BrontozoumBron`to*zo"um\, n. [NL., fr. Gr. ? thunder + ? animal.] (Paleon.) An extinct animal of large size, known from its three-toed footprints in Mesozoic sandstone. Note: The tracks made by these reptiles are found eighteen inches in length, and were formerly referred to gigantic birds; but the discovery of large bipedal three-toed dinosaurs has suggested that they were made by those reptiles.