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A scan of the skull of a 319-million-year-old fossil Fish has led to the discovery of the oldest example of a well-preserved vertebrate brain and shed new light on the early evolution of teleosts.
The fossil of the skull of the extinct Coccocephalus wildi was found in a coal mine in England more than a century ago, according to researchers to learn published in the journal Nature on Wednesday.
The fossil is the only known specimen of the fish species, so scientists at the University of Michigan in the US and the University of Birmingham in the UK used the non-destructive imaging technique of computed tomography (CT) to look inside its skull and examine its internal physical structure .
A surprise came. The CT image showed an “unidentified spot,” according to a press release from the University of Michigan.
The striking 3D object had a well-defined structure with features found in vertebrate brains: it was bilaterally symmetrical, contained cavities that resembled ventricles in appearance, and had extending filaments that resembled cranial nerves.
“This is such an exciting and unexpected find,” study co-author Sam Giles, a vertebrate paleontologist and senior research associate 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 sure it was actually a brain. Aside from being just a conservation curiosity, the brain anatomy in this fossil has major implications for our understanding of brain development in fish,” she added.
C. wildi was an early ray-finned fish — with a backbone and fins supported by bony rods called “rays” — thought to be 6 to 8 inches long that swam in an estuary and ate small aquatic animals and aquatic insects, the researchers said.
According to the study, the brains of living ray-finned fish have structural features not seen in other vertebrates, notably a forebrain made up of neural tissue that folds outward. In other vertebrates, this neural tissue folds inward.
C. wildi lacks this characteristic feature of ray-finned fish because the configuration of a part of its forebrain called the “telencephalon” more closely resembles that of other vertebrates such as amphibians, birds, reptiles and mammals, the study authors said.
“This indicates that the telencephalon configuration observed in living ray-finned fish must have evolved much later than previously thought,” said the study’s lead author Rodrigo Tinoco Figueroa, a graduate student at the University of Michigan Museum of Paleontology.
He added that “our knowledge of the evolution of the vertebrate brain is largely limited to what we know of living species,” but “this fossil helps us fill in important knowledge gaps that could only be gleaned from extraordinary fossils like this one.” .”
Unlike hard bones and teeth, scientists say brain tissue — which is soft — is rarely found preserved in vertebrate fossils, the researchers said.
However, the study found that the brain of C. wildi was “extraordinarily” well preserved. Though invertebrate brains as old as 500 million years have been found, they’re all flattened, said Giles, who added that this vertebrate brain “is the oldest three-dimensional fossil brain of anything known.”
The skull was found in layers of soapstone. According to Figueroa, low oxygen concentration, rapid burial through fine-grained sediment, and a very compact and protective cranium played key roles in the preservation of the fish’s brain.
The braincase created a chemical microenvironment around the enclosed brain that may have helped replace its soft tissue with dense mineral that preserved the fine details of the brain’s 3D structures.
Giles said: “The next steps are to figure out exactly how such delicate features as the brain can be preserved for hundreds of millions of years and to look for other fossils that also preserve the brain.”
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