What it is
A small fish that died about 319 million years ago. It was an early ray-finned fish, the group that makes up almost every kind of fish alive today. It swam in river mouths and likely ate small shrimp-like animals, small squid relatives and insects. Its skull came out of the roof of a coal mine in northern England, and it is the only one of its kind ever found.
Why it matters
A brain is soft. It rots fast, and it almost never turns to stone. But an X-ray scan showed this skull still has its brain inside, with its nerves, about an inch long. A dense mineral took the place of the soft tissue and kept its shape. No older brain has been found in any animal with a backbone.
Folded the other way
Every ray-finned fish alive today, from a goldfish to a tuna, grows its brain by folding the tissue outward. This brain folds inward, the way yours does and the way every other animal with a backbone does. So the outward fold came later. Of living fish, its brain looks most like a sturgeon's or a paddlefish's.