According to dictionary.com, there are some options for the plural of "brontosaurus".
http://dictionary.reference.com/browse/brontosaurus

"–noun, plural -sau·rus·es, -sau·ri"

However, the DOL Dinosaur Omnipedia offers some more wisdom in their writing guide:
http://www.dinosauria.com/dml/writing.html

"It is perfectly acceptable, however, to form plurals of vernacular names. A writer might use the vernacular reptile, dinosaur or tyrannosaur to refer to a single specimen of Reptilia, Dinosauria or Tyrannosaurus. The writer may use the vernacular plurals reptiles, dinosaurs or tyrannosaurs to refer to more than one individual of Reptilia, Dinosauria or Tyrannosaurus."

This implies that -saurs is an acceptable ending for the plural of Tyrannosaurus.

Multiple sources representing "official" use of language imply that a variety of spellings could be valid to name a plurality of giant lizards.