They are members of the Order Medullosales — closely related to modern-day cycads.
These extinct ferns lived 310 million years ago during the Great Coal Age — a timeframe that includes the Pennsylvanian and Mississippian periods when much of the Earth's coal was formed.
As ferns, trees and other plant matter decayed, vast deposits of peat accumulated. Floodwaters brought silt deposits, covering and intermingling with the decaying peat. Time and pressure turned that mixture to the coal we mine today.
Back when they were alive, Pennsylvania was largely a tropical swamp with ferns towering at more than 50 feet high. They dominated the landscape, lived and died a full 185 million years before the first of our lovely flowering plants had even arrived.