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 (), meaning "bird", and (), meaning "hip". However, as birds are theropod dinosaurs, they are only distantly related to this group.
Ornithischians with well known anatomical adaptations include the ceratopsians or "horn-faced" dinosaurs (e.g. Triceratops), the pachycephalosaurs or "thick-headed" dinosaurs, the armored dinosaurs (Thyreophora) such as stegosaurs and ankylosaurs, 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.
Description Ornithischia is a very large and diverse group of dinosaurs, with members known from all continents, habitats, and a very large range of sizes. They are primarily herbivorous browsers or grazers, but some members may have also been opportunistic omnivores. Ornithischians are united by multiple features of the skull, teeth, and skeleton, including especially the presence of a and , an increased number of , the absence of , and an . Early ornithischians ranged around in length, with them increasing in size over time so that the largest armoured ornithischians were around and , the largest horned ornithischians were around and , and the largest crested ornithischians were around and .
Early ornithischians were relatively small dinosaurs, averaging about 1–2 meters in body length, with a triangular skull that had large circular orbits on the sides. This suggests that early ornithischians had relatively huge eyes that faced laterally. The forelimbs of early ornithischians are considerably shorter than their hindlimbs. A small forelimb such as those present in early ornithischians would not have been useful for locomotion, and it is evident that early ornithischians 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 early ornithischians show that the tibia is considerably longer than the femur, a feature that suggests that early ornithischians were adapted for bipedality, and were fast runners.Colbert, E. H. (1981). A primitive ornithischian dinosaur from the Kayenta Formation of Arizona. Museum Northern Arizona Bull. 53, 1-61
Skulls
Much of the knowledge of early ornithischian anatomy comes from Lesothosaurus, which is a taxon known from multiple skulls and skeletons from the Early Jurassic of Lesotho. The rear of its skull is box-like, while the snout tapers to a point. The is small, the that opens from the side of the skull into the palate is large, shallow and triangular, the is large and round and has a palpebral creating a brow, and the lower jaw has a large .
The skulls of Emausaurus and Scelidosaurus, two early members of the armoured group Thyreophora, show similarities in the box-like skull that tapers to the front. The antorbital fossa is smaller and forming an elongate oval in both taxa, and the palpebral which is elongate and slender in Lesothosaurus is widened in Emausaurus and completely incorporated into the skull as a flat bone in Scelidosaurus. Skulls in members of the thyreophoran group Stegosauria are much longer and lower, with the width at the back being greater than the height in Stegosaurus. The snout and lower jaw are long and deep, and in some genera the does not have any teeth. As in Scelidosaurus, the palpebral forms the top border of the orbit as a flat brow bone, but the antorbital fossa is reduced to the point of absence in some genera.
Ankylosaurs, the other group of armoured ornithischians, have very robust, immobile skulls, with three significant features that separate them from other groups. The antorbital fossa, and mandibular fenestra are all closed, the sutures separating skull bones are almost completely obliterated by surface texturing, and there is bony armour above the orbits, and at the top and bottom corners of the back of the skull. Teeth are sometimes absent from the premaxilla, and both the upper and lower jaws have deeply inset teeth creating large cheeks. Ankylosaurs also have very extensive and complicated network of sinuses, formed by bone growth in the palate.
The skulls are known from many early ornithopods and some heterodontosaurids, showing similar general features. Skulls are relatively tall with shorter snouts, but the snout is elongated in some later taxa like Thescelosaurus. The orbit and antorbital fossa are large, but the nasal opening is small, and while teeth are present in the premaxilla, there is a toothless front tip that likely formed a keratinous beak. The premaxillary teeth and the first lower tooth in Heterodontosaurus are enlarged into sizeable canines. In later ornithopods, the skulls are more elongate and sometimes fully rectangular, with a very large nasal opening, and a thin, elongate palpebral that can extend the entire way across the orbit. Teeth are almost always absent from the premaxilla, the antorbital fossa is reduced and round to slit-like, the tip of the snout is sometimes flared to form a broad beak. Members of the ornithopod family Hadrosauridae show further adaptations, including the formation of where teeth are continuously replaced, and in many genera the development of prominent cranial crests formed by multiple different bones of the skull.
