Most fossil remains consist solely of bones, with nothing else preserved. Only a handful retain something far more delicate. More than 300 million years ago, a tiny fish no bigger than a minnow sank into the mud of a prehistoric swamp near Tourden, a village in Lancashire, northwest England. The findings of a new study have been published in the journal Proceedings of the National Academy of Sciences.
Its body settled into soft sediments between coal seams in the Burnley coalfields. What happened next was improbable. Through a combination of chemical processes, a stroke of luck, and sheer chance, the fish preserved far more than just its skeleton.
The soft tissue of its brain was also preserved. This single fact has drawn intense attention from paleontologists. The brain is ephemeral. It decays faster than almost any other tissue, making fossilization an extreme rarity.
“Preservation of soft tissues is generally uncommon in the fossil record, and structures like skin or muscles are what typically survive. Nervous tissues are preserved extremely rarely because they decompose very quickly,” explains study lead author Abigail Caron from the University of Chicago. “Therefore, the significance of this specimen lies in the fact that we can now study brain evolution using such fossils, where we previously only had bony parts or their infills.”
This creature has its own name—Trawdenia planti. It was a ray-finned fish, a group defined by slender fins supported by bony spines. This group is no minor offshoot. Ray-finned fish make up nearly 99% of the more than 30,000 fish species alive today, and roughly half of all living vertebrates.
When the team examined the fish’s skull, the preserved brain turned out not to be rattling around in an oversized cavity. It almost perfectly filled the space. This contradicts a common trend where fossilized brains appear too small for the casings that once housed them.
This snug fit carries a useful lesson. If the brain filled the case in this fish, then the inside of the case can serve as a proxy for the brain even after the soft tissues are gone.
This idea turns thousands of fossilized bony skulls into potential evidence of brain shape and size. Suddenly, the available data for studying early brains seems much more abundant. The fish that proliferated after the Devonian period have long puzzled researchers trying to link them to modern groups.
Michael Coates from the University of Chicago calls this confusion “like a bush at the base of the evolutionary tree.”
Trawdenia planti helps trim that bush a bit. Its brain possesses features pointing to similarities with modern sturgeons and paddlefish, including a portion of the cerebellum that envelops the midbrain.
Coates notes a shift in how these fish brains should be interpreted.
“By studying the shape of soft tissues, we realize that what matters isn’t their relative sizes, but how they are arranged inside the skull,” Coates said. “We may be witnessing the earliest radiation of fish that today are represented by paddlefish and sturgeons.”
The story of this fossil is almost as dramatic as the scientific account. It was first discovered in 1888. Miners in the Lancashire area collected fossils as curiosities. Geologists also sought them out, as plant fossils helped map the richest coal seams underground.
The stone nodule containing the fish was split into two halves. Both halves were sent to the Natural History Museum in London and stored there as separate specimens. For years, no one connected them. Eventually, Dr. Coates realized the two halves belonged to the same animal.
Coates began studying the fossil in the 1990s and described its skeleton in a 1999 paper. He returned to the topic using CT scanning for a 2018 publication, in collaboration with Kristen Tietjen, a scientific illustrator who then worked in his lab and now is at the University of Kansas.
Sharper images and more computationally intensive processing revealed the outer and inner membranes of nervous tissue lining the braincase. The scans also detected structures of the ventricles that once moved cerebrospinal fluid through the head. Three-dimensional models built from this data confirmed the brain had filled that cavity.
This discovery offers researchers a new perspective. As imaging techniques improve, new examples of early fish brain structures may be uncovered, even in specimens that did not preserve their soft tissues.
“Maybe we just didn’t have the technology before to look for such traits, but this kind of preservation is only possible under very special circumstances,” says Caron. “Undoubtedly, there are far more specimens with sufficiently good braincase morphology than there are those with good soft tissue preservation. So, this significantly expands the dataset that can be used to study brain evolution based on various fossil remains.”
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