Brain anatomy provides important evidence for the relationships among ray-finned fishes. However, two major limitations limit our understanding of neuroanatomical evolution in this large vertebrate group. First, the most deeply 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 gross picture provided by cranial endocasts: natural or virtual fillings of empty spaces in the skull.

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

The brain and cranial nerves of an extinct bluegill-sized fish are about an inch long. The finding offers new insights into the early evolution and neuronal architecture of the most important group of fish 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, owns the CT-scanned brain that was examined for the new study. Ray-finned fish have bony rods called rays that support their spine 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 faces right, with the jaws visible in the lower right portion of the fossil. The eye socket is the round, bumpy feature above the jaws. This fish would have been six to eight 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 with a thick mineral during the fossilization process, which preserved its three-dimensional structure down to the finest detail.

UM paleontologist Matt Friedman, a senior author of the new study and director of the Museum of Paleontology, said: “An important takeaway is that these kinds of soft parts can be preserved, and they can be preserved in fossils that we’ve had for a long time — this is a fossil that’s been known for over 100 years.”

UM PhD candidate Rodrigo Figueroa said: “This superficially unimpressive 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 the brains of only living species needs to be reworked.”

“With the widespread availability of modern imaging techniques, I wouldn’t be surprised if we find that fossil brains and other soft parts are much more common than we previously thought. Our research group and others are now looking at fossil fish heads with a new and different perspective.”

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

During the investigation, scientists used only non-destructive techniques, as the cranial fossil is the only known specimen of its kind.

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

In the case of C. wildi, scientists weren’t looking for a brain when they activated 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 in the skull.

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

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

The unidentified object showed several features common to vertebrate brains, including bilateral symmetry, hollow areas resembling ventricles, and numerous filaments extending into openings in the braincase that resembled the cranial nerves that pass through such canals in living animals. species go.

Friedman said, “It had all these features and I said to myself, ‘Is this a brain I’m looking at? So I zoomed in on that part of the skull to do a second scan at a higher resolution, and it was very clear that that was exactly what it needed to be, and precisely because this was such an unambiguous example, we decided to move forward with it.”

There are more than 30,000 species of ray fin whales, which make up about half of all vertebrate species. The other half consists of terrestrial vertebrates, such as birds, mammals, reptiles and amphibians, and less diversified fish species, such as 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 with modern fish specimens from the UM 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 cranial nerves resemble a miniature crustacean with outstretched arms, legs, and claws, like a lobster or crab.

Scientists noted, “In particular, the brain structure of Coccocephalus suggests a more complicated pattern of fish brain evolution than 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 fish, the brain of Coccocephalus folds inward, so this fossil records a time before that signature feature of the brain of ray-finned fish evolved. This gives us some constraints on when this trait evolved – something we didn’t have a good handle on before the new data on Coccocephalus.”

“Here we found remarkable preservation in a fossil that has been explored several times before by multiple people over the past century. But because we have these new tools to look inside fossils, it reveals another layer of information to us.”

“That’s why sticking to the physical ones is so important. Because who knows what people will be able to do with the fossils in our collections in 100 years.”

Magazine reference:

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