“This misshapen pebble is actually the first ever dinosaur brain fossil ever found.”

Vox takes a close look at a fossilized iguanodon brain:

On a dark winter night in 2004, Jamie Hiscocks spotted an odd-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.

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.

Photo at the link.