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The Dinosauria clade was divided into the two orders Ornithischia and Saurischia by Harry Seeley in 1887. This division, which has generally been accepted, is based on the evolution of thepelvis into a more bird-like structure (although birds did not descend from these dinosaurs), details in the vertebrae and armor and the possession of a 'predentary' bone. The predentary is an extra bone in the front of the lower jaw, which extends the dentary (the main lower jaw bone). The predentary coincides with the premaxilla in the upper jaw. Together they form a beak-like apparatus used to clip off plant material.

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