Fossils have been recovered from a number of strange and surprising places, including in museum drawers and deep in today’s deserts. More than a century ago, a 319-million-year-old fossilized fish was found at the Mountain Fourfoot coal mine in Lancashire, England. It was safely kept in the Manchester Museum and scientists are still learning from it 125 years later.
A CT scan of the fossil has revealed that it contains the oldest example of a well-preserved vertebrate brain. The findings are documented in a study published in the journal on February 1 Nature. The brain and cranial nerves are about a centimeter long and are extinct Coccocephalus wildi (C. wildi). This was an early ray-finned fish that likely ate small crustaceans, cephalopods, and aquatic insects while swimming around estuaries. Backbones and fins supported by bony rods called rays are a feature of all ray-finned fish.
[Related: One wormy Triassic fossil could fill a hole in the evolutionary story of amphibians.]
Surprisingly, Friedman wasn’t looking for a brain when examining it C. wildi skull fossil.
“I scanned it, then I loaded the data into the software we use to visualize these scans, and found that there was an unusual, unique object inside the skull,” said the co-author and University of UC paleontologist Michigan (UM) Matt Friedman in a statement.
The unusual, unidentified spot was brighter on the CT image, meaning it was likely denser than skull bone or rock surrounding the fossil. It also displayed several features common to vertebrate brains, including bilateral symmetry, cavities resembling ventricles, and multiple filaments.
These softer parts of vertebrate fossils are preserved through a unique fossilization process that replaced the soft tissues that made up the brain and cranial nerves of fish with a dense mineral that preserved the three-dimensional structure in remarkable detail.
The skull fossil is the only known one C. wildi specimen of its kind, so the team could only use non-destructive techniques to study it. The team used CT scans to look inside these early ray-finned fish skulls to learn more about their anatomy and infer evolutionary relationships.
“Given the widespread availability of modern imaging techniques, I wouldn’t be surprised if we found that fossil brains and other soft tissues are much more common than we previously thought. From now on, our research group and others will look at fossil fish heads from a new and different perspective,” said co-author and UM graduate student Rodrigo Figueroa in a statement.
[Related: Spinosaurus bones hint that the spiny dinosaurs enjoyed water sports.]
Specimens of early ray-finned fish such as Coccocephalus can fill in the gaps about the early evolutionary stages of the approximately 30,000 ray-finned fish species alive today. According to the authors, the brain structure of Coccocephalus shows that there is a more complicated pattern of fish brain evolution than suggested by what is found in living species alone.
“This superficially nondescript and small fossil not only shows us the oldest example of a fossilized vertebrate brain, but it also shows that much of what we thought about the evolution of brains from living species alone needs to be revised,” Figueroa said.
The results also highlight why it’s important to store samples and clear out junk drawers from time to time.
“Here we have found a remarkable preservation in a fossil that has been examined multiple times by multiple people over the past century,” Friedman said. “That’s why it’s so important to capture the physical rehearsals. Because who knows what people might do with the fossils in our collections 100 years from now.”