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A scan of a 319 million year old fossilized skull fish led to the discovery of the oldest known example of a well-preserved vertebrate brain, shedding new light on the early evolution of bony fishes.
A fossilized skull of the extinct Coccocephalus wildi was found in a coal mine in England more than a century ago, according to researchers. to learn was published Wednesday in the journal Nature.
The fossil is the only known example of the fish species, so scientists from the University of Michigan in the US and the University of Birmingham in the UK used the destructive imaging technique of computed tomography (CT) scanning to look inside the skull and examine its internal organs. body structure.
After doing that, a surprise came. A University of Michigan press release said the CT scan showed an “unidentified spot.”
The distinct, 3D object had a well-defined structure with features found in vertebrate brains: It was bilaterally symmetrical, with hollow spaces similar to ventricles and elongated filaments resembling cranial nerves.
“This is a very interesting and unexpected finding,” study co-author Sam Giles, a vertebrate paleontologist and senior researcher at the University of Birmingham, told CNN on Thursday. studying the skull.
“It was so unexpected that it took us a while to be sure it was actually a brain. “The anatomy of the brain in this fossil is not only of conservation interest, but has major implications for our understanding of brain evolution in fish.”
C. wildi was an early ray-finned fish — with a spine and fins supported by bony rods called “rays” — thought to be 6 to 8 inches long, swimming in estuaries, feeding on small aquatic animals and aquatic insects. , according to researchers.
According to the study, the brains of living ray-finned fishes have structural features not seen in other vertebrates, notably a forebrain composed of neural tissue that folds outward. In other vertebrates, this nerve tissue folds inward.
According to the study authors, C. wildi lacks this characteristic of ray-finned fishes, the configuration of the part of its forebrain called the telencephalon being more similar to that of other vertebrates such as amphibians, birds, reptiles and mammals. .
“This suggests that the telencephalon configuration seen in living ray-finned fishes emerged much later than previously thought,” said Rodrigo Tinoco Figueroa, a doctoral student at the University of Michigan’s Museum of Paleontology.
He added that “our knowledge of vertebrate brain evolution is largely limited to what we know from living species,” but “this fossil helps us fill important gaps in knowledge that can only be obtained from exceptional fossils like this one.”
Unlike hard bones and teeth, scientists rarely find soft brain tissue in vertebrate fossils, the researchers said.
However, the study noted that C. Wildi’s brain was “exceptionally” well preserved. Although invertebrate brains have been found up to 500 million years ago, they are all flattened, Giles said, adding that this vertebrate brain is “the oldest three-dimensional fossil brain of anything we know of.”
The skull was found in layers of soapstone. According to Figueroa, low oxygen concentrations, rapid burial by fine sediment, and a very compact and protective cerebral cortex played a key role in protecting the fish’s brain.
The brain box created a chemical micro-environment around the enclosed brain that could help replace its soft tissue with a dense mineral that holds the fine details of the brain’s 3D structures.
Giles said: “The next steps are to determine exactly how delicate features like the brain can be preserved over hundreds of millions of years, and to look for more fossils that also preserve the brain.”
