Iguanodon bernissartensisWIKIMEDIA, GHEDOGHEDOAt the height of the Cretaceous Period, one hapless herbivore—likely related to Iguanodon—fell backwards into a bog, smacked its head, and addled its brain. Now, around 133 million years later, scientists have uncovered a sediment cast of that brain, etched across eons into the dinosaur’s mineralized skull. The fossil, described in a study published last week (October 27) in Earth System Evolution and Early Life, is incredibly well preserved and, according to the authors, may be the only extant impression of dinosaur brain tissue.

The endocast was first discovered in 2004, when a fossil hunter in England came across a pebble with odd ripples along its surface. Experts agreed that the pebble was a dinosaur fossil, but they hardly expected to find that it contained the mineralized remnants of soft tissue—much less the first recorded dinosaur brain. While happy accidents of rapid mineralization have...

After examining the pebble under a scanning electron microscope, Norman and colleagues were able to make out blood vessels, capillaries, cortical tissue, and even the remnants of meninges. They discovered that the brain was structurally similar to those of modern birds and reptiles, and proposed that the bog’s fast-acting acidic waters probably preserved its finer features.

Although the dinosaur’s brain appears larger than expected, the researchers caution that this is likely an artifact of the decaying process. “As we can’t see the lobes of the brain itself, we can’t say for sure how big this dinosaur’s brain was,” Norman said in a statement.

“Conditions were just right in order to allow preservation of the brain tissue,” Norman added in the statement. “Hopefully, this is the first of many such discoveries.”

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