The Collaborative International Dictionary of English v.0.48:

Mastodonsaurus \Mas`to*don*sau"rus\, n. [NL., fr. E. Mastodon +
   Gr. say^ros a lizard.] (Paleon.)
   A large extinct genus of labyrinthodonts, found in the
   European Triassic rocks.
   [1913 Webster]