimens of dinosaurs, as well as mammals, turtles, crocodilians, and fish from the Morrison Formation. Because of this, Como Bluff is considered to be one of the major sites for the early discovery of dinosaur remains. Among the species discovered is the only known specimen of Coelurus. Significant discoveries were made in 22 different areas scattered along the entire length of the ridge. It is included on the National Register of Historic Places as well as the National Natural Landmark list.

Delapparentia

Delapparentia is a genus of iguanodont dinosaur from the Early Cretaceous period of Galve, Teruel Province, Spain. It may be a synonym of Iguanodon bernissartensis.

Dinosaur classification

Dinosaur classification began in 1842 when Sir Richard Owen placed Iguanodon, Megalosaurus, and Hylaeosaurus in "a distinct tribe or suborder of Saurian Reptiles, for which I would propose the name of Dinosauria." In 1887 and 1888 Harry Seeley divided dinosaurs into the two orders Saurischia and Ornithischia, based on their hip structure. These divisions have proved remarkably enduring, even through several seismic changes in the taxonomy of dinosaurs.

The largest change was prompted by entomologist Willi Hennig's work in the 1950s, which evolved into modern cladistics. For specimens known only from fossils, the rigorous analysis of characters to determine evolutionary relationships between different groups of animals (clades) proved incredibly useful. When computer-based analysis using cladistics came into its own in the 1990s, paleontologists became among the first zoologists to almost wholeheartedly adopt the system. Progressive scrutiny and work upon dinosaurian interrelationships, with the aid of new discoveries that have shed light on previously uncertain relationships between taxa, have begun to yield a stabilizing classification since the mid-2000s. While cladistics is the predominant classificatory system among paleontology professionals, the Linnean system is still in use, especially in works intended for popular distribution.

Evolution of dinosaurs

This article gives an outline and examples of dinosaur evolution. For a detailed list of interrelationships see Dinosaur classification.

Dinosaurs evolved within a single lineage of archosaurs 243-233 Ma (million years ago) from the Anisian to the Carnian ages, the latter part of the middle Triassic. Dinosauria is a well-supported clade, present in 98% of bootstraps. It is diagnosed by many features including loss of the postfrontal on the skull and an elongate deltopectoral crest on the humerus.In March 2017, scientists reported a new way of classifying the dinosaur family tree, based on newer and more evidence than available earlier. According to the new classification, the original dinosaurs, arising 200 million years ago, were small, two-footed omnivorous animals with large grasping hands. Descendants (for the non-avian dinosaurs) lasted until 66 million years ago.

Garden Park, Colorado

Garden Park is a paleontological site in Fremont County, Colorado, known for its Jurassic dinosaurs and the role the specimens played in the infamous Bone Wars of the late 19th century. Located 10 km (6.2 mi) north of Cañon City, the name originates from the area providing vegetables to the miners at nearby Cripple Creek in the 19th century. Garden Park proper is a triangular valley surrounded by cliffs on the southeast and southwest and by mountains to the north; however, the name is also refers to the dinosaur sites on top and along the cliffs. The dinosaur sites now form the Garden Park Paleontological Resource Area, which is overseen by the Bureau of Land Management.

Genasauria

Genasauria is a clade of extinct beaked, primarily herbivorous dinosaurs. Paleontologist Paul Sereno first named Genasauria in 1986. The name Genasauria is derived from the Latin word gena meaning ‘cheek’ and the Greek word saúra (σαύρα) meaning ‘lizard.’ Genasauria is the most inclusive clade within the order Ornithischia. According to Sereno (1986), Genasauria represents all ornithischians except for the most primitive ornithischian, Lesothosaurus. Sereno’s formal definition is, “Ankylosaurus, Triceratops, their most recent common ancestor and all descendants.” It is hypothesized that Genasauria had diverged from Lesothosaurus by the Early Jurassic. Cranial features that characterize Genasauria include a medial offset of the maxillary dentition, a sprout-shaped mandibular symphysis, moderately sized coronoid process, and an edentulous (without teeth) anterior portion of the premaxilla. A distinguishing postcranial feature of Genasauria is a pubic peduncle of the ilium that is less robust than the ischial peduncle.

