Scientists revealed the oldest example of a well-preserved vertebrate brain

The discovery opens a window into neural anatomy.

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Brain anatomy provides key evidence for the relationships between ray-finned fishes. However, two major limitations limit our understanding of neuroanatomical evolution in this major vertebrate group. First, the deepest branching living lineages are separated from the group’s common ancestor by hundreds of millions of years, indicating that aspects of their brain morphology are specialized relative to primitive conditions. Second, there are no direct constraints on brain morphology in the earliest ray-finned fishes beyond the coarse picture provided by cranial endocasts: natural or virtual infillings of void spaces within the skull.

In a new study, scientists from the University of Michigan reported brain and cranial nerve soft-tissue preservation in Coccocephalus wildi, an approximately 319-million-year-old ray-finned fish. Scientists performed CT scans on the skull of fossilized fish. The results revealed the oldest example of a well-preserved vertebrate brain.

An extinct bluegill-sized fish’s brain and cranial nerves measure around an inch in length. The finding provides new insights into the early evolution and neuronal architecture of the major group of fishes that are still alive today, the ray-finned fishes.

Coccocephalus wildi, an early ray-finned fish that swam in an estuary and presumably dined on small crustaceans, aquatic insects, and cephalopods, a group that currently includes squid, octopuses, and cuttlefish, is the owner of the CT-scanned brain that was examined for the new study. Fish with ray fins have bony rods called rays supporting their backbones and fins.

fossilized skull of Coccocephalus wildi
The fossilized skull of Coccocephalus wildi, an early ray-finned fish that swam in an estuary 319 million years ago. The fish is facing to the right, with the jaws visible in the lower right portion of the fossil. The eye socket is the circular, bumpy feature above the jaws. This fish would have been 6 to 8 inches long, about the size of a bluegill. Photo credit: Jeremy Marble, University of Michigan News.

When the fish died, the soft tissues of its brain and cranial nerves were replaced by a thick mineral during the process of fossilization, which retained its three-dimensional structure in perfect detail.

U-M paleontologist Matt Friedman, a senior author of the new study and director of the Museum of Paleontology, said, “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 doctoral student Rodrigo Figueroa said, “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.”

“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.”

 Coccocephalus wildi skull
CT scan of the Coccocephalus wildi skull, showing the fossilized brain and associated structures. The brain and cranial nerves appear bright white in the center of the images. The fish is facing to the left. The eye sockets are the large, black oval areas surrounded by bone, and the jaws are below the eye sockets. Image credit: Figueroa et al. in Nature, February 2023.

During the study, scientists used only nondestructive techniques as the skull fossil is the only known specimen of its species.

Scientists used computed tomography (CT) scanning to look inside the skulls of early ray-finned fishes. The larger study aims to obtain internal anatomical details that provide insights about evolutionary relationships.

In the case of C. wildi, scientists were not looking for a brain when they fired up a micro-CT scanner and examined the skull fossil. They scanned it and then loaded the data into the software. The software visualized these scans and noticed an unusual, distinct object inside the skull.

The unexplained blob appeared brighter on the CT scan and was most likely denser than the skull’s bones or the nearby rock.

Friedman said, “It is common to see amorphous mineral growths in fossils, but this object had a clearly defined structure.”

The unidentified object exhibited several characteristics common to vertebrate brains, including bilateral symmetry, hollow regions resembling ventricles, and numerous filaments extending towards openings in the braincase that resembled the cranial nerves that pass through such canals in living species.

Friedman said, “It had all these features, and I said to myself, ‘Is this 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.”

There are over 30,000 species of ray-finned fish, making up about half of all vertebrate species. The remaining half comprises land vertebrates, such as birds, mammals, reptiles, and amphibians, and less diversified fish species, like cartilaginous and jawless fish.

The brain of Coccocephalus has three main regions that roughly correspond to the forebrain, midbrain, and hindbrain in living fish, according to a thorough analysis of the fossil and comparisons to modern-fish specimens from the U-M Museum of Zoology collection. The central body of the brain is the size of a raisin and has a raisin-like shape.

Both sides of the central body send out cranial nerves. When combined, the main body and the cranial nerves resemble a miniature crustacean with outstretched arms, legs, and claws, like a lobster or a crab.

Scientists noted“Notably, the brain structure of Coccocephalus indicates a more complicated pattern of fish-brain evolution than is suggested by living species alone.”

“These features give the real fossil value in understanding patterns of brain evolution, rather than simply being a curiosity of unexpected preservation.”

“Unlike all living ray-finned fishes, the brain of Coccocephalus folds inward, So this fossil is capturing a time before that signature feature of ray-finned fish brains evolved. This gives us some constraints on when this trait evolved—something we did not have a good handle on before the new data on Coccocephalus.”

“Here, we’ve found remarkable preservation in a fossil examined several times before by multiple people over the past century. But because we have these new tools for looking inside 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.”

Journal Reference:

  1. Figueroa, R.T., Goodvin, D., Kolmann, M.A. et al. Exceptional fossil preservation and evolution of the ray-finned fish brain. Nature (2023). DOI: 10.1038/s41586-022-05666-1