 Despite the animal 's overall size , the braincase of Stegosaurus was small , being no larger than that of a dog . A well @-@ preserved Stegosaurus braincase allowed Othniel Charles Marsh to obtain , in the 1880s , a cast of the brain cavity or endocast of the animal , which gave an indication of the brain size . The endocast showed the brain was indeed very small , the smallest proportionally of all dinosaur endocasts then known . The fact that an animal weighing over 4 @.@ 5 metric tons ( 5 short tons ) could have a brain of no more than 80 g ( 2 @.@ 8 oz ) contributed to the popular old idea that all dinosaurs were unintelligent , an idea now largely rejected . Actual brain anatomy in Stegosaurus is poorly known , but the brain itself was , however , small even for a dinosaur , fitting well with a slow , herbivorous lifestyle and limited behavioural complexity . 
