Occurrence and paleoecology of Marsilea from the Eocene Wasatch Formation,Johnson County, Wyoming

Fossil Marsiliaceae, referable to Marsilea (also known as water clover or p epperwort), were found as impressions in baked claystone (clinker) of the E ocene Wasatch Formation near Buffalo, Wyoming. This is the first documented evidence of Marsilea foliage in Tertiary strata. The fossil is in the Bure au of Land Management Petrified Tree Environmental Education Area where str ata are associated with the Healy and Walters coal beds. Previous palynolog ical analyses, the presence of the ferns Salvinia preauriculata and Lygodiu m kaulfussi, and the stratigraphic position of the deposits indicate an Ear ly Eocene age for the fossils. Leaf impressions of a variety of pteridophyt es and spermatophytes, in conjunction with more than 100 pollen and spore t axa, indicate deposition on a tropical to subtropical fluvial floodplain. L eaf morphology, including leaf size, shape, and venation, are very similar to living species of Marsilea. The preservation in thermally altered sedime ntary rock that was derived from claystone is consistent with the plants' c urrent preference for moist clay soils. The conservative morphology and env ironmental preferences of Marsilea indicate that it is an ecologically cons ervative genus that can be a reliable indicator of quiet freshwater habitat s with clay substrata.