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 qualities, and I was like, ‘Is this really a brain I’m looking at?'” says Friedman from the University of Michigan.
“So I zoomed in on that region of the skull to do a second, higher-resolution scan, and it was very clear that it had to be just that. And just because this was such a clear example, we decided to take it further.”
Usually the only remaining traces of such ancient life are of harder parts of animals, such as their bones, that are easier to preserve, since soft tissues are rapidly degraded.
But in this case, a dense mineral, possibly pyrite, seeped in, replacing tissue that had likely been preserved longer in a low-oxygen environment. This allowed for scans that look like the small fish’s cranial nerves and soft-tissue details. Coccocephalus wildi.
The ancient specimen is the only one of its kind. 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 methods of investigation.
“Here we have found a remarkable preservation in a fossil that has been examined several times over the past century by several people.” explained Friedman.
“But because we have these new tools to look inside fossils, it reveals another layer of information.”
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This prehistoric estuary fish likely preyed on insects, small crustaceans, and cephalopods, hunting them with fins supported by bony rods called rays.
Ray-finned fish, subclass Actinopterygiiform over half of all animals alive today have backbonesincluding tuna and seahorse and 96 percent of all fish.
This group split off from the lobe-finned fishes, some of which eventually became our own ancestors, about 450 million years ago. C. wildi then embarked on its own evolutionary path from the fish groups that survive today about ten million years ago.
“Analysis places this taxon outside the clade that contains all living ray-finned fish species,” says University of Michigan paleontologist Rodrigo Figueroa and colleagues write on your paper.
“Details of brain structure in coccocephalus therefore have implications for the interpretation of neural morphology during the early evolutionary stages of a large vertebrate lineage.”
Some brain features would have been lost through the decay and preservation process, but the team was still able to discern specific morphological details. This allowed them to see that the way this prehistoric forebrain evolved resembled ours more than the rest of the ray-finned fish alive today.
“Unlike all living ray-finned fish, the brain of coccocephalus folds inwards”, Notebook Friedman. “So this fossil captures a time before this characteristic feature of ray-finned fish brains evolved. This gives us some limitations on the development of this trait – something we didn’t have a good handle on prior to the new data coccocephalus.”
This fold is called a protruding forebrain – like ours, the two cerebral hemispheres end up enclosing a cavity like a “c” and joined together as a mirror image. In comparison, everted forebrains seen in extant ray-finned fish have instead two inflated lobes with only a thin gap between them.
The researchers are keen to scan other fish fossils in the museum’s collections to see what other signs of soft tissue might be hiding inside.
“A key conclusion is that this type of soft tissue can be preserved, and it 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.” says Friedman.
“That’s why it’s so important to stick with the physical specimens. Because who knows what people might do with the fossils in our collections 100 years from now.”
This study was published in Nature.
