World’s oldest vertebrate brain found in 319-million-year-old fossil
Most of what we know about ancient extinct animals comes from their bones, since soft tissues don’t usually fossilize well. But now, scientists have discovered the oldest preserved vertebrate brain, in a fossilized fish almost 320 million years old.
After an animal dies, its flesh and organs usually disappear very quickly, thanks to scavengers or decomposition. That leaves just the bones, which in some cases can fossilize to become the museum exhibits we’re familiar with. But if a carcass avoids exposure to the elements, such as being buried quickly or getting encased in materials like amber, soft tissues like skin or feathers can survive to the present day.
Now, scientists have discovered the oldest known fossilized brain in a vertebrate animal. The record belongs to a 319-million-year-old fish known as Coccocephalus wildi, an early ancestor of ray-finned fishes, which make up the biggest group of modern vertebrates. The previous record-holder was a shark dating back 300 million years, but other notable fossil brains include a 310-million-year-old horseshoe crab and a 133-million-year-old dinosaur. It’s got nothing on the oldest known heart though, which was found in a 380-million-year-old fish fossil.
The fossil of C. wildi isn’t a new discovery however – it was dug out of a coal mine almost a century ago. But in the new study, researchers conducted a CT scan of the specimen to examine its insides without damaging it, and spotted a bright blob in the skull, indicating a denser mineral, perhaps pyrite. This blob looked suspiciously brain-like, the team said: it was symmetrical down the middle, had hollow spaces that resembled ventricles, and filaments that seemed to be cranial nerves.
“It had all these features, and I said to myself, ‘Is this really a brain that I’m looking at?’” Said Matt Friedman, senior author of the study. “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.”
The team says the pristine preservation likely occurred because the fish was buried beneath sediments very quickly after it died, with little oxygen present. A chemical micro-environment that was conducive to fossilizing soft tissues also seemed to have formed inside its skull.
The discovery can also fill in some blanks in the story of the evolution of fish. According to the researchers, the brain of C. wildi is most similar to that of sturgeons and paddlefish, which are considered primitive because they diverged from other ray-finned fishes over 300 million years ago.
“Unlike all living ray-finned fishes, the brain of Coccocephalus folds inward,” said Friedman. “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.”
The team believes that this kind of brain preservation occurs more often than previously thought. In future work they plan to start looking for them.
New Atlas, 5 February 2023