PALEOBOTANICAL DIVERSITY AT THE BLUE RIM SITES OF THE EOCENE BRIDGER FORMATION, SOUTHWESTERN WYOMING
The Blue Rim sites are unusual in that multiple plant organs are preserved together in the same horizons. In modern vegetation, reproductive structures are the primary organs used to classify plants. These structures are rarely preserved in fossil floras, yet Blue Rim has fruits, seeds, and several flower types with well-preserved pollen in the anthers. These Eocene fossils, representing an interval approximately midway in time between the initial burst of angiosperms and the present day, include both modern and extinct genera of flowering plants.
While isolated paleobotanical specimens and taxa of the Bridger Formation have been described previously, the entire diversity of the flora has yet to be examined. Two floral horizons have been found in the Blue Rim area to date. The lower horizon, more thoroughly collected thus far, is dominated by approximately 35 species of angiosperms such as Populus cinnamomoides and species that display a vine habit including the heterophyllous climbing fern Lygodium kaulfussi. Silicified wood stumps of Anacardiaceae and three additional unknown dicotyledonous woods were also found near the lower horizon. Populus spp. are shared with the nearby Green River Formation, while Lygodium kaulfussi is geographically widespread throughout the Eocene. The upper horizon at Blue Rim has some members similar to the lower, but it also preserves other taxa like Macginitea wyomingensis, Cedrelospermum nervosum, and Chaneya.
The presence of leaves suitable for physiognomic analysis as well as a variety of other plant organs for which nearest modern relatives can be determined, is a good basis for assessing the climatic conditions that supported the Bridger Formation vegetation. This flora, together with faunal sites in the same formation, provides an opportunity to reconstruct the ecosystem of southwestern Wyoming ~50 Ma.