A taxonomic, quantitative, and biostratigraphic analysis is presented for a macrofloral assemblage collected from below the Rock Island (No. 1) Coal Member at the historical Friendship Farm locality in Rock Island County, on the northwestern margin of the Illinois Basin. The Middle Pennsylvanian (middle Moscovian) fossiliferous strata involve the middle Tradewater Formation, and are situated a short distance below the Atokan–Desmoinesian Stage boundary as defined by marine microfossils and palynology. The assemblage of 14 fossil-taxa is overwhelmingly dominated by pteridosperms, including Laveineopteris rarinervis, Neuropteris flexuosa, Alethopteris serlii, Macroneuropteris scheuchzeri, and Mariopteris nervosa, whereas lycopsids, sphenopsids, and ferns are comparatively rare. Homotaxial comparison with the macrofloral biozonation scheme established in Europe indicates an Asturian age for the assemblage, in all likelihood early Asturian (Linopteris obliqua Biozone). Similar comparisons with range limits documented in the Appalachian Basin yield a more ambiguous age, but an early Asturian age is equally probable. These findings indicate that the Bolsovian–Asturian Substage boundary of western Europe lies below the Atokan–Desmoinesian Stage boundary in the Illinois Basin, boundaries that have traditionally been correlated by palynology. The Middle Pennsylvanian interval near these boundaries is being increasingly recognized as one of significant environmental change, reflected in marked lithological and biological modifications throughout tropical Euramerica. However, the fact that the Bolsovian–Asturian and Atokan–Desmoinesian boundaries are not synchronous, albeit being broadly equivalent, suggests that the environmental perturbation was a protracted event that may have had global consequences.