Ornithischia (/ɔːrnɪˈθɪskiə/ or-ni-THISS-kee-ə) is an extinct clade of mainly herbivorous dinosaurs characterized by a

pelvic structure similar to that of birds. 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 unrelated, instead being theropod dinosaurs.

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.There is strong evidence that certain groups of ornithischians lived in herds, often segregated by age group, with juveniles forming their own flocks separate from adults. 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 TianyulongPsittacosaurus, and Kulindadromeus may have been primitive feathers.

Currently, the exact placement of Ornithischia within the dinosaur lineage is a contentious issue. Traditionally, the group is considered sister to Saurischia (which contains Theropoda and Sauropodomorpha). In the alternative evolutionary hypothesis for dinosaurs that was proposed by Baron, Norman & Barrett in the journal Nature in 2017, Ornithischia as well as Theropoda were grouped together in the clade Ornithoscelida. The work by Baron and colleagues 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.. A further 2017 study found some support for the previously abandoned Phytodinosauria model, which classifies ornithischians together with sauropodomorphs.

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