ENDEMISM IN FOREST ECOSYSTEMS DURING THE EARLY EOCENE CLIMATIC OPTIMUM IN WYOMING

During the Early Paleogene hothouse, mid-latitude North America exhibited a warm, equable climate that was likely conducive to high beta diversity and endemism, as observed in the modern tropics and subtropics. To test this, we compared the flora and herbivorous insect feeding traces from the Early Eocene Climatic Optimum in the neighboring Bighorn (BHB) and Wind River (WRB) Basins of Wyoming. We conducted unbiased floral counts at eleven localities along a laterally extensive carbonaceous shale bed in the BHB and at eight localities along a lithologically similar horizon in the WRB. The insect herbivore fauna was assessed by censusing insect feeding damage on fossil leaves at seven of the BHB and eight of the WRB localities. Geochemical analyses were conducted to complement the macrofossil data.

Paleoclimate estimates indicate similar mean annual temperatures and slightly wetter conditions in the WRB. The most abundant species at both sites are taxa common throughout Eocene deposits in the Western Interior (ferns Lygodium kaulfussi, Salvinia preauriculata, and Equisetum sp.; monocot Zingiberopsis isonervosa; and dicots Platycarya castaneopsis, “Dombeya” novi-mundi, and Populus wyomingiana); five of the top ten most abundant species at each site are shared, although rank orders are different. However, there are important differences between basins. The most abundant species in the pooled BHB data is a conifer, whereas not a single conifer macrofossil or terpenoid was found in the WRB carb shale. Alnus, an important component in the BHB and many other Eocene floras, is also absent in the WRB censuses. Overall, of the 44 plant taxa found in the WRB sites, only 13 are also found in the BHB, indicating little overlap in rare species. Floral diversity is higher in the WRB, but, interestingly, damage diversity is lower. The range in damage diversity and prevalence among host plants is greater in the BHB, as would be expected for a drier environment. Damage composition on individual hosts is somewhat different at the two sites, but no significant differences in damage diversity or prevalence exist on the abundant species found in both basins. Our results support high endemism in the temperate zone during the height of EECO warming.