\Bron`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.