The Spencer Farm Flora is a compression-impression flora of early Pennsylvanian age (Namurian B, or possibly Namurian C) from Brown County, west-central Illinois. The plant fossils occur in argillaceous siltstones and sandstones of the Caseyville Formation that were deposited in a ravine eroded in Mississippian carbonate rocks. The plant-bearing beds are the oldest deposits of Pennsylvanian age yet discovered in Illinois. They were formed before extensive Pennsylvanian coal swamps developed.
The flora consist of 29 species and a few problematical forms. It represents an unusual biofacies, in which the generally rare genera Megalopteris, Lesleya, Palaeopteridium, and Lacoea are quite common. Noeggerathiales, which are seldom present in roof-shale floras, make up 20% of the specimens. The Spencer Farm Flora is and extrabasinal (= upland ) flora that was growing on the calcareous soils in the vicinity of the ravine in which they were deposited.
It is suggested here that Noeggerathiales may belong to the Progymnosperms and that Noeggerathialian cones might be derived from Archaeopteris-like fruitifications. The cone genus Lacoea is intermediate between Noeggerathiostrobus and Discinites in its morphology. Two new species, Lesleya cheimarosa and Rhodeopteridium phillipsii, are described, and Gulpenia limburgensis is reported from North America for the first time
