A 300 million-year-old fossilized fish brain was discovered during a routine computed tomography (CT) scan, according to a study published today in the Proceedings of the National Academy of Sciences. Until now, scientists assumed that brains rarely—if ever—turned into fossils. Other soft tissue fossils, such as muscles and kidneys, have been found that date back longer than 350 million years ago, but because the brain is delicate and consists mostly of water, it's much less likely to be preserved in fossil form, says study co-author John Maisey, a curator in the paleontology division of the American Museum of Natural History in New York.  But "It's more than just a curiosity," he says. "Modern technology has revealed a fossil that we really didn't know about before." High-powered scans using x-ray synchrotron microtomography (which, like a CT, uses x-rays to image cross-sections of an object) allowed scientists to peer into the rock-solid skull to see the 0.06-by-0.28-inch (1.5 by 7 mm) brain.

The fossil was from an iniopterygian, an ancient extinct fish that is a relative of sharks, rays and ratfish. What surprised researchers even further is that it showed a brain similar to that of modern-day shark.

In the fossilization process, the brain itself was replaced with hard minerals, which preserved the shape of the original organ, and the rest of the cavity was filled with sediment, Maisey says. He notes that researchers found several fossilized craniums, each resembling a little "broken bowl of rock," in rock from the Upper Carboniferous period in Kansas and Oklahoma. But only one has yielded a preserved brain structure.

"It's quite possible that brain fossils are actually more common, and we simply haven't been able to find them," says Maisey, who noted that researchers may now try to check out other fossilized skulls with the high-tech scanners to see if they contain mineralized brains. Of course, this finding also means that paleontologists  may have to  stretch their own brains a bit to include things other than bones. "Now we have to learn new things about brains," Maisey joked, "that we didn't have to bother with [before]."

Image of fossilized brain courtesy of A. Pradel