Oldest brain in a backboned animal found in fish fossil

A fossilised fish originally dug up more than a century ago in an English coal mine has been shown to hold the world’s oldest brain in a vertebrate animal.

CT scans revealed the new internal features including a brain and cranial nerves about 2 centimetres long.

Soft tissue such as internal organs decay very quickly and very rarely fossilise.

However, the internal skull structure and soft tissue of this fish were preserved in exquisite detail when it died. This suggests the animal was buried quickly in sediment with little oxygen present.

Called Coccocephalus wildi, the fish is known only from its skull, but scientists estimate it would have been 15-20 centimetres long. It is also believed that the creature would have been a carnivore.

At 319 million years old, the fish lived 100 million years before the first dinosaurs roamed the Earth. This period of Earth’s history is known as the Pennsylvanian (the second major interval of the Carboniferous Period) and was characterised by high oxygen levels.

The higher oxygen content in the atmosphere saw the evolution of giant arthropods including dragonflies with metre-long wingspans, spiders the size of cats and millipedes the length of a car.

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Among Pennsylvanian fossils are the first emergence of reptiles in the palaeontological record.

Artist’s interpretation of a remarkable 319-million-year-old fish that preserves the earliest fossilized brain of a backboned animal. Credit: Márcio L. Castro.

The team say that the prehistoric brain bears resemblance to the nervous systems of living fish.

“Comparisons to living fishes showed that the brain of Coccocephalus is most similar to the brains of sturgeons and paddlefish, which are often called ‘primitive’ fishes because they diverged from all other living ray-finned fishes more than 300 million years ago,” says Dr Sam Giles from the University of Birmingham, UK.

Giles adds that the discovery can help scientists flesh out the evolution of today’s fishes.

“This unexpected find of a three-dimensionally preserved vertebrate brain gives us a startling insight into the neural anatomy of ray-finned fish,” Giles comments. “It tells us a more complicated pattern of brain evolution than suggested by living species alone, allowing us to better define how and when present day bony fishes evolved.”

The researchers were not looking for the brain when they examined the fossil for the first time, but saw an unusual object inside the skull.

3D animation of CT-scanned images of the Coccocephalus wildi fossilized skull (light yellow), followed by the fossilized fish brain and cranial nerves (pink) and otolith ear-stones (white) inside the skull. Credit: University of Michigan Museum of Paleontology.

They found that the brain was symmetrical and contained hollow spaces. They also saw filaments extending into openings in the braincase, similar to cranial nerves observed in living species.

However, Coccocephalus’s brain folds inward, unlike in living ray-finned fish species which have outward-folding brains.

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“Not only does this superficially unimpressive and small fossil show us the oldest example of a fossilised vertebrate brain, but it also shows that much of what we thought about brain evolution from living species alone will need reworking,” says the US University of Michigan PhD student Rodrigo Figueroa.

The discovery is detailed in a paper published in Nature.

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