the idea that "stegosaurus" had a "brain the size of a walnut" (an oft-repeated phrase), was not quite accurate. it had a brain the size of a dog's, but in proportion to its body, the brain was very small. in the 1880s a well-preserved "stegosaurus" braincase allowed othniel charles marsh to get a cast of the brain cavity, which gave an idea of the brain size. the endocast showed that the brain was indeed small: relative to body size, maybe the smallest among the dinosaurs. the fact that an animal weighing over 4.5 metric tonnes (5 short tons) could have a brain of no more than 80 grams (2.8 oz) led to the idea that dinosaurs were unintelligent, an idea now largely rejected.