Article: Morphology of fertile Pecopteris unita from the Middle Pennsylvanian of Illinois

Ironstone nodules from the Francis Creek Shale (Middle Pennsylvanian) near Morris, Illinois, have yielded both fertile and sterile pinnae of Pecopteris unita Brongniart. Partly petrified pinnae bear decurrent pinnules at approximately 90° angles. Pinnules have entire downturned margins, taper slightly apically, measure 4-8 mm long, 2-3 mm wide, and have unbranched, apically curved lateral veins. Synangia are abaxial and composed of 5-7 radially arranged, laterally appressed sporangia that are free at their apices. Sporangia surround solid tissue basally, a central hollow area distally, and dehisce longitudinally along the midline of the inward-facing sporangia! walls. Spores are monolete, reniform, measure 14-20 nanometers long, and possess a papillate sculpturing. These synangia compare favourably with the organ genera Scolecopteris Zenker and Cyathotrachus Watson, and are not similar to Ptychocarpus Weiss, a poorly understood genus with bilaterally symmetrical synangia.