Isopod from the Pennsylvanian of Illinois

Hesslerella shermani is described as the oldest representative of the crustacean superorder Peracarida, order Isopoda, suborder Phreatoicidea. This description is based on a single specimen of exceptional preservation from the Middle Pennsylvanian of Illinois. The existence of isopods in the Pennsylvanian raises some questions concerning peracarid and eumalocostracan evolution.