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Any of various medium-sized to large dinosaurs of the group Hadrosauroidea of the Cretaceous Period. Hadrosaurs had a duck-like bill and a mouth containing many series of rough grinding teeth for chewing tough plants. They walked on two legs or on all fours and had hoofed feet. Many hadrosaurs bore hollow crests on their skulls. They were the last and largest ornithopod dinosaurs. Also called duck-billed dinosaur.