‘Dinosaurs’ (Dinosauria, originally coined by Richard Owen in 1842) were a superorder of dominant vertebrate reptiles that dominated the terrestrial ecosystems of Earth from the Triassic to the Cretaceous periods (between 230 to 65 million years ago).
The superorder Dinosauria was later broken down by British paleontologist Harry G. Seeley into two orders, Saurischia and Ornithischia. Until Seeley, paleontologists had tried breaking down the Dinosauria superorder into smaller orders based on the structure of dinosaurian feet and teeth. Seely suggested however, dinosaur orders be based on the evolution of their hip structure.
His work has been accepted since being published and is generally used to classify dinosaurs by paleontologists all over the world today. Saurischian means ‘lizard hipped’ while Ornithischian mean ‘bird hipped’, although it has been well proven by modern paleontology that birds have actually evolved from the Saurischian family (modern birds evolved from a group of Saurischian dinosaurs called Coelurosaurs).
Ornithischian order of dinosaurs are further broken down into several families and sub-orders. Some of the Ornithischian families and sub-orders include (but are not limited to) Cerapoda, Thyreophora, Fabrosauride, and Heteredontosauride. To make it easier to understand what kind of dinosaurs fell under Ornithischia, some good examples would be Ceratopsians (ex. Triceratops), the duck-billed Hadrosaurs (ex. Edmontosaurus), the armored/club-tailed Ankylosaurs (ex. Ankylosaurus), and the Stegosaurs (ex. Stegosaurus) with prominent plates and spines along their back.
All Ornithischians were herbivores, and a lot of them were secondarily quadripedals. As mentioned before, Ornithischians had their pelvis (hip) evolved to be more like ‘bird shaped’. And from the examples listed above, we can also see some of the other important physical characteristics that Ornithischians had such as thick ‘armored’ skin, duck-bill, and plates on their back. They also possessed a peculiar feature. Their predentary bone which is basically an extra bone in the front part of the lower jaw. It extends the primary lower jaw bone and coincides with the premaxilla (a pair of small extended bones) in the upper jaw. In Ornithischians, they were covered by a horny plate, forming a beak-like mouth structure that would be used to clip of tough plant parts.
They had atleast one palpebral also known as ‘eyelid bone’ and reduced skull openings near the eyes and in the lower jaw. They also had five or more sacral vertebrae (based in the base of the spinal column). (1)
Very much in contrast to Saurischian, Ornithischian pubis bones point downwards and back towards the tail and were parallel with the ishchium (which form the lower and back part of the hip bone), and would work towards supporting the abdomen region. Ornithischians also had a more evolved, stable pelvis (unlike the lizard-like pelvis that Saurischians had).
Brief Characteristics (2):
herbivorous, with blunt or leaf-shaped teeth predentary bone in lower jaw [D&R p. 173] rostral tip of premaxilla toothless, forming a beak.ossified tendons along at least part of spinal columnreduced antorbital fenestrabuccal emargination of the jawsat least 5 sacral vertebraeantorbital fenestra reduced or absentpremaxillae expanded, form part of naresornithischian (tetraradiate) pelvis with a prepubic process, process of pelvis pointing backwards to make room for the long digestive tract