Paleontologist Matt Friedman was surprised to discover a remarkably detailed 319-million-year-old fish brain fossil while testing micro-CT scans for a larger project.
“It had all these features, and I said to myself, ‘Is this really the brain I’m looking at?’ he says Friedman of the University of Michigan.
“So I zoomed in on that area of the skull for a second, higher-resolution scan, and it was very clear that this was exactly what it was meant to be. And because it was an unambiguous pattern, we decided to take it further.”
Usually, the remaining traces of such ancient life are hard parts of animals, such as bones, which are easier to preserve because soft tissues break down quickly.
But in this case, a dense mineral, probably pyrite, which had been stored longer in a low-oxygen environment, seeped into and replaced the tissue. These scans allowed the cranial nerve and soft tissue details of the small fish to be picked out. 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, the feature has remained hidden because scientists would not risk invasive research methods.
“Here we found remarkable preservation in a fossil that had been previously examined several times by several people in the last century.” explains Friedman.
“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, chasing them with fins supported by bony rods called rays.
Ray-finned fish, subclass Actinopterymakeup more than half of all vertebrates living todayincluding tuna and seahorses and 96 percent of all fish.
This group diverged from the lobe-finned fishes, some of which eventually became our ancestors, about 450 million years ago. C. wildi then tens of millions of years ago, it took its evolutionary path from the fish groups that still live today.
“The analysis excludes this taxon from the group that includes all living ray-finned fish species,” said University of Michigan paleontologist Rodrigo Figueroa and colleagues. let them write on their papers.
“Details of brain structure Coccocephalus therefore has implications for interpretations of neural morphology during early evolutionary stages of major vertebrate lineages.”
Some brain features would have been lost through the decay and preservation process, but the team was still able to extract specific morphological details. This allowed them to see that the development of this prehistoric forebrain was more similar to ours than that of other ray-finned fish living today.
“Unlike all living ray-finned fish, the brain Coccocephalus folds in” notes Friedman. “So this fossil covers a period before the evolution of this signature feature of ray-finned fish brains. It gives us some constraints on when this feature evolved—something we didn’t have a good handle on before the new data. Coccocephalus.”
This inner fold is known as the hollowed out forebrain – as in our case, the two cerebral hemispheres surround a hollow space like a “c” and its mirror image merges. By comparison, instead of the everted forebrains seen in extant ray-finned fishes, there are two swollen lobes with only a thin slit between them.
The researchers are interested in scanning other fish fossils in the museum’s collections to see what other signs of soft tissue might be hiding inside.
“An important finding is that these kinds of soft parts can be preserved, and they can be preserved for a long time in the fossils we have – this is a fossil that has been known for over 100 years.” he says Friedman.
“That’s why it’s so important to capture physical specimens. Because who knows what people will be doing with the fossils in our collections now in 100 years.”
This study was published nature.
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