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A scan of the skull of a 319-million-year-old fossilized fish has led to the discovery of the oldest example of a well-preserved vertebrate brain, shedding new light on the early evolution of bony fish.
The fossil of the skull of the extinct Coccocephalus wildi was found more than a century ago in a coal mine in England, according to researchers of the study published Wednesday in the journal Nature.
The fossil is the only known specimen of the fish species, so scientists from 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) scanning to look inside its skull and determine its to examine the interior. bodily structure.
When he did, a surprise came. The CT image showed an “unidentified blob,” according to a press release from the University of Michigan.
The separate 3D object had a well-defined structure with features found in the vertebrate brain: it was bilaterally symmetrical, contained cavities resembling ventricles in appearance, and had extending filaments resembling cranial nerves.
“This is such an exciting and unexpected find,” study co-author Sam Giles, a vertebrate paleontologist and senior research fellow at the University of Birmingham, told CNN on Thursday, adding that they had “no idea” there was a brain when they decided to study the skull.
“It was so unexpected that it took us a while to be sure it was really a brain. The anatomy of the brain in this fossil is not only a curiosity from a conservation point of view, but also has major implications for our understanding of the evolution of the brain in fish,” she added.
C. wildi was an early ray-finned fish — with a spine and fins supported by bony rods called “rays” — believed to have been 6 to 8 inches (15 to 20 cm) long, swam in an estuary and ate small aquatic animals and aquatic insects, according to the researchers.
The brains of live ray-finned fish display structural features not found in other vertebrates, most notably a forebrain composed of neural tissue that folds outward, according to the study. In other vertebrates, this nerve tissue folds inward.
C. wildi lacks this distinctive feature of ray-finned fishes, with 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 authors of the research .
“This indicates that the telencephalon configuration observed in living ray-finned fishes must have evolved much later than previously thought,” said lead study author Rodrigo Tinoco Figueroa, a doctoral student at the University of Michigan Museum of Paleontology.
He added that “our knowledge of vertebrate brain evolution is largely limited to what we know of living species,” but “this fossil helps us fill important knowledge gaps, which could only be obtained from exceptional fossils like this one.”
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. While invertebrate brains have been found that date back up to 500 million years, they’re all flattened, said Giles, who added that this vertebrate brain is “the oldest three-dimensional fossil brain of anything we know of.”
The skull was found in layers of soapstone. Low oxygen concentration, rapid burial by fine-grained sediment and a very compact and protective braincase played a key role in preserving the fish’s brains, according to Figueroa.
The braincase created a chemical microenvironment around the enclosed brain that could have helped replace the 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 how such delicate features as the brain can be preserved for hundreds of millions of years, and find more fossils that also preserve the brain.”
