Butt Wars: The Final Conflict by Andy Griffiths
The stinky scoundrels from international bestsellers THE DAY MY BUTT WENT PSYCHO! and ZOMBIE BUTTS FROM URANUS! are back--and determined to wipe away the earth! Zack Freeman (and his butt) have twice saved the world from total reek-dom. But now the young butt-fighter faces his nastiest challenge yet: Hundreds of thousands of Great White Butts attacking the earth with giant brown blobs are about to cause Buttageddon. In order to stop them, Zack will have to hitch a ride in a time-traveling buttmobile, back to the reign of the prehistoric buttosaurs. Can Zack battle the Tyrannosore-arses, juggle a giant arseteroid, and put the butts-gone-bad back in their place? Or will the entire world be abutterated?