An outcrop of the Mississippian Hartselle Sandstone in north-central Alabama preserves in situ, erect cormose lycopsids, assigned to Hartsellea dowensis gen. and sp. nov., in association with a low diversity bivalve assemblage dominated by Edmondia. The isoetalean lycopsids are rooted in a silty claystone in which the bivalve assemblage occurs, representing the transition from tidal flat and tidal channel regime into a poorly developed inceptisol. Two paleosols are preserved in the sequence and each is overlain by a fine-grained quartz arenite, responsible for casting aerial stems and cor-mose bases of the entombed plants. The massive quartz arenites are in sharp contact with interpreted O-horizons of the paleosol, and the lower sandstone displays a lobate geometry. The plant assemblages are interpreted as back-barrier marshes, the first unequivocal marshlands in the stratigraphic record, preserved by overwash processes associated with intense storm surges in a Transgressive Systems Tract. A sample suite curated in the National Museum of Natural History, collected by David White at the turn of the last century in the Greenbrier Limestone of West Virginia, preserves rooting structures, leaves and sporophylls, and sporangia and megaspores of H. downensis in a mixed carbonate mud (micrite). The presence of isoetalean lycopsids in both siliciclastic and carbonate peritidal environments within nearshore shelf settings of the Early Carboniferous indicates that adaptation to periodic brackish water, if not tolerance to infrequent fully marine-water inundation during storm surges, had evolved in these marsh plants by the late Paleozoic.