High Quality Content by WIKIPEDIA articles! The predentary is an `extra` bone in the front of the lower jaw, which extended the dentary (the main lower jaw bone). It is found in the fossilised remains of ornithischian dinosaurs, which were herbivorous. The predentary coincided with the premaxilla in the upper jaw. Together they formed a beak-like apparatus used to clip off plant material. In ceratopsian dinosaurs, it opposes the rostral bone.