The "oldest example of a well-preserved vertebrate brain" has been identified in a 319 million-year-old fish fossil.
The fossilised Coccocephalus wildi was found in a coal mine in Lancashire more than a century ago and had been sitting in the archives of Manchester Museum.
New research by teams from universities in Birmingham and Michigan its skull contained a brain and cranial nerves.
Palaeontologist Dr Sam Giles said the "unexpected find" gave a "startling insight" into brain evolution.
The fossil of the now-extinct fish was originally recovered from a layer of soapstone in the roof of the Lancashire mine and was first scientifically described in 1925.
Though only its skull was recovered, scientists from the University of Birmingham (UoB) and the University of Michigan (UoM) believe it would have been 6 to 8ins (15 to 20cm) long and was probably a carnivore.
A UoB representative said soft tissues like brains normally decay quickly and "very rarely fossilise", but when this fish died, it was "probably quickly buried in sediments with little oxygen present" as such environments "can slow the decomposition of soft body parts".
They said the skull fossil was the only known specimen of its species, so only non-destructive techniques were used during the study.
They added that the research team had not been looking for a brain when examining the skull fossil, but found the distinct object which had features found in vertebrate brains, such as bilateral symmetry and hollow spaces.
Dr Giles said the "unexpected find of a three-dimensionally preserved vertebrate brain gives us a startling insight into the neural anatomy of ray-finned fish".
"It tells us a more complicated pattern of brain evolution than suggested by living species alone, allowing us to better define how and when present day bony fishes evolved," she added.
UoM researcher Rodrigo Figueroa said the "superficially unimpressive and small fossil [not only] shows us the oldest example of a fossilised vertebrate brain, but it also shows that much of what we thought about brain evolution from living species alone will need reworking".
Why not follow BBC North West on Facebook, Twitter and Instagram? You can also send story ideas to firstname.lastname@example.org