Fossil Marsiliaceae, referable to Marsilea (also known as water clover or pepperwort), were found as impressions in baked claystone (clinker) of the Eocene Wasatch Formation near Buffalo, Wyoming. This is the first documented evidence of Marsilea foliage in Tertiary strata. The fossil is in the Bureau of Land Management's Petrified Tree Environmental Education Area where strata are associated with the Healy and Walters coal beds. Previous palynological analyses, the presence of the ferns Salvinia preauriculata and Lygodium kaulfussi, and the stratigraphic position of the deposits indicate an Early Eocene age for the fossils. Leaf impressions of a variety of pteridophytes and spermatophytes, in conjunction with more than 100 pollen and spore taxa, indicate deposition on a tropical to subtropical fluvial floodplain. Leaf morphology, including leaf size, shape, and venation, are very similar to living species of Marsilea. The preservation in thermally altered sedimentary rock that was derived from claystone is consistent with the plants' current preference for moist clay soils. The conservative morphology and environmental preferences of Marsilea indicate that it is an ecologically conservative genus that can be a reliable indicator of quiet freshwater habitats with clay substrata.