EVIDENCE OF MAMMALIAN BODY SIZE CHANGE ACROSS THE EARLY EOCENE ETM2 HYPERTHERMAL EVENT, BIGHORN BASIN, WY

During the early Paleogene, Earth experienced a series of significant and rapid global warming events, known as hyperthermals, which were characterized by global atmospheric carbon isotope excursions. The largest of these events, known as the Paleocene-Eocene Thermal Maximum (PETM), is also characterized by transient mammalian dwarfism. Recent high-resolution studies suggest that body size decreased due to the rise in atmospheric temperatures and increased levels of atmospheric CO₂. Understanding hyperthermals and their effects on terrestrial biotas is important for predicting future ecological changes in response to present day CO₂ and temperature increases.

In order to decipher the relationship between mammal body size, atmospheric temperatures, and atmospheric CO₂ levels, it is important to analyze these variables across multiple hyperthermals. Until recently, hyperthermal events subsequent to the PETM have not been observed in terrestrial sedimentary deposits. However, recent analysis of paleosol carbonates in the Bighorn Basin of Wyoming has revealed the existence of two early Eocene hyperthermals that are smaller in magnitude than the PETM, and previously only known in marine records. These events, occurring roughly 2 million years after the PETM, are known as ETM2 and H2. This study analyzed body size change in a commonly appearing early Eocene fossil of the Bighorn Basin, Hyracotherium, previously shown to be sensitive to the temperature and CO₂ increases of the PETM.

Several studies have shown that mean tooth area in mammals positively correlates with mean body size. Thus, any significant change in tooth size over time within a mammalian lineage should reflect a change in body size. Based on this information, Hyracotherium teeth were analyzed for body size change across ETM2 by measuring the length and width of individual teeth spanning the Bighorn Basin ETM2 stratigraphic section. Results suggest that Hyracotherium body size significantly decreased during the ETM2 hyperthermal event, followed by a post-ETM2 body size rebound.

Future work will focus on collecting a higher resolution Hyracotherium tooth record across the ETM2 and H2 hyperthermals, with the goal of resolving a possible empirical relationship between the magnitude of hyperthermal warming and extent of mammal body size change.