The oldest preserved vertebrate brain has been found in a 319-million-year-old fossilized fish skull that was removed from an English coal mine over a century ago.

Revealed by a CT scan in a study led by University of Michigan researchers, the roughly inch-long brain and cranial nerves belonged to an extinct member of the large ray-finned fishes group, which today includes species like tuna, salmon, goldfish and cod.

"Here we've found remarkable preservation in a fossil examined several times before by multiple people over the past century," senior author and University of Michigan paleontologist Matt Friedman said in a news release. "But because we have these new tools for looking inside of fossils, it reveals another layer of information to us."

Friedman, who is also the director of his university's Museum of Paleontology, says the unexpected discovery was made with a micro-CT scanner, which can provide detailed internal images just like the larger medical CT scanners found in hospitals and clinics.

"I scanned it, then I loaded the data into the software we use to visualize these scans and noticed that there was an unusual, distinct object inside the skull," Friedman recalled. "It is common to see amorphous mineral growths in fossils, but this object had a clearly defined structure."

The object was brighter on the CT image than the surrounding bone or rock, meaning it was likely denser. It also had characteristics resembling the brains found in vertebrates, which are animals with spines.

"It had all these features, and I said to myself, 'Is this really a brain that I'm looking at?'" Friedman said. "So I zoomed in on that region of the skull to make a second, higher-resolution scan, and it was very clear that that's exactly what it had to be. And it was only because this was such an unambiguous example that we decided to take it further."

The study was published on Feb. 1 in the scientific journal Nature.

The fossil was discovered in a layer of soapstone at a coal mine in Lancashire in North West England and was first scientifically described in 1925. The only known specimen of its species, Coccocephalus wildi likely lived in an estuary, where it could have dined on aquatic insects, small crustaceans and other invertebrates, like the ancestors of squid and snails. Although only its skull was found, scientists believe the fish would have been about 15 to 20 centimetres long. Its jaw shape and teeth show it was probably a carnivore.

After the fish died, researchers say it was likely quickly buried in sediments with little oxygen; an environment that can slow the decomposition of soft tissue. The animal's brain would have then been replaced during fossilization with a dense mineral that preserved its three-dimensional structure in exceptional detail.

"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," Friedman said. "There seems to be, inside this tightly enclosed void in the skull, a little micro-environment that is conducive to the replacement of those soft parts with some kind of mineral phase, capturing the shape of tissues that would otherwise simply decay away."

The discovery also sheds light on the evolution of ray-finned fishes, which have web-like fins supported by bony spines and represent about half of all living vertebrate species.

"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," the study's lead author, University of Michigan doctoral student Rodrigo Figueroa, said in the news release. "With the widespread availability of modern imaging techniques, I would not be surprised if we find that fossil brains and other soft parts are much more common than we previously thought. From now on, our research group and others will look at fossil fish heads with a new and different perspective."

Friedman and Figueroa both think their work highlights the need to preserve previously discovered fossils for future study.

"That's why holding onto the physical specimens is so important," Friedman said. "Because who knows, in 100 years, what people might be able to do with the fossils in our collections now."