Fern fossil (Pecopteris sp.). This fern dates from the Carboniferous period (350-290 million years ago). The Carboniferous period was a time when ferns dominated the land and when they died, they formed large beds of coal (a form of carbon). Fossils appear when rocks form around objects that have been buried in mud or ash. As the rock forms, the shape and anatomy of buried animals or plants can be preserved. Found in Illinois, USA.