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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, shining a new light on the early evolution of bony fish.
The fossil of the skull belonging to the extinct Coccocephalus wildi was found in a coal mine in England more than a century ago, according to researchers of the study published in the journal Nature on Wednesday.
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 nondestructive imaging technique of computed tomography (CT) scanning to look inside its skull and examine its internal bodily structure.
Upon doing so, came a surprise. The CT image showed an “unidentified blob,” a University of Michigan press release said.
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 extending filaments that resembled cranial nerves.
“This is such an exciting and unanticipated find,” study coauthor Sam Giles, a vertebrate paleontologist and senior research fellow at the University of Birmingham, told CNN Thursday, adding that they had “no idea” 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 that it actually was a brain. Aside from being just a preservational curiosity, the anatomy of the brain in this fossil has big implications for our understanding of brain evolution in fishes,” she added.
Fills ‘important gaps’ in knowledge
C. wildi was an early ray-finned fish – possessing a backbone and fins supported by bony rods called “rays” – that is thought to have been 6 to 8 inches long, swum in an estuary, and ate small aquatic animals and aquatic insects, according to the researchers.
The brains of living ray-finned fish display structural features not seen in other vertebrates, most notably a forebrain consisting of neural tissue that folds outward, according to the study. In other vertebrates, this neural tissue folds inward.
C. wildi lacks this hallmark feature of ray-finned fish, 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 study authors.
“This indicates that the telencephalon configuration seen in living ray-finned fishes must have emerged much later than previously thought,” lead study author Rodrigo Tinoco Figueroa, a doctoral student at the University of Michigan’s Museum of Paleontology, said.
He added that “our knowledge on the evolution of the vertebrate brain is mostly restricted to what we know from living species,” but “this fossil helps us fill important gaps in the knowledge, 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, according to the researchers.
However, the study noted that C. wildi’s brain was “exceptionally” well preserved. While there are invertebrate brains up to 500 million years old that have been found, 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. Low oxygen concentration, rapid burial by fine-grained sediment, and a very compact and protective braincase played key roles in preserving the brain of the fish, according to Figueroa.
The braincase created a chemical micro-environment around the enclosed brain that could have helped to replace its soft tissue with dense mineral that maintained 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 look for more fossils that also preserve the brain.”