Scientist accidentally discovers the oldest brain of all vertebrates: ScienceAlert

Scientist accidentally discovers the oldest brain of all vertebrates: ScienceAlert

Paleontologist Matt Friedman was surprised to discover a remarkably detailed 319-million-year-old fish brain fossil while testing micro-CT scans for a wider project.

“It had all these features and I said to myself, ‘Is this really a brain I’m looking at?'” say Friedman of the University of Michigan.

“So I zoomed in on that part of the skull to do a second scan at a higher resolution, and it was very clear that that’s exactly what it needed to be. And just because this was such an unequivocal example, they decided to take us further.”

Usually, the only remaining traces of such ancient life come from easier-to-preserve hard parts of animals, such as their bones, as soft tissues break down quickly.

But in this case, a dense mineral, possibly pyrite, seeped in and replaced tissue that was likely preserved longer in a low-oxygen environment. This allowed scans to pick up what appears to be cranial nerve and soft tissue details from the little fish, Coccocephalus wildi.

The ancient specimen is the only one of its kind, so despite being in the hands of researchers since it was first described in 1925, this feature remained hidden because scientists didn’t want to risk invasive research methods.

“Here we found remarkable preservation in a fossil that has been explored several times before by multiple people over the past century,” explains Friedman.

“But because we have these new tools to look inside fossils, it reveals another layer of information for us.”

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This prehistoric estuary fish probably hunted insects, small crustaceans and cephalopods, chasing them with fins supported by bony rods called rays.

Ray-finned fish, subclass Actinopterygiicome up with more than half of all living animals with a backbone are alive todayincluding tuna and seahorses, and 96 percent of all fish.

This group split from lobe-finned fishes about 450 million years ago – some of which eventually became our own ancestors. C. wanted then took its own evolutionary path from the groups of fish still alive today about tens of millions of years ago.

“Analysis places this taxon outside the group that contains all living ray-finned fish species,” University of Michigan paleontologist Rodrigo Figueroa and colleagues write in their newspaper.

“Details of brain structure in Coccocephalus therefore have implications for interpretations of neural morphology during the early evolutionary stages of an important lineage of vertebrates.”

illustrated gaping fish with close-up insert of its brain structure
Artistic interpretation of the 15 to 20 centimeters long (6 to 8 inches long) fish and its brain structure. (Marcio L. Castro)

Some brain features would have been lost through decay and the preservation process, but the team was still able to discern specific morphological details. This allowed them to see that the way these prehistoric forebrains evolved was more like ours than the rest of the living ray-finned whales living today.

“Unlike all living ray-finned fish, the brain of Coccocephalus folds in,” notes Friedman. “So this fossil records a time before that signature fish brain feature with ray fins evolved. This gives us some constraints on when this trait evolved — something we didn’t have a good grasp of before the new data on Coccocephalus.”

This inward fold is known as an evaginated forebrain – as with us, the two hemispheres eventually end in a ‘c’ and the mirror image embraces a hollow space. In comparison, inward-turned forebrains seen in still-living ray-finned fishes instead have two inflated lobes, with only a thin gap between them.

The researchers would like to scan other fish fossils in the museum’s collections to see what other signs of soft tissue might be hiding there.

“An important takeaway is that these kinds of soft parts can be preserved, and they can be preserved in fossils that we’ve had for a long time — this is a fossil that’s been known for over 100 years,” say Friedman.

“That’s why it’s so important to hold onto the physical specimens. Because who knows, 100 years from now, what people might be able to do with the fossils in our collections now.”

This research was published in Nature.

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