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an order of fossil reptiles of the superorder Dinosauria. The pubic bone of the tetraradiate pelvis had a developed posterior appendage (as in birds). The anterior, toothless part of the jaws was clad in a horny jacket and resembled a bill. The teeth were laterally compressed, with leaflike crowns. The distal phalanges of the digits in late ornithischians were flattened into hooves. The reptiles were herbivorous, with the earliest ones possibly being omnivorous. Many possessed defensive adaptations in the form of cutaneous scutella, spines, or horns. There were six (or four) suborders, including Stegosau-ria, Ankylosauria, and Ceratopsia. Ornithischians lived in the Jurassic and Cretaceous periods.