by Paleontologist Matt Friedman was surprised to discover a remarkably detailed 319-million-year-old fish brain fossil while testing micro-scans for a larger project.
“He had all these characteristics, and I was like, ‘Is this really a brain I’m looking at? “,” says Friedman of the University of Michigan.
“So I zoomed in on that area of the skull to do a second, higher resolution scan, and it was very clear that was exactly what it should be. And that’s only because it was an example so clear that we decided to go further.”
Usually, the only remaining traces of this ancient life come from the hard parts of animals that are easier to preserve, such as their bones, because soft tissue breaks down quickly.
But in this case, a dense mineral, possibly pyrite, infiltrated and replaced tissue that had probably been kept longer in an oxygen-poor environment. This allowed scans to pick up what looks like cranial nerve and soft tissue details from the tiny fish, Wild Coccocephalus.
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 would not risk invasive investigative methods.
“Here we found remarkable preservation in a fossil that had been examined many times before by multiple people over the past century,” Friedman says.
“But because we have these new tools to look inside fossils, it reveals another layer of information to us.”
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This prehistoric estuarine fish probably hunted insects, small crustaceans and cephalopods, hunting them with fins supported by bony rods called rays.
Ray-finned fishes, subclass Actinopterygii, make up more than half of all living backbone animals alive today, including tunas and seahorses, and 96% of all fish.
This group split off from lobe-finned fish – some of which eventually became our own ancestors – around 450 million years ago. C.wildi then followed its own evolutionary path from groups of fish still living today some tens of millions of years ago.
“The analyzes place this taxon outside the group containing all living species of ray-finned fishes,” University of Michigan paleontologist Rodrigo Figueroa and colleagues write in their paper.
“Details of the structure of the brain in Coccocephalus therefore have implications for interpretations of neural morphology during the early evolutionary stages of a major vertebrate lineage.”
Some brain features would have been lost due to the decay and preservation process, but the team could still distinguish specific morphological details. This allowed them to see that the way this prehistoric forebrain developed was more like ours than the rest of the ray-finned fish alive today.
“Unlike all living ray-finned fishes, the brain of Coccocephalus folds inward,” notes Friedman. “So this fossil captures a moment before this characteristic feature of the brain of ray-finned fish evolved. This gives us some constraints on when this trait evolved – something we didn’t understand long before the new data on Coccocephalus.”
This inward fold is known as the evaginated forebrain – as with us, the two cerebral hemispheres eventually embrace a hollow space like a “c” and its mirror image comes together. In comparison, the everted forebrains seen in surviving ray-finned fish instead have two swollen lobes, with only a thin crevice between them.
The researchers want to analyze other fish fossils in the museum’s collections to see what other signs of soft tissue might be hiding inside.
“An important conclusion is that these kinds of soft parts can be preserved, and they can be preserved in fossils that we have had for a long time – this is a known fossil for over 100 years,” Friedman says.
“That’s why preserving physical specimens is so important. Because who knows, 100 years from now, what people might be doing with the fossils in our collections today.”
This research was published in Nature.
