Paleontologists find dinosaur bones on a fairly regular basis, but rarely do they find fossilized soft tissues, which decay over time and eventually vanish from existence – unless they are stored in some very special conditions, that is. For the first time, scientists have described fossilized dinosaur brain tissue which most likely belonged to a Iguanodon-like species that plunged its noggin into a swamp around 133 million years ago, "pickling" the sample in the process.

Tissue samples will degrade over time as bacteria breaks them down, and exposure to oxygen has a big part to play in this. So it would take a special set of conditions to preserve dinosaur brain tissue, and for one giant earth-walker, that special place seemed to have been a stagnant, shallow pond.

The scientists at the University of Cambridge and the University of Oxford who made the discovery believe the dinosaur is closely related to the Iguanodon, a herbivore that existed in the Cretaceous period. It is thought that when the dinosaur died, it came crashing to the ground and its head turned over, leaving the top of its skull resting against the bottom of the pond.

This highly acidic and low-oxygen water then essentially "pickled" the dinosaur's brain, the scientists say, which allowed the tissue to become mineralized before decaying away. Using scanning electron microscope techniques, the scientists were able to identify meninges in the tissue, which are tough membranes that surround the brain, along with strands of collagen and blood vessels. They also discovered signs of structures that may represent tissue from the brain cortex, interlaced with delicate capillaries.

Their close examination of the brain revealed similarities with modern-day descendants of dinosaurs, such as crocodiles and birds, and also found that it appeared to be pressed against the skull. This is in contrast to typical reptiles, whose sausage-shaped brains sit inside a dense pillow of blood vessels and sinuses and only take up around half of the cranial cavity.

This raises the possibility that some dinosaurs had larger brains than we thought, but the researchers say it is more probable that when the dinosaur came down to rest that final time, its overturned head partnered with gravity to press its brain against the roof of the skull.

"As we can't see the lobes of the brain itself, we can't say for sure how big this dinosaur's brain was," says Dr David Norman from the University of Cambridge. "Of course, it's entirely possible that dinosaurs had bigger brains than we give them credit for, but we can't tell from this specimen alone. What's truly remarkable is that conditions were just right in order to allow preservation of the brain tissue – hopefully this is the first of many such discoveries."

You can hear from Norman in the video below, about the fossil that was originally discovered in 2004 and just released in a Special Publication of the Geological Society of London.

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