Neuropteris flexuosa Sternberg, 1823 fossil plant from the Pennsylvanian of Illinois, USA (public display, FMNH PF 46204, Field Museum of Natural History, Chicago, Illinois, USA).

One of the most remarkable soft-bodied fossil deposits (lagerstätten) on Earth is the Pennsylvanian-aged Mazon Creek Lagerstätte near Chicago, Illinois. In the Mazon Creek area, the Francis Creek Shale consists of concretionary gray shales. The Francis Creek concretions are composed of argillaceous ironstone, and can be fossiliferous or nonfossiliferous. The fossiliferous concretions contain land plants and terrestrial & marine animals, including nonmineralizing organisms.

This is Neuropteris flexuosa, a fossil seed fern. Seed ferns are not true ferns. Seed ferns were odd, usually arborescent (tree-like) plants that had woody trunks, large fronds bearing fern-like foliage, and reproduced using seeds (not the spores of true ferns).

Classification: Plantae, Pteridospermophyta, Medullosales

Stratigraphy: Mazon Creek Lagerstätte, Francis Creek Shale Member, Carbondale Formation, Desmoinesian Stage (= Westphalian D), upper Middle Pennsylvanian