The CT-scanned skull of a 319-million-year-old fossilized fish discovered in an English coal mine more than a century ago has shown the earliest example of a well-preserved vertebrate brain.
The brain and cranial nerves of an extinct bluegill-sized fish are about an inch long.
According to the authors of a study sponsored by the University of Michigan, the discovery provides insight into the brain structure and early development of the primary group of fishes alive today, the ray-finned fishes.
319-million-year-old fish preserves the earliest fossilized brain
The chance discovery also sheds light on the preservation of soft tissues in backboned animal fossils.
The majority of animal fossils in museum collections are made up of hard body components like bones, teeth, and shells.
Coccocephalus wildi, an early ray-finned fish that swam in an estuary and presumably fed on tiny crustaceans, aquatic insects, and cephalopods, a group that today includes squid, octopus, and cuttlefish, had its brain CT-scanned for the current study.
Ray-finned fishes have bony rods called rays that support their backbones and fins.
When the fish died, the soft tissues of its brain and cranial nerves were replaced throughout the fossilization process with a thick material that maintained its three-dimensional structure in perfect detail.
Rodrigo Figueroa, a Ph.D. student in the Department of Earth and Environmental Sciences at the University of Michigan, is the primary author.
This visually unimpressive and little fossil not only gives us the oldest example of a fossilized vertebrate brain but also demonstrates that much of what we assumed about brain evolution based on live species alone would need to be reworked, according to Figueroa.
With the broad availability of contemporary imaging methods, Figueroa wouldn't be shocked if he discovered that fossil brains and other soft organs are far more prevalent than previously imagined.
The study team and others will examine prehistoric fish heads from a new and unusual angle.
"An key conclusion is that these sorts of soft parts can be maintained, and they may be kept in fossils that we've had for a long time - this is a fossil that's been known for over 100 years," said U-M paleontologist and Museum of Paleontology director Matt Friedman.
Because the skull fossil from England is the sole known example of its kind, the U-M-led study could only utilize nondestructive procedures.
The Coccocephalus study is part of a larger effort by Friedman, Figueroa, and colleagues to see into the skulls of early ray-finned fishes using computed tomography (CT) scanning.
The bigger study's purpose is to collect interior anatomical information that will give insights into evolutionary links.
Friedman was not searching for a brain when he studied the skull fossil of C wildi with his micro-CT scanner.
A chemical micro-environment might have played an important role in the preservation
The mystery object had several characteristics found in vertebrate brains: it was bilaterally symmetrical, it had hollow spaces similar to ventricles, and it had multiple filaments extending toward openings in the braincase, similar to cranial nerves, which travel through such canals in living species, as per Interesting Engineering.
Though preserving brain tissue in vertebrate fossils is rare, scientists have had more luck with invertebrates.
The full brain of a 310-million-year-old horseshoe crab, for example, was discovered in 2021, and scans of amber-encased insects showed brains and other organs.
Brains and other elements of the nervous system have even been discovered in flattened specimens dating back more than 500 million years.
Though sedimentary layers slowed the disintegration of soft bodily components, Figueroa believed a chemical micro-environment inside the skull's braincase may have aided in the preservation of fragile brain tissues and the replacement of them with a dense mineral, presumably pyrite.
Friedman claimed that the brain of Coccocephalus folds inward, unlike the brains of all extant ray-finned fishes.
As a result, this fossil represents a time before the distinctive feature of ray-finned fish brains originated.
This gives us some restrictions on when this characteristic arose, something we didn't have a strong hold on before the new Coccocephalus data.
Related article: Fish Fossil Found in China May Have the World's Oldest Teeth