FAUNAS IN FLUX DURING THE SUCCESSIVE HYPERTHERMALS OF THE EARLY EOCENE

Several hyperthermal events (5-10 °C increase in <20 ka) occurred during the early Eocene, including the Paleocene/Eocene Thermal Maximum (PETM, ~56.3 Ma), Eocene Thermal Maximum 2 (ETM2, ~53.8 Ma) and H2 (~53.7 Ma). The carbon isotope excursions of all three events are known from terrestrial sections in Wyoming’s Bighorn Basin, but related faunal change has only been previously studied at the PETM. The dense, highly-resolved, well-documented mammal record from the Fifteenmile Creek (FC) part of the Bighorn Basin permits high resolution analysis of faunal response to ETM2 and H2, and direct comparison of these events with the PETM.

More than 30,000 mammal specimens previously collected (1975-2010) from 410 fossil localities tied to the FC section were binned into four series of the shortest-possible, equal-time (~30-~50 ka) data bins, exhaustively standardized, and averaged by meter level across all bins. Parameters modeling faunal dynamics were calculated from the binned specimen data and compared with signals from the unbinned data to test resolution. Parameters include turnover (rates of species appearances and disappearances), diversity (the interplay of species richness and the evenness of abundance distributions), and relative body size (approximated by the size of lower first molars).

Faunal response at ETM2 and H2 is nearly identical and differs substantially from response to the PETM as follows. ETM2 and H2 are characterized by equally high species first and last appearances, high beta richness (differentiation between assemblages), and increases and decreases in body size. In contrast, faunal change at the PETM includes a burst of species appearances, high alpha richness (average within assemblage richness), and decreases or no change in body size. These differences are probably related to different conditions during the hyperthermals. PETM warming in the Bighorn Basin was accompanied by notable drying, which led to immigration across northern land routes and metabolic effects on body size. In contrast, the later hyperthermals occurred during a period of significant long-term evolutionary, environmental and climatic change. Increased productivity and habitat complexity may have led to morphological disparity and spatial heterogeneity of species.