The mineralized brain matter, appearing like a brown pebble, probably came from a large dinosaur roughly on the order of five meters long that grazed on plants during the Early Cretaceous some 133 million years ago. It was likely a relative of iguanodon, and it shares some distinct characteristics with its living relatives: birds and crocodiles.
When paleontologists look for dinosaur fossils, they typically look for bones or teeth — hard tissues that are more readily preserved in the fossil record. Soft stuff, such as muscle, organs and skin, quickly decays. Sometimes the impressions of feathers and skin are found, and occasionally they or other bits of tissue get directly preserved. But for the most part, scientists have to take educated guesses, based on the skeletons, of what those soft parts may have looked like.
This fossil brain fragment, however, seems to have been preserved under highly unusual circumstances. Discovered in 2004 near Bexhill in southeast England by fossil hunter Jamie Hiscocks, it drew the attention of study co-author Martin Brasier of Oxford University, who asked study co-author David Norman, a dinosaur paleobiologist at Cambridge University, to take a look.
“I recognized it immediately as part of the endocranial cast of an iguanodon-like animal, which is a dinosaur, and was also struck by the curious sort of encrusted nature of its surface — and got very interested in what it might mean,” Norman said.
The scientists think this is not a full brain: It’s probably just the tough membranes known as the meninges, along with some of the capillaries and adjacent cortical tissues.
Norman said this brain tissue likely survived because of the strange circumstances in which the animal died: It probably ended up flipped over in a swampy area, so that the top of its skull was submerged in shallow, highly acidic, low-oxygen water.
“In effect, that environment pickled the upper part of the head of the animal,” he explained.
The sausage-shaped brains of today’s reptiles usually take up only about half the available space in their skulls; the fact that the dinosaur’s fossil brain seems to have been pressed up against the skull might lead some to consider whether it had a much bigger brain. But Norman said this was probably not the case: The top of the brain was probably pressed up against the skull due to gravity.
“It was just the circumstances of decay that led to the brain being pushed down,” he explained.
Scientists can’t say much about the nature of this dinosaur noggin — especially not how it thought or whether it was intelligent, the researcher said.
“All I can say definitively is that dinosaurs had a brain,” Norman added.
As rare as this fossil tissue may be, is it possible that there are other soft-tissue fossils that have flown under the radar?
“Yes, you wonder,” Norman said. “And I won’t be the only person wondering. A lot of people who work on fossils might be looking at them in a slightly different light now, when they realize there is some curious structure on the surface.”
The dinosaur brain fragment was described in a special issue of the Geological Society of London honoring study co-author Martin Brasier, who died in 2014.
Image Credit: Jamie Hiscocks/University of Cambridge.