The brains of stegosaurs were very small ( about 0.001% of their total mass), this was determined by molds made of the brain casing (because brains are soft and would not be preserved). The size of the brain leads us to the conclusions that stegosaurs weren't too smart, having one of the smallest brains of all dinosaurs. When molds were made of the stegosaurs' neural canal it was discovered to have an enlarged cavity in its sacral region. This cavity was thought to possibly be a second brain or a space for an enlarged nervous system to help the tiny brain in controlling the enormous body, but the same feature was found in birds which use the cavity as a glycogen revisor to supply glycogen to the nervous system and this is what the cavity in the stegosaurs is believed to function as.