Oldest preserved vertebrate brain found in 319-million-year-old fish fossil

Illustrative image: fish fossil. Photo: James St. John, Creative Commons Attribution 2.0

As scientists were studying a 319-million-year-old fossil, the only discovered specimen of its species, using noninvasive CT scanning, they were surprised to find that the structures of the brain were perfectly preserved, something extremely rare when it comes to soft tissue.

Coccocephalus wildi was a ray-finned fish, likely carnivorous, that lived over 300 million years ago. Only one specimen, and only the skull, was ever discovered, in a coal mine in England, about a century ago.

Since the skull is one of a kind, scientists who want to conduct research on it must use non-invasive methods, such as CT scanning.

Imagine the surprise of the scientists, who scanned the skull and found an unusual, distinct object they did not expect to find inside it, which displayed several features found in vertebrate brains: bilateral symmetry, hollow spaces similar in appearance to ventricles, and multiple filaments extending toward openings in the braincase, similar in appearance to cranial nerves.

Just like other soft tissues, brain and nerve tissue usually decay quickly and very rarely fossilize.

But in this case, the soft tissues of the fish’s brain and cranial nerves were replaced during the fossilization process with a dense mineral, preserving their three-dimensional structure in exquisite detail.

Researchers at the University of Birmingham and the University of Michigan who conducted the study, believe that the discovery opens a window into the neural anatomy and early evolution of ray-finned fishes, a major group of fishes alive today, and more importantly one that makes up about half of all vertebrate animals, a clade that includes all backboned creatures, from amphibians to mammals, including humans.

“This unexpected find of a three-dimensionally preserved vertebrate brain gives us a startling insight into the neural anatomy of ray-finned fish. 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, said senior author Sam Giles of the University of Birmingham.

“An important conclusion is that these kinds of soft parts can be preserved, and they may be preserved in fossils that we’ve had for a long time — this is a fossil that’s been known for over 100 years,” senior author Matt Friedman, from the University of Michigan, commented.

Rodrigo Figueroa, also from the University of Michigan and the lead author of the article on the study said:

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

To put a long story short, a fish biscuit well beyond its “best by” date may turn what we have so far known about the evolution of the nervous system on its head.