PALEOBOTANICAL INVESTIGATION OF THE GREEN RIVER LACUSTRINE FLORA OF FOSSIL BUTTE, WYOMING

The Eocene biota of the Fossil Butte vicinity, southwestern Wyoming, is well known for its diverse fish fauna, but the associated flora has received little attention. We have investigated leaves and plant reproductive structures of the lacustrine sediments to gain insight into floristic composition and diversity and for comparison with the younger Green River flora of the Parachute Creek Member in Utah and Colorado. This assemblage, which we term the Twin Creek flora, includes an aquatic component (the floating fern, Salvinia and the floating dicotyledons, Ceratophyllum and Nelumbo) and a more diverse terrestrial component. The climbing fern, Lygodium, is represented by fertile structures as well as leaves. The main elements of the leaf assemblage include palm (Sabalites), large araceous laminae, and several kinds of deciduous dicotyledons (notably Cedrelospermum nervosum, Platycarya castaneopsis, "Allophylus" flexifolia, "Cardiospermum" coloradensis, Syzygioides americana, and Rhus sp.). Distinctive fruits include Ailanthus, Cedrelospermum, Paleoplatycarya, and others as yet undetermined. With about 20 leaf types, the Twin Creek flora lacks the high diversity seen in the Parachute Creek member of the Green River Formation in Colorado and Utah (more than 50 leaf types). This might be due in part due to taphonomy, and to differences in age and climate. Some of the more abundant leaf types of the Parachute Creek flora, e.g., Populus, Salix, and Macginitiea, are absent or rare in the Twin Creek flora. Conversely, Platycarya castaneopsis, which is common in the Twin Creek and other early Eocene floras of Wyoming, is missing from later Eocene floras including that of the Parachute Creek member.