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Description edit

 
Skull of Lesothosaurus, an early ornithischian

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.[1] Ornithischians are united by multiple features of the skull, teeth, and skeleton, including especially the presence of a predentary and palpebral, an increased number of sacral vertebrae, the absence of gastralia, and an opisthopubic pubis.[2] 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 nasal opening is small, the antorbital fossa that opens from the side of the skull into the palate is large, shallow and triangular, the orbit is large and round and has a palpebral creating a brow, and the lower jaw has a large mandibular fenestra.[3] 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.[3][4] 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 premaxilla 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.[5] 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, supratemporal fenestra 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.[6] 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.[7] 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.[8] Members of the ornithopod family Hadrosauridae show further adaptations, including the formation of dental batteries where teeth are continuously replaced, and in many genera the development of prominent cranial crests formed by multiple different bones of the skull.[9] Pachycephalosauria, at one time thought to be close to ornithopods and know known to be related instead to ceratopsians, show very 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 frontal and parietal 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.[10] Ceratopsians, the sister group to pachycephalosaurs, also display many cranial adaptations, most importantly the evolution of a bone called the rostral that forms the top beak opposite the predentary. The jugal 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.[11] 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 parietal fenestrae 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 2 m (6.6 ft) long.[12]

References edit

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