Which dinosaur is often said to have a brain the size of a walnut?

The dinosaur often said to have had a brain the size of a walnut was Stegosaurus. The comparison comes from the animal’s very small brain relative to its massive body, although “walnut-sized” is a catchy approximation rather than a precise measurement.

Stegosaurus was a large, herbivorous dinosaur that lived during the Late Jurassic, about 150 million years ago. It is recognized by the two rows of large plates along its back and the four spikes on its tail, commonly called the thagomizer.

Older popular accounts also claimed that Stegosaurus had a second brain or enlarged nerve center near its hips. That structure was more likely a glycogen-rich body cavity or enlargement associated with the spinal cord, not a second brain. The dinosaur’s braincase suggests limited cognitive capacity, but brain size alone does not fully measure intelligence or behavior.

The walnut phrase is sometimes confused with claims about other large dinosaurs. Tyrannosaurus rex had a much larger brain, while Iguanodon and Brachiosaurus are not the standard answers to this classic trivia question.