ANN ARBOR – Modern imaging technologies have given researchers at the University of Michigan insight into a previously held belief: soft tissue cannot be fossilized.
A new CT scan of the skull of a 319-million-year-old fish revealed soft parts of vertebrates can, in fact, be fossilized. The research team said the fish is the oldest known example of a vertebrate brain that’s been well preserved.
The fish is an extinct species that’s the size of a bluegill. Its brain and cranial nerves are approximately one inch long, and the imaging has provided researchers with insights into the early evolution and neural anatomy of ray-finned fishes, a major group of fishes that exist today.
When the fish died, its soft brain tissue and cranial nerves were replaced with a dense mineral during the fossilization process that preserved its three-dimensional form in remarkable detail.
“An important conclusion is that these kinds of soft parts can be preserved, and they may be preserved in fossils that we’ve had for a long time—this is a fossil that’s been known for over 100 years,” U-M paleontologist, director of the Museum of Paleontology and senior author of the new study Matt Friedman said in a statement.
The work was part of U-M doctoral student Rodrigo Figueroa’s dissertation. The lead author studies under Friedman in the Department of Earth and Environmental Sciences
“Not only does this superficially unimpressive and small fossil show us the oldest example of a fossilized vertebrate brain, but it also shows that much of what we thought about brain evolution from living species alone will need reworking,” Figueroa said in a statement.
“With the widespread availability of modern imaging techniques, I would not be surprised if we find that fossil brains and other soft parts are much more common than we previously thought. From now on, our research group and others will look at fossil fish heads with a new and different perspective.”
The research team determined the brain belongs to an early ray-finned fish called the Coccocephalus wildi, which swam in estuaries and ate aquatic insects, small crustaceans and cephalopods, a group whose modern descendants includes cuttlefish, octopuses and squid.
The fossil was found in a coal mine in England more than a century ago and is currently on loan from the Manchester Museum.
Friedman, who with Figueroa and other colleagues scans skulls of early ray-finned fishes, said he did not expect to find a brain when he went to image the skull fossil with his micro-CT scanner.
“I scanned it, then I loaded the data into the software we use to visualize these scans and noticed that there was an unusual, distinct object inside the skull,” he said in a statement. “It is common to see amorphous mineral growths in fossils, but this object had a clearly defined structure.
“It had all these features, and I said to myself, ‘Is this really a brain that I’m looking at?’ So I zoomed in on that region of the skull to make a second, higher-resolution scan, and it was very clear that that’s exactly what it had to be. And it was only because this was such an unambiguous example that we decided to take it further.”
Scientists suspect the fish was quickly buried in sediments after it died. In an environment with such little oxygen, the decomposition of soft body parts can be slowed, they said. Figueroa added that the skull’s braincase with a chemical micro-environment could have replaced the animal’s brain tissues with a dense mineral like pyrite, helping to preserve it further.
“There seems to be, inside this tightly enclosed void in the skull, a little micro-environment that is conducive to the replacement of those soft parts with some kind of mineral phase, capturing the shape of tissues that would otherwise simply decay away,” Friedman said in a statement.
The findings gave the scientists insights into brain evolution in ray-finned fishes.
“Unlike all living ray-finned fishes, the brain of Coccocephalus folds inward,” Friedman said in a statement. “So, this fossil is capturing a time before that signature feature of ray-finned fish brains evolved. This provides us with some constraints on when this trait evolved—something that we did not have a good handle on before the new data on Coccocephalus.”
The research team said the finding emphasizes why preserving specimens is so important.
“Here we’ve found remarkable preservation in a fossil examined several times before by multiple people over the past century,” Friedman said in a statement. “But because we have these new tools for looking inside of fossils, it reveals another layer of information to us.
“That’s why holding onto the physical specimens is so important. Because who knows, in 100 years, what people might be able to do with the fossils in our collections now.”
Other authors include Michael Coates and Abigail Caron of the University of Chicago; Sam Giles of London’s Natural History Museum and the University of Birmingham; Danielle Goodvin and Matthew Kolmann of the U-M Museum of Paleontology.
To read the full study, published in Nature, click here.