Credit: PROC. NATL ACAD. SCI. USA/P. JANVIER/CNRS

Proc. Natl Acad. Sci. USA doi: 10.1073/pnas.0807047106 (2009)

A remarkably well-preserved brain has been discovered in a 300-million-year-old fossil of a fish from Kansas.

Philippe Janvier of the National Museum of Natural History in Paris and his colleagues used X-rays from the European Synchrotron Radiation Facility in Grenoble, France, to peer inside the skulls of iniopterygians, extinct relations of modern sharks and ratfish.

In one skull (pictured), they imaged a dense calcium phosphate structure. Its shape and relationship to nerve locations suggest that it is a brain, mineralized by phosphate-fixing microbes before the soft tissue could decay. The researchers hope that other vertebrates fossilized in similar conditions might yield further preserved organs, potentially throwing light on brain evolution.