Paleontologist Matt Friedman was surprised to discover a highly detailed brain fossil of a 319-million-year-old fish while testing microcomputed tomography for a larger project.

“It had all these features and I was like, ‘Is this really a brain I’m looking at,'” Friedman of the University of Michigan said. I asked,” he says.

“So I zoomed in on that area of ​​the skull to do a second higher resolution scan, and it was very clear that this was exactly what it was supposed to be. And just because it was such a clear sample, we decided to go ahead.”

Often, traces of such ancient life remain in the hard parts of animals that are more easily preserved, such as bones, as the soft tissues rapidly decompose. But in this case, a dense mineral, possibly pyrite, leached out and likely replaced the fabric, which had been preserved longer in the low-oxygen environment. This made it possible to reveal details of the cranial nerves and soft tissues of the small fish during the scan. Coccocephalus wildi

The ancient specimen is the only one of its kind, so although it has been in the hands of researchers since it was first described in 1925, this feature has remained hidden because scientists have not put it at risk using invasive research methods.

“Here we found a wonderful preservation of fossils that had been studied several times over the past century by several people,” explains Friedman.

“But as we have new tools for finding fossils, it opens up another layer of information.”

This prehistoric estuarine fish likely preyed on insects, small crustaceans and cephalopods by chasing them with fins supported by bony rods called stingers. Rayfin fish, a subclass of Actinopterygii, account for more than half of all living vertebrates today, including tuna and seahorses, and 96 percent of all fish.

This group diverged from the blade-finned fish, some of which later became our ancestors, about 450 million years ago. Then C. wildi about tens of millions of years ago, it followed its own evolutionary path from fish groups still living today.

“The analysis places this taxon outside the group that includes all current species of ray-finned fish,” University of Michigan paleontologist Rodrigo Figueroa and colleagues write in their paper.

“So the details of the brain structure cococephaly It is important for the interpretation of neuronal morphology in the early stages of evolution of the main vertebrate lineage.”

Some features of the brain may have been lost due to the decay and preservation process, but the team was still able to distinguish certain morphological details. This allowed them to see that the development of this prehistoric forebrain was more similar to ours than to other ray-finned fish alive today.

“Unlike all living ray-finned fish, the brain is coxcephaly “So this fossil records the time before the brain feature characteristic of ray-finned fish emerged. This gives us some constraints on when the feature evolved – something we can’t get a good handle on until new data about it. cococephaly”

This inner fold is known as the drained forebrain – like us, the two hemispheres of the brain occupy a hollow space like the letter “c” and its mirror image is joined together. By comparison, the outward-turned forebrain of radial-finned fish has two swollen lobes with only a thin space between them.

The researchers want to scan other fish fossils in the museum’s collections to see what other signs of soft tissue lurk inside.

“The key implication is that such soft pieces can be preserved and preserved in fossils we have long had—these are fossils that have been known for over 100 years,” Friedman says. “That’s why it’s so important to keep physical specimens. Because 100 years from now, who knows what humans will be able to do with the fossils in our collection.”