Scientists studying a prehistoric fish that once lived in what is now Kansas and Oklahoma have uncovered a 300-million-year-old fossilized brain.

"Fossilized brains are unusual, and this is by far the oldest known example," said John Maisey, curator in the division of paleontology at the American Museum of Natural History in New York.

"Soft tissue has fossilized in the past, but it is usually muscle and organs like kidneys," Maisey said in a statement.

Maisey and co-authors report in the online edition of Proceedings of the National Academy of Science that the brain was discovered in Kansas in a fossilized iniopterygian, which they had sent for scanning at the European Synchrotron Radiation Facility in Grenoble, France.

Iniopterygians are extinct relatives of modern ratfishes, also known as ghost sharks.

The scan found a fossilized blob inside the braincase and closer study revealed it was the fossilized brain of the ancient creature.

"Now that we know that brains might be preserved in such ancient fossils, we can start looking for others. We are limited in information about early vertebrate brains, and the evolution of the brain lies at the core of vertebrate history," Maisey said.

His co-authors included Alan Pradel of the National Museum of Natural History in Paris and Paul Tafforeau at the European Synchrotron Radiation Facility.