Pachycephalosauria, at one time thought to be close to ornithopods and now known to be related instead to ceratopsians, show a unique skull anatomy that is unlike any other ornithischian. The bones of the top of the skull are thickened and in many taxa expanded significantly to form round bony domes as the top of the head, as well as possessing small nodes or elongate spikes along the back edge of the skull. Many taxa are only known from these thick skull domes, which are fused from the and bones. As in many other ornithischians, the snout is short and tapering, the nasal opening is small, the antorbital fossa is sometimes absent, and there are premaxillary teeth, though only three. The two palpebrals are also incorporated into the skull roof as in thyreophorans, rather than free.
Ceratopsians, the sister group to pachycephalosaurs, also display many cranial adaptations, most importantly the evolution of a bone called the that forms the top beak opposite the predentary. The bones flare to the sides to create a pentagonal skull seen from above, the nasal opening is closer to the top of the snout than the teeth, and while the snout tapers in some taxa, it is very deep and short in Psittacosaurus. The ceratopsian palpebral is generally triangular, and the back edge of the skull roof forms a flat frill that is enlarged in more derived ceratopsians. The ceratopsian family Ceratopsidae progresses on these features with the addition of horns above each orbit and on the top of the snout, as well as substantial elongation of the frill and in many genera the development of two large forming holes in the frill. The skull and frill elongation makes the skulls of Torosaurus and Pentaceratops the largest of any known terrestrial vertebrate, at over long.
"Bird-hip"
The ornithischian pelvis was "opisthopubic", meaning that the pubis pointed down and backwards (posterior), parallel with the ischium, while in the saurischian pelvis, the pelvis jutted forwards as in lizards. 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).
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. It has also been argued that the opisthopubic condition is basal to maniraptorans (including among others birds, therizinosauroids and dromaeosaurids), with some clades having later experienced a reversal to the propubic condition.Holtz, T.R. and Osmólska, H. (2004). "Saurischia." In Weishampel, Dodson and Osmólska (eds.), The Dinosauria, second edition. Berkeley: University of California Press.
Integument
Exceptionally preserved specimens of ornithischians preserved integument, the external tissues scuh as skin, scales, and filaments. Many ornithischians are known to have been covered in scales, similar to those of other dinosaurs. Unlike the overlapping scales of modern lizards, typical ornithischian scales are mosaics of non-overlapping "basement scales", small and of polygonal shape. In many species, larger feature scales would be interspersed between these basement scales. An especially abundant record of these scales is known from hadrosaurid mummies, where distinct patterns of scalation can be observed between species. Other large ornithischians such as ceratopsids, ankylosaurs, iguanodontids and stegosaurs are also known from traces or preserved scales. Several other types of scalation are found on localized parts of the body in specific taxa. For some examples, hadrosaurs possess "midline feature scales" running along the top of the animal, Kulindadromeus and Haolong possess large overlapping scales over the tail, and reticulate scales have been documented on hands and feet of ceratopsids and Kulindadromeus. In Psittacosaurus and Borealopelta, the melanosomes of especially well preserved scales have been used to approximate their colour in life. Some portions of the body would have instead been covered in hard keratin like that found on the claws and beaks of modern birds and reptiles. Examples of these include beaks, the nails on the hands and feet, the horns of horned dinosaurs, the plates of stegosaurs, and the armour of ankylosaurs. The osteoderms found in armored dinosaurs would have been embedded externally within the skin, and covered much of the animal.
Despite similarities in external scalation, differences in skin thickness can be observed between ornithischians. The small ceratopsian Psittacosaurus had skin thick, close to the skin thickness of modern rhinos and elephants, whereas large hadrosaurs had especially thin skin only thick unlike mammalian megafauna.