Paleontologists don't go looking for brains, and I'm not surprised. I once got to hold a fresh brain in my hands (it was at a medical school — nothing fishy, I promise), and I can vouch that they are marvelously delicate: a custard for thinking.
When any vertebrate with a brain dies, be it human, turtle, or guppy, that fragile greasy clump of neurons is one of the first organs to vanish. Scientists must infer what ancient brains were like very often by examining the case that held it — that is, if they can find a relatively intact braincase.
In recent years, scientists have been able to get important clues about brains by scanning the brain cases. They can create virtual fossils in their computers that reveal a wealth of details.
Alan Pradel of the Museum National d'Histoire Naturelle in Paris and his colleagues recently scanned a 300-million-year old fossil of an ancient relative of sharks called Sibyrhynchus denisoni. They recognized many details of the skull. But when they looked closer, they saw something they could not quite believe. They saw something that looked like a fossilized brain.
Even without a brain, Sibyrhynchus is very interesting. It belonged to a group known as iniopterygians, whose closest living relatives are ratfish. While there are few species of ratfishes today, 300 million years ago they enjoyed a much bigger diversity. Iniopterygians were small (6 inches long) and had big eyes and pectoral fins, along with a club on their tail.
Pradel and his colleagues were pleased enough to see the braincase of Sibyrhynchus, but they were stunned to see a chunk of rock deep inside that looked like a very small fish brain (and I do mean small — its length was 7 mm, or a quarter of an inch).
Fossils sometimes form strange structures, but Pradel and his colleagues are pretty sure that they're actually seeing a brain. It has the shape of a ratfish brain, including the various sections of a ratfish brain. And it even has nerves that extend to the right places to connect to the eyes and ears.
You may be struck by how small the brain (yellow) is compared to the braincase (red). If the scientists are right, it's a cautionary tale for those who would estimate the size and shape of ancient fish brains from their braincases.
But perhaps, in the future, researchers will find more actual brains, and will be able to chart the evolution of these delicate organs in greater detail.
Source: Skull and brain of a 300-million-year-old chimaeroid fish revealed by synchrotron holotomography