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An analysis of the skull of a 319-million-year-old fossilized fish has led to the discovery of the oldest well-preserved example of a vertebrate brain, shedding new light on the early evolution of bony fish.
The skull fossil belonging to the extinct Coccocephalus wildi was discovered in a coal mine in England more than a century ago, according to researchers in the study published Wednesday in the journal Nature.
The fossil is the only known specimen of the fish species. Scientists from the University of Michigan in the US and the University of Birmingham in the UK therefore used the non-destructive imaging technique of computed tomography (CT) to look inside his skull and examine internal sound. body structure.
In doing so, came a surprise. The CT image showed an “unidentified spot,” according to a University of Michigan press release.
The distinct 3D object had a clearly defined structure with features found in vertebrate brains: it was bilaterally symmetrical, contained hollow spaces similar in appearance to ventricles, and had extended filaments that resembled cranial nerves.
“This is such an exciting and unexpected finding,” study co-author Sam Giles, a vertebrate paleontologist and principal investigator at the University of Birmingham, told CNN on Thursday, adding that they had “no idea” that there was a brain inside when they decided to study the skull.
“It was so unexpected that it took us a while to be certain it was indeed a brain. As well as being just a preservation curiosity, the brain anatomy in this fossil has great implications for our understanding of brain evolution in fish,” she added.
C. wildi was an early ray-finned fish—possessing a backbone and fins supported by bony rods called “rays”—that is said to have been 6 to 8 inches long, swam in an estuary, and ate small aquatic animals and aquatic insects, according to the researchers.
According to the study, the brains of living ray-finned fish have structural features not seen in other vertebrates, including a forebrain made up of neural tissue that folds outward. In other vertebrates, this neural tissue folds inward.
C. wildi lacks this characteristic characteristic of ray-finned fishes, the configuration of a part of its forebrain called the “telencephalon” more closely resembling that of other vertebrates, such as amphibians, birds, reptiles and mammals, according to the study authors. .
“This indicates that the telencephalon configuration observed in living ray-finned fishes must have emerged much later than previously thought,” said the study’s lead author, Rodrigo Tinoco Figueroa, a doctoral student at the Museum of University of Michigan Paleontology.
He added that “our knowledge of vertebrate brain evolution is mostly limited to what we know of living species,” but “this fossil helps us fill important knowledge gaps that could only be obtained. ‘from exceptional fossils like this’.
Unlike hard bones and teeth, scientists rarely find brain tissue — which is soft — preserved in vertebrate fossils, the researchers say.
However, the study noted that the C. wildi brain was “exceptionally” well preserved. Although there are invertebrate brains dating back 500 million years, they are all flattened, said Giles, who added that this vertebrate brain is “the oldest three-dimensional fossil brain of anything we know.” .
The skull was found in layers of soapstone. According to Figueroa, a low oxygen concentration, rapid burial by fine-grained sediments, and a very compact and protective braincase played a key role in preserving the fish’s brain.
The puzzle created a chemical microenvironment around the encased brain that could have helped replace its soft tissue with a dense mineral that maintained the fine details of the brain’s 3D structures.
Giles said: “The next steps are to determine exactly how such delicate features as the brain can be preserved for hundreds of millions of years, and to search for more fossils that also preserve the brain.”
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