Using fossil plants to understand global change: Evidence for Paleocene-Eocene warming in the Greater Green River Basin of southwestern Wyoming
The fossil record offers the only opportunity to observe the effects of global change in an extended time series. I analyze two methods for estimating past mean annual temperature and precipitation from assemblages of fossil leaves: leaf-margin and leaf-area analysis, respectively. These and other approaches are applied to diverse and well preserved paleofloras from the Greater Green River Basin of southwestern Wyoming in order to study climatic and biotic change during an interval of pronounced global warming, the late Paleocene (Tiffanian and Clarkforkian) and early Eocene (Wasatchian and earliest Bridgerian). Vegetational response to climate change in the study area was strongly pronounced. The Clarkforkian was humid and subtropical, following cooler humid conditions in the Tiffanian. Many plant families with modern tropical affinities immigrated in the Clarkforkian, but diversity remained as low as in the Tiffanian. Temperatures peaked in the mid-Wasatchian, the time of the Cenozoic thermal maximum, and a turnover of over 80% of species occurred from the Clarkforkian to the Wasatchian that included the immigration of a second wave of plant families with modern tropical affinities. Mid-Wasatchian vegetation was more diverse than that of the Clarkforkian and was derived from humid, subtropical swamp forest. A second major species turnover occurred from the Wasatchian to the earliest Bridgerian, accompanying severe drying and slight cooling. The source vegetation consisted of subtropical scrub growing on the margins of a playa lake system in a more arid and seasonal climate. Predominantly frost-free conditions were present throughout the study interval, with the exception of the Tiffanian and possibly portions of the Graybullian. Although the Eocene Green River lake system has been suggested as a factor contributing to mild southern Wyoming winters, this study shows that equable climates existed during several time intervals when lakes were either not present or not fully developed.