Early Jurassic–Late Cretaceous, 200–66 Ma (Possible Late Triassic record)
|A collection of ornithischian fossil skeletons. Clockwise from upper left: Heterodontosaurus tucki (a heterodontosaurid), Stegosaurus stenops (a plated stegosaur), Euoplocephalus tutus (an armored ankylosaur), Edmontosaurus annectens (a duck-billed hadrosaur), Stegoceras validum (a thick-headed pachycephalosaur), and Triceratops horridus (a horned ceratopsian).|
Ornithischia (// 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 Tianyulong, Psittacosaurus, 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.
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 share a unique bone called the predentary (Figure 2). This unpaired bone is found at the front of the lower jaws and presumably aided ornithischians in cropping vegetation. In 2017 Baron & Barrett suggested that Chilesaurus may represent an early diverging ornithischian that had not yet acquired the predentary of all other ornithischians.
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. 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).
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.
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).
However, not all ornithischians were strictly herbivorous. Some groups, like the heterodontosaurids, were likely omnivores. 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.
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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