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Tyrannosaurus rex has a reputation for being one of the biggest and fiercest dinosaurs ever to have lived. But it probably wasn鈥檛 the brightest: it had a simpler brain than an earlier, smaller tyrannosaur. The change could be a consequence of growing so large.

Fossils suggest that T. rex could reach 12 metres in length and between 8 and 14 tonnes in weight. It belonged to a family of dinosaurs called the tyrannosaurs 鈥� but not all of them were large. For example, Dilong paradoxus 鈥� which lived 50 million years before T. rex evolved 鈥� was only 1.6 metres long and about knee-high to a human.

Martin Kundr谩t at Pavol Jozef Safarik University in Slovakia, and his colleagues, want to know what happened to tyrannosaur brains聽as the dinosaurs evolved from small predators to beasts the size of T. rex. In their search for clues they looked inside the skull of a 125-million-year-old Dilong fossil found in Liaoning, northern China to work out the size and shape of its brain. Then they did the same for a 67-million-year-old T. rex fossil 鈥� a famous specimen nicknamed 鈥楽ue鈥�.

Brain shape

The team found a shift in brain shape as tyrannosaurs evolved and enlarged 鈥� from an S-shaped arrangement in Dilong to what Kundr谩t describes as a 鈥渟impler鈥�, more linear shape in T. rex.

What鈥檚 more, although T. rex鈥檚 brain was 62 times bigger than Dilong鈥檚, T. rex had a relatively smaller forebrain and midbrain. These are brain regions connected with controlling agility. T. rex also had comparatively smaller eyes and the brain region associated with vision was relatively smaller than in Dilong 鈥� although the region of the brain associated with the sense of smell was relatively larger in T. rex than in Dilong.

Kundr谩t has observed similar brain shape changes in modern crocodiles, one of the closest living relatives of dinosaurs. He says small, juvenile crocodiles have S-shaped brains that look superficially like the Dilong brain, whereas large adults have a more linear, T. rex-like brain. As such, Kundr谩t speculates that T. rex might have had a simpler, elongate brain as a direct consequence of its gigantic size. 鈥淏ut how much [this] affected their behaviour is difficult to say for the time being,鈥� he says.