UM scientists discover 319 million-year-old fish brain. It could be used to unlock evolution secrets

Carol Thompson
The Detroit News

An ancient, well-preserved fish brain analyzed recently by University of Michigan scientists could unlock new discoveries about evolution and the formation of fossils.

The brain, which the university describes as the oldest "well-preserved vertebrate brain" on record, was an unexpected discovery. It is contained within the skull of a fossilized fish that was discovered more than 100 years ago in a coal mine in England.

The fossilized fish is old — 319 million years old. It's the only specimen of its kind, Coccocephalus wildi, a ray-finned fish that is the predecessor to most living fish species, a diverse group of 30,000 fish species that includes tuna, herring and gar.

UM paleontologists were scanning the fish as part of a study using computed tomography, or CT scanning, to learn more about the evolution of early ray-finned fishes without cracking the fossils open to see what's inside. They hope understanding the old fishes' anatomy will teach them about the ways they changed over millennia, said Rodrigo Figueroa, a UM doctoral student conducting the research.

This is 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.

They knew the scan would show bones and scales. They didn't expect a brain. Soft tissue tends to decompose over millions of years.

Not this time. The original tissue was replaced with a dense mineral during the fossilization process, and that mineral preserved the brain's three-dimensional structure.

It isn't perfectly clear why the fish's brain was so marvelously preserved, Figueroa said. The fish's body fossilized in fine sediment, which preserves things with greater detail. It also fossilized in a low-oxygen environment, which warded off microbes and prevented normal decomposition.

It was an amazing find, Figueroa said. Almost unthinkable.

"It's hard to believe at first," he said. "You're like, 'Alright, now am I fooling myself here?' But then you start putting the pieces together: It's inside the brain case. It's where the brain should be. It's symmetrical. It has the morphological features.

"Then, you're like 'Alright, I'm not fooling myself. It is right there.'"

Matt Friedman, University of Michigan Museum of Paleontology director and Figueroa's adviser, first noticed the crab-shaped brain.

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

Figueroa, Friedman and other researchers from UM's paleontology museum, London's Natural History Museum and the University of Chicago published a study about the brain in the journal Nature on Wednesday.

The team realized scientists appear to have misunderstood the evolution of fish. The 319-million-year-old brain was not what they expected, suggesting evolution took a more complicated path to reach the structures of modern fish brains.

The brain's tissues folded inward, not outward like modern ray-finned fishes. That makes the ancient fish more similar to sturgeon or paddlefish than its other ancestors.

"The fossil brain shows different characteristics than we would expect to find, considering the group it comes from," Figueroa said. "It's changing considerably the framing that we have for the evolution of the ray-finned fish brain."

To learn more about the evolution of modern fish, Figueroa is going to scan more fossils to look for brains hiding within them. Then, he'll study the brains to see how they varied and changed over time.

"Soft tissue preserved in the fossil record might be more common than we imagined," he said. "They can be preserved and be informative, not only cool. They actually tell us something."