319-million-year-old fish skull holds well-preserved vertebrate brain

The accidental discovery revealed the oldest brain found in any vertebrate.
Deena Theresa
Artist's interpretation of a remarkable 319-million-year-old fish that preserves the earliest fossilized brain of a backboned animal.
Artist's interpretation of a remarkable 319-million-year-old fish that preserves the earliest fossilized brain of a backboned animal.

Márcio L. Castro 

More than a century ago, a CT-scanned skull of a 319-million-year-old fossilized fish was found on the roof of the Mountain Fourfoot coal mine in Lancashire, England. It was first scientifically described in 1925. Today, scientists uncovered the oldest example of a well-preserved vertebrate brain from the fish. 

The discovery was accidental; University of Michigan paleontologist Matt Friedman, a senior author of the new study and director of the Museum of Paleontology, was not looking for a brain when he readied his micro-CT scanner and examined the skull fossil.

"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

That distinct object was brighter and denser than the bones of the skull or the surrounding rock. "It is common to see amorphous mineral growths in fossils, but this object had a clearly defined structure," Friedman said.

The object was found to share features with vertebrate brains. 

"It had all these features, and I said to myself, 'Is this really a brain that I'm looking at?'" Friedman said. "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."

319-million-year-old fish skull holds well-preserved vertebrate brain
The fossilized skull of Coccocephalus wildi, an early ray-finned fish that swam in an estuary 319 million years ago.

Intact soft parts from a fossil known for over 100 years

The CT-scanned brain analyzed is that of Coccocephalus wildi, an early ray-finned fish that swam in an estuary and likely dined on small crustaceans, aquatic insects, and cephalopods, a group that today includes squid, octopuses, and cuttlefish. Ray-finned fishes have backbones and fins supported by bony rods called rays.

According to the researchers, the brain and its cranial nerves are an inch long and belong to an extinct bluegill-size fish. The discovery sheds light on the neural anatomy and early evolution of ray-finned fishes.

Post-death, the soft tissues of the fish's brain and cranial nerves were replaced with a dense mineral that preserved their 3D structure.

"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," said Friedman.

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The revelation was made possible thanks to modern imaging techniques. The research team used computed tomography (CT) scanning to look inside the skulls of early ray-finned fishes. And as the skull fossil from England is the only known specimen of its species, only nondestructive techniques could be used during the study.

A chemical micro-environment could have been key in the preservation

Though sedimental deposits have slowed down the decomposition of soft body parts, it is likely a chemical micro-environment inside the skull's braincase may have helped to preserve the delicate brain tissues and to replace them with a dense mineral, possibly pyrite, Figueroa said.

"Unlike all living ray-finned fishes, the brain of Coccocephalus folds inward," Friedman said. "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."

These details permitted the scientists to understand that this prehistoric forebrain developed was more like ours in comparison to the rest of the living ray-finned fishes alive today.

 According to the researchers, the discovery highlights the importance of preserving physical specimens.

"Here we've found remarkable preservation in a fossil examined several times before by multiple people over the past century," Friedman said. "But because we have these new tools for looking inside fossils, it reveals another layer of information to us.

The study is published in Nature.

Study Abstract:

Brain anatomy provides key evidence for the relationships between ray-finned fishes, but two major limitations obscure 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, with indications that aspects of their brain morphology—like other aspects of their anatomy—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. Here we report brain and cranial nerve soft-tissue preservation in Coccocephalus wildi, an approximately 319-million-year-old ray-finned fish. This example of a well-preserved vertebrate brain provides a window into neural anatomy deep within ray-finned fish phylogeny. Coccocephalus indicates a more complicated pattern of brain evolution than suggested by living species alone, highlighting cladistian apomorphiesand providing temporal constraints on the origin of traits uniting all extant ray-finned fishes. Our findings, along with a growing set of studies in other animal groups point to the importance of ancient soft tissue preservation in understanding the deep evolutionary assembly of major anatomical systems outside of the narrow subset of skeletal tissue

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