Though it is rare, soft tissue does fossilize under the right conditions. However brain tissue rarely does due to its high water content. As such, researchers have to rely on the shape of the brain case of well preserved specimens. But recently, a preserved brain was discovered within the fossil skull of a fish known as an Iniopterygian, specifically, a member of the genus Sibyrhynchus. These unusual cartilaginous fish were most closely related to the modern Chimeras and lived from the Devonian to the Carboniferous. They had large, dorsally situated pectoral fins that were thought to be used to 'fly' through the water as well as possibly in mating. This discovery marks the oldest known fossil of any brain, or as I like to call it, the oldest bit of zombie rock candy.
The fossil skull was found in Kansas in pristine shape. Many have already noted the irony of finding such an important fossil, especially considering that it is in the form of a brain, within a state with some of the highest percentages of creationists in the country.
Many fossils become flattened due to the pressures of the rock above, but in some cases, non-compressed fossils are found. In these instances, the samples are sent off for imaging to determine the internal structures of the specimen and in hopes of finding some hidden goodies. Upon being taken to European Synchrotron Radiation Facility (ESRF) a type of industrial grade CT scan, known as x-ray synchrotron microtomography was performed. This technique revealed a denser structure within the brain case, prompting to researchers to try a newer technique designed to reveal finer structures as well as put the specimen in a flashy 3-D view. This technique, know as X-ray holotomography revealed the fossilized brain in all its glory.
With this discovery, many are wondering if fossil brains could be more common but have been overlooked due to imprecise imaging techniques. If this is so, a more detailed understanding of the evolution of the brain may be possible.