A Stegosaurus brain had much more in common with the brain of a lizard than with the brain of a mammal. The front of the brain, which included the olfactory bulbs (sense of smell) was unusually large, but the cerebrum (higher thinking) and cerebellum (complex voluntary musclular movement) were quite small.

The medulla oblongata (autonomic nervous system) was long and large. An endocast of Stegosaurus displaces about 56 milliliters of water making it very small when compared to the size of the animal. For a fully grown house cat we find that the brain displaces about 30 milliliters of water. In spite of its small brain, stegosaurs were very successful land animals for at least 100 million years.