The Stegosaurus had a brain the size of a walnut.

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Dinosaurs, like Winnie the Pooh, have traditionally been depicted as having very small brains, and therefore not being very intelligent creatures.

It is true that, in general, dinosaurs' brains were much smaller than the brains of mammals possessing heads of comparable size. Whereas in a human skull, most of what is under the immediate bone surface are brain matter, a dinosaur skull's key facet is its jaw structure; much of the available space was occupied by powerful biting muscles, with the brain being buried under a thick casing to keep it well protected.

A rough system of estimating dinosaurs' (and other creatures') intelligence is known as the Encephalisation Quotient, or EQ. Developed by the American palaeoneurologist Harry Jerison in the 1970s, a dinosaur's EQ is the ratio of its brain weight relative to the brain weight of a "typical" animal of similar body weight. Typically, warm-blooded mammals and birds have much higher EQ ratings than cold-blooded reptiles of the same size, and there is a wide variation in the estimated EQ ratios of different dinosaur types, which reflects their differing lifestyles and (possibly) metabolic rates.

Most dinosaurs have an EQ similar to those of modern reptiles. Typically, herbivores such as the large sauropods, armored ankylosaurs, and stegosaurs are at the lowest end of the scale. It is often said that Stegosaurus had a brain the size of a walnut - in fact, it was more like the size of a lime, or a dog's brain, but still relatively small for a dinosaur that grew up to nine meters long. Later ornithischian herbivores of the Cretaceous period, such as Edmontosaurus, possessed slightly bigger brains, but still smaller relative to carnivorous dinosaurs.

Predatory theropods are thought to have relatively larger brains (and excellent eyesight), evolved because of their need to hunt prey at speed. T. rex, for example, had a particularly large brain even compared to other dinosaurian predators its size.

However, at the top of the scale (by a distance) are those theropod dinosaurs most closely related to modern-day birds - small- and medium-sized carnivorous dromaeosaurids such as Velociraptor, and troodontids such as Troodon.

Troodontids' brains were comparable in size to those of today's flightless birds, and they had large eyes that pointed forward to give them binocular (three-dimensional) vision, and also an acute sense of hearing to help locate their prey.

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