This fish skull still has its brain inside. And the fish died about 319 million years ago.
That should be impossible. A brain is soft. It rots fast, and it almost never turns to stone. What a fossil keeps is bone, teeth and shell.
The skull came out of the roof of a coal mine in northern England. It went to a museum in Manchester, and it was first described in 1925. It is the only one of its kind ever found.
So nobody was going to cut it open. A team in Michigan put it in an X-ray scanner instead, to study the skull. They weren't looking for a brain.
But the scan showed something inside. It had two matching halves. It had hollow chambers. And thin threads ran out from it, into the holes in the skull where nerves go.
It was the brain, and its nerves, about an inch long. A dense mineral had taken the place of the soft tissue, and kept its shape.
No older brain has been found in any animal with a backbone. And for about a hundred years it sat in a museum, and nobody knew it was there.