Source : Webster's Revised Unabridged Dictionary (1913)

Brontozoum \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.