Mega Mechanasaur was constructed by Kooky von Koopa to be used in an assault on Dome City. Mega Mechanasaur resembled an enormous, robotic Tyrannosaurus Rex with giant wheels in place of legs. Mega Mechanasaur was powered by Tyrannosaurus Rex running on a treadmill, Mega Mechanasaur's flame breath was provided by a large Dino-Torch hidden in the machine's mouth.
Bursting through the gates of Dome City, Kooky and Bowser proceeded to rampage through the city, destroying everything in their way. After causing some wide-scale destruction, Mega Mechanasaur was finally destroyed by becoming trapped in a large pot-hole and by being blasted by several fireballs.
Kooky, Bowser and the Mega Mechanasaur's T-Rex "engine" were last seen fleeing from the collapsed machine; the Dino-Torch in Mega Mechanasaur's mouth evidently wasn't so lucky.