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, and Kulindadromeus may have been primitive feathers. In the alternative evolutionary hypothesis for dinosaurs that was proposed by Baron, Norman & Barrett in the journal Nature in 2017, the presence of such features in Ornithischia as well as Theropoda was interpreted as possible evidence for feathers being an ancestral condition of the clade Ornithoscelida.
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). The two most notable traits are a "bird-like" hip and beak-like predentary structure though they shared other features as well.
The ornithischian pelvis is "opisthopubic" meaning that the pubis points down and back (posterior) parallel with the ischium (Figure 1a). Additionally, the pelvis has a forward-pointing process to support the abdomen. This results in a four-pronged pelvic structure. In contrast to this, the saurischian pelvis is "propubic" meaning the pubis points toward the head (anterior), as in ancestral reptiles (Figure 1b).
The opisthopubic pelvis independently evolved at least three times in dinosaurs (in ornithischians, birds and therizinosauroids). Some argue that the opisthopubic pelvis evolved a fourth time in the clade Dromaeosauridae but this is controversial as other authors argue that dromeosaurids are mesopubic.
Ornithischians shifted from bipedal to quadrupedal posture at least three times in their evolutionary history and have been shown to have been capable of adopting both postures early in their evolutionary history.
Most ornithischians were herbivorous. In fact, most of the unifying characters of Ornithischia are thought to be related to this herbivory. 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. The smallest known Ornithischians are Fruitadens haagarorum. The largest Fruitadens individuals reached just 65–75 cm. Previously, only carnivorous, saurischian theropods were known to reach such small sizes. At the other end of the spectrum, the largest known ornithischians reach about 15 meters (smaller than the largest saurischians).
There is strong evidence that some ornithischians lived in herds. This evidence consists of multiple bones beds with large numbers of the same species in different age classes who died simultaneously.
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^Zhao, Q. (2013). "Juvenile-only clusters and behaviour of the Early Cretaceous dinosaur Psittacosaurus". Acta Palaeontologica Polonica. doi:10.4202/app.2012.0128.
^ abRichard 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)
^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. PMID19295609.
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.
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