On a dark winter night in 2004, Jamie Hiscocks spotted an oddly shaped stone on a beach by his home in Sussex, England. “I could see in my torchlight structured detail on the surface of the object,” Hiscocks, a fossil hunter by trade, told me in an email. “Immediately I knew this wasn't any ordinary pebble.”

Hiscocks showed the specimen to Martin Brasier, a top paleobiologist at Oxford University. Braiser identified it as a dinosaur endocast — a fossil that forms when sediment fills up the inside of an animal’s skull — belonging to an iguanodon, a dinosaur that lived during the Cretaceous Period.

But this wasn’t your standard endocast. For one, it wasn’t smooth. “It looked a bit corrugated almost; there were ridges and grooves,” Alexander Liu, a former student of Brasier, told me. (Brasier died in a car accident in 2014.)

That got the researchers excited. Closer analysis revealed a few-millimeter-thick layer of structures that looked like blood vessels. There were also traces of meninges, the tough outer layer that protects the brain, preserved in mineral form.

Brains usually decompose very quickly after death. So quickly that no piece of fossilized brain had ever been discovered from a vertebrate living on land.

That made this the very first dinosaur brain fossil ever to be found, as Liu and co-authors describe in a special publication of The Geological Society of London, published in October.

The discovery — which was a side project Brasier and others pursued slowly for years — lends some hope that one day paleobiologists could crack the mystery of dinosaur intelligence. But perhaps even more remarkable is that this piece of brain was even fossilized in the first place.

Soft tissues rarely ever fossilize

There are a few reasons no one has ever discovered a fossilized dinosaur brain before.

For one, dinosaurs lived a very, very long time ago. Nearly all of them perished completely; their bodies and bones decomposed, leaving no trace.

A tiny fraction managed to die under fortuitous circumstances that allowed their tissue and bones to be fossilized in rock. In these rare cases when organic tissue fossilizes, minerals come in to replace the tissue. But soft tissues like brains break down especially quickly — making it about the least likely tissue to be preserved.

So what might have allowed this particular brain to evade the microbes that would have otherwise devoured it? Liu, Brasier, and colleagues had to become forensic investigators to figure it out.

Here’s the story they pieced together.

It begins around 133 million years ago, in the Cretaceous Period, when the Earth was a much stranger place. It was much warmer, there was very little ice at the poles, and around a third of the land that we inhabit today was covered by water. The continents were just beginning their drift apart from one another.

In this strange world — in prehistoric Britain — roamed a species of dinosaur that looked like a huge (30 foot long, and 7 feet high) reptilian horse with a tail and a peaked ridge along its back.

When this iguanodon died, several serendipitous things occurred for its brain to become a fossil.

1) When this animal died, it likely fell headfirst into water, where its skull turned upside down. That limited the exposure to air. (Brains quickly decompose in the presence of oxygen.)

2) The skull stayed intact, so when a small portion of the brain began to decompose, the chemicals it leeched stayed within the brain case. That decomposition “released nutrients and enzymes, rich in things like iron and phosphate,” Liu explained.

3) Those nutrients and enzymes essentially pickled the other portion of the brain, preserving it. Those nutrient and enzymes also contained the right chemicals to begin the process of mineralization.

4) Perhaps within a few days, Liu explained, the preserved portion — a section just a few millimeters thick that was pressed up against the skull — began to be replaced by phosphate and carbonate minerals. A chemical reaction allowed the minerals to transform the organic material, mimicking their structure.

Over time, that fossilized brain separated from the rest of the body. It was carried by tides and storms and found a home in tidal pool in the UK. (The British shoreline, with its exposed sedimentary rock, is a favorite of fossil hunters.) When a winter storm uncovered a previously submerged section of beach in 2004, it was lucky that a pro like Hiscocks was walking by. “Being such a fragile thing, the next storm could have destroyed it,” Hiscocks said.

The scientists discovered blood vessels, the telltale sign that this was indeed part of a brain

For years, this fossil was a side project for Brasier (who typically studied much older fossils, closer to the origin of life on Earth). Most of the research was performed by his students, Liu says, who also were engaged in other projects. So it took a long time to confirm their hunch.

“The most convincing thing in the data we’ve got are the blood vessels,” Liu said. “They’re incontrovertible; they can’t be anything else. They have the right diameters, they branch in the right way, they’re hollow, and they are in the right places.”

You can clearly see the vessels on the electron microscope images. Here, the arrows point to evidence that the vessels were hollow.

And here you can see the vessels branch like any ordinary capillary would.

That the blood vessels are so clear make the scientists confident that they also have seen meninges. Liu says there’s some evidence of fossilized cortical (grey matter) tissue as well, but they’re not as confident.

According to Liu, the paper was nearly ready to publish in 2010, but Brasier held off. He wanted Hiscocks to guarantee that the fossil would eventually end up in a museum, for all to study and see. When a fossil is in a museum, it allows other to replicate the study results. The exact institution where the fossil will end up has not yet been determined.

Can the fossil tell us anything about dinosaur intelligence?

In many ways, the discovery doesn’t actually tell us all that much about dinosaurs.

“Of course we knew dinosaurs had brains,” David Norman, a paleontologist at Cambridge and a co-author on the paper, said. This paper, he says, is more a proof of concept: Tissues as soft and squishy as brains can, under the right circumstance, fossilize.

And that should inspire fossil curators to take a look back at their collections, Liu said. “Now that we know that these soft tissues can be preserved,” he said, “people can go back in museum collections and look out for them, reexamine them, see if it is more common.”

More dinosaur brain specimens could help solve a big mystery about dinosaur intelligence: Were their brains more like modern-day reptiles or more like modern-day birds?

In modern reptiles, the brain typically does not take up all the space in the skull. It’s much smaller than the skull, supported by tissue that pads it. In birds, however, the brain does typically take up most of the skull.

A more birdlike brain would suggest dinosaurs were more intelligent than typical reptiles.

There’s some evidence from this iguanodon brain that it did take up most of the skull, but it’s inconclusive. Because the animal likely died upside down, the brain tissue could have spread out on the roof of the skull. “You can’t say it’s evidence dinosaur brains are bigger than we thought,” Norman said. At the very least, Liu says, the size of the brain indicates iguanodons were at least as smart as modern-day crocodiles (not the brightest in the animal kingdom, but a cunning enough hunter).

The overarching question that Norman, Liu, and their colleagues are after going forward is not “how smart were dinosaurs?” It’s a lot bigger than that. “It’s unraveling the history of the Earth,” Norman said. These animals lived on a planet that was a lot different from the one we live on today. What was it about their biology, about their physiology, that allowed them to thrive? “The questions are almost endless,” he said. And this fossil just sparks more of them.