|A collection of ornithischian fossil skeletons. Clockwise from upper left: Heterodontosaurus (Heterodontosauridae), Nipponosaurus (Ornithopoda), Borealopelta (Ankylosauria), Triceratops (Ceratopsia), Stegoceras (Pachycephalosauria), and Stegosaurus (Stegosauria).|
Ornithischia (//) is an extinct clade of mainly herbivorous dinosaurs characterized by a pelvic structure superficially 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 only distantly related to this group as birds are theropod dinosaurs. Two groups of ornithischians survived till the K–Pg extinction event 66 million years ago; the ankylosaurs and the cerapods.
Ornithischians with well known anatomical adaptations include the ceratopsians or "horn-faced" dinosaurs (e.g. Triceratops), armored dinosaurs (Thyreophora) such as stegosaurs and ankylosaurs, pachycephalosaurs 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.
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 was "opisthopubic", meaning that the pubis pointed down and backwards (posterior), parallel with the ischium (Figure 1a). Additionally, the ilium had a forward-pointing process (the preacetabular process) to support the abdomen. 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).
Figure 1b - Saurischian propubic pelvic structure (left side)
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 dromaeosaurids are mesopubic.
Ornithischians shared a unique bone called the predentary (Figure 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.
- 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).
- Ornithischians developed a narrow "eyebrow", or palpebral bone, across the outside of the eye socket.
- Ornithischians had reduced, or even closed-off, antorbital fenestrae (the fenestra in front of the eye socket).
- Ornithischian jaw joints were lowered below the level of the teeth, bringing the teeth into simultaneous occlusion.
- Ornithischians had "leaf-shaped" cheek teeth.
- 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.
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).
Currently, the exact placement of Ornithischia within the dinosaur lineage is a contentious issue. Traditionally, Ornithischia is considered the sister group of Saurischia (which contains Theropoda and Sauropodomorpha). 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. 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. A further 2017 study found some support for the previously abandoned Phytodinosauria model, which classifies ornithischians together with sauropodomorphs.
The Fabrosauridae is an obsolete group of basal ornithischian dinosaurs from the Early to Middle Jurassic, originally proposed by Galton (1972). Recent studies indicate that Fabrosauridae is not a natural grouping of dinosaurs, and instead consist of unrelated genera.
The proposed "Fabrosaurs" descended from a Lesothosaurus-like animal. Proposed fabrosaurids were 1–2 meters long, and were lightly built and bipedal. Their skulls were triangular and had very large eye sockets. They were herbivorous and would have used agility to escape predators.
Fabrosaurids are known from southern Africa in the Upper Triassic and Lower Jurassic, as well as in China from the Middle and Upper Jurassic periods. Fabrosaurids bear a semblance to many other members of Ornithischia, and have attracted much interest in phylogenetic studies. However, most genera that were previously referred to as Fabrosauridae are only known from fragmented and partial remains, with most based on no more than isolated teeth, and their place in Fabrosauridae is questionable.
Peter Malcolm Galton in 1972 originally proposed the creation of Fabrosauridae based on the finding of cheek teeth. The successive discovery (1990) of a Middle Jurassic primitive ornithischian, Agilisaurus louderbacki, found in the Xiashaximiao Formation of Zigong, Sichuan Basin, China, has led to paleontologists learning more about the phylogeny of fabrosaurids.
Fabrosaurids have many characters that are unique to their physiology, particularly unique tooth morphology and structure, and short forelimb lengths compared to hindlimbs. Unlike many other primitive Ornithischians, such as heterodontosaurids and hypsilophodontids, the teeth of fabrosaurids are very thinly and uniformly enameled. Compared to Agilisaurus, Fabrosaurus is much more primitive in that it has six premaxillary teeth, and a stout prepubis. Agilisaurus differs from Fabrosaurus in that it appears to be larger and more well developed in structure.
The Upper Jurassic ornithischian dinosaur, Gongubusaurus, from China, is thought to be more closely related to fabrosaurids than hypsilophodontids. The type species, G. shiyii, is from the Shangshaximiao Formation in the Rongxian County in Sichuan Basin, has been classified from only a few scattered teeth.
Fabrosaurids were relatively small ornithischian dinosaurs, averaging about 1–2 meters in body length, with a triangular skull that had large circular orbits on the sides. This suggests that fabrosaurids had relatively huge eyes that faced laterally. The forelimbs of fabrosaurids are considerably shorter than their hindlimbs. A small forelimb such as those present in Fabrosauridae would not have been useful for locomotion, and it is evident that fabrosaurids 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 fabrosaurids show that the tibia is considerably longer than the femur, a feature that suggests that fabrosaurids were adapted for bipedality, and were fast runners.
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 ornithischian is 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 bone beds where large numbers of individuals of the same species and of different age groups died simultaneously.
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|Wikispecies has information related to Ornithischia.|
- Ornithischia, from Palæos. (cladogram, characteristics)