This is a difficult question to answer, not least because brains do not fossilise.
From the newscientist.com
How plants and animals live, die and fossilise is the basis of stratigraphy.
From the newscientist.com
And digging sticks, used to unearth tubers and bulbs, do not fossilise.
From the newscientist.com
A large mammal ancestor might simply have failed to fossilise.
From the newscientist.com
Unlike my more recent ancestors these earliest remains had no skeletons so did not fossilise well.
From the newscientist.com
Tendons do not fossilise well, but they leave traces called Sharpey's fibers where they attach to the bone.
From the newscientist.com
The brain itself does not fossilise, but the inside of the cranium retains an impression of its contours.
From the economist.com
They fossilise an industrial structure that needs to change.
From the economist.com
It is an EU rule that unintended consequences, left for a few years, fossilise into special interests.
From the economist.com
More examples
Fossilize: convert to a fossil; "The little animals fossilized and are now embedded in the limestone"
Fossilize: become mentally inflexible
(fossilised) fossilized: set in a rigidly conventional pattern of behavior, habits, or beliefs; "obsolete fossilized ways"; "an ossified bureaucratic system"
(fossilisation) fossilization: the process of fossilizing a plant or animal that existed in some earlier age; the process of being turned to stone
Fossils (from Latin fossus, literally "having been dug up") are the preserved remains or traces of animals, plants, and other organisms from the remote past. ...
Alternative spelling of fossilize
(Fossilised) Objects which have been turned to stone, sometimes used in jewellery e.g petrified wood
(fossilised) [semantics,pragmatics] A term referring to any phrase or expression which is non-productive and which does not vary in its form. This applies for example to idioms like topsy turvy where the words only exist in this combination.