The structure of the skull (foreground) of a 300-million-year-old iniopterygian fish from Kansas remotely related to living ratfish is elucidated thanks to holotomography, a technique based on synchrotron X-ray phase contrast imaging (background), and yields the first hint at an exceptional mineralization of the brain (orange). (Credit: PNAS/Philippe Janvier (CNRS, Museum National d'Histoire Naturelle))
I think this must be the 300 million year old fish brain that was mentioned on Tuesday. Although it is the first time that the soft tissue of such an old fossil brain has ever been found it sounds like it is also one of the first times people have used sophisticated imaging techniques to look inside the fossil skull.
(S)cientists used the technique of absorption microtomography to study different samples. One sample, stemming from Kansas (US), revealed a peculiar structure: it was denser than the surrounding matrix that fills the braincase, and which is made of crystalline calcite. In order to elucidate its structure in detail, they decided to use a second technique, X-ray holotomography. Surprisingly, the results showed a symmetrical and elongated object placed in the same position as a brain would have been.