Temporal range: Late Triassic – Late Cretaceous
Ornithischia, meaning 'bird hip joint' in Greek, is one of two classification orders used to distinguish and classify dinosaurs, the other being Saurischia. The name is derived from the method used to classify the animals, as dinosaurs classified under the ornithischia branch are done so because of their bird-like hip structure, which evolved from the "lizard-hipped" saurischians. Ornithischians were typically herd-oriented herbivores, and hence outnumbered the carnivorous saurischians.
The two-order system of classification for dinosaurs was developed in 1887 by Harry Seeley. The ornithischia division was developed based on the bird-like structure of these dinosaurs' hip bones. Despite this similarity, modern-day birds did not develop and evolve from ornithischians. The ornithischian hip bone differs from the suarischian counterpart in that the pubis bone points downward and backwards, facing the tail, parallel with the ischium. This creates a forward-pointing process that is used to support the animal's abdomen. It also gives the animal a four-pronged hip structure, which differs from the saurischian's three-pronged structure. The saurischians also had a pubis bone which points downward and forward, as is with ancestral lizard types. The ornithischian pelvis was also much more balanced and stable than a saurischian version.
The ornithischians also had an extra predentary bone in the front of their lower jaw, which extended the main jaw bone. This bone coincides with the upper jaw to create a beak-like structure used for clipping plants and fauna. Saurischians, as carnivorous animals, did not have this structure.
While the saurischia/ornithischia classification system is generally accepted among scholars and paleontologists, there does exist a minority theority proposed by paleontologist Robert Bakker that suggests classifying the theropods alone in their own order, while grouping the sauropodomorphs along with the ornithischians in the second order. In effect, grouping the carnivorous dinosaurs together, and the herbivorous dinosaurs together in a seperate order. This classification system identifies the carnivorous group as Theropoda, while classifying the herbivorous dinosaurs as either Phytodinosauria, or "plant dinosaurs", as suggested by Bakker, or as Ornithischiformes.
- Order Ornithischia
- Family Pisanosauridae
- Family Fabrosauridae
- Suborder Thyreophora - (armored dinosaurs)
- Suborder Cerapoda