Genasauria is commonly divided into Neornithischia and Thyreophora. Neornithischia is characterized by asymmetrical distributions of enamel covering the crowns of the cheek teeth, an open acetabulum, and a laterally protruding ischial peduncle of the ilium. Neornithischia includes ornithopods, pachycephalosaurs, and ceratopsians. Thyreophora is characterized by body armor and includes stegosaurs, ankylosaurs, Scelidosaurus, and Scutellosaurus.

Hadrosauroidea

Hadrosauroidea is a clade or superfamily of ornithischian dinosaurs that includes the "duck-billed" dinosaurs, or hadrosaurids, and all dinosaurs more closely related to them than to Iguanodon.They are from Asia, Europe and Africa. Many primitive hadrosauroids, such as the Asian Probactrosaurus and Altirhinus, have traditionally been included in a paraphyletic (unnatural grouping) "Iguanodontidae". With cladistic analysis, the traditional Iguanodontidae has been largely disbanded, and probably includes only Iguanodon and perhaps its closest relatives.

Haya griva

Haya is an extinct genus of basal neornithischian dinosaur known from Mongolia.

Jeholosauridae

Jeholosaurids were herbivorous neornithischian dinosaurs from the Cretaceous Period (Aptian - Santonian, with a possible Campanian record) of Asia. The family was first proposed by Han et al. in 2012. The jeholosaurids were defined as those ornithischians more closely related to Jeholosaurus shangyuanensis than to Hypsilophodon foxii, Iguanodon bernissartensis, Protoceratops andrewsi, Pachycephalosaurus wyomingensis, or Thescelosaurus neglectus. The Jeholosauridae includes the type genus Jeholosaurus and Yueosaurus.

Lexovisaurus

Lexovisaurus is a genus of stegosaur from mid-to-Late Jurassic Europe, 164.7 mya. Fossils of limb bones and armor fragments have been found in middle to late Jurassic-aged strata of England and France.

Minmi paravertebra

Minmi is a genus of small herbivorous ankylosaurian dinosaur that lived during the early Cretaceous Period of Australia, about 119 to 113 million years ago.

Neornithischia

Neornithischia ("new ornithischians") is a clade of the dinosaur order Ornithischia. They are the sister group of the Thyreophora within the clade Genasauria. Neornithischians are united by having a thicker layer of asymmetrical enamel on the inside of their lower teeth. The teeth wore unevenly with chewing and developed sharp ridges that allowed neornithischians to break down tougher plant food than other dinosaurs. Neornithischians include a variety of basal forms historically known as "hypsilophodonts", including the Parksosauridae; in addition, there are derived forms classified in the groups Marginocephalia and Ornithopoda. The former includes clades Pachycephalosauria and Ceratopsia, while the latter typically includes Hypsilophodon and the more derived Iguanodontia.

Parksosauridae

Parksosauridae is a clade or family of small ornithischians which have previously been generally allied to hypsilophodontids. Parksosauridae is often considered a synonym of Thescelosauridae, but the two groups cannot be synonyms because Parksosauridae is defined as a stem, while Thescelosauridae is defined as a node.

Saurischia

Saurischia ( saw-RIS-kee-ə, meaning "reptile-hipped" from the Greek sauros (σαῦρος) meaning 'lizard' and ischion (ἴσχιον) meaning 'hip joint') is one of the two basic divisions of dinosaurs (the other being Ornithischia). ‘Saurischia’ translates to lizard-hipped.

In 1888, Harry Seeley classified dinosaurs into two orders, based on their hip structure, though today most paleontologists classify Saurischia as an unranked clade rather than an order.

Yueosaurus

Yueosaurus is an extinct genus of basal ornithopod dinosaur known from Zhejiang Province, China.

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