As imposing as it was, Ankylosaurus was powered by an unusually small brain--which was about the same walnut-like size as that of its close cousin Stegosaurus, long considered to be the most dim-witted of all the dinosaurs. As a rule, slow, armored, plant-munching animals don't require much in the way of gray matter, especially when their main defensive strategy consists of flopping down on the ground and lying motionless (and perhaps swinging their clubbed tails).
