Date of Award

Spring 2018

Project Type


Program or Major

Earth and Environmental Sciences

Degree Name

Doctor of Philosophy

First Advisor

William C. Clyde

Second Advisor

Rosemarie E. Came

Third Advisor

Joseph M. Licciardi


Abrupt perturbations of the global carbon cycle during the early Eocene are associated with rapid global warming events. Recent studies have observed mammal dwarfing during the most severe of these ancient global warming events (or “hyperthermals”), known as the Paleocene-Eocene Thermal Maximum (PETM, ~56 mya). Chapter I of this dissertation establishes a stratigraphic framework around two subsequent and smaller-magnitude warming events known as ETM2 and H2 (~53.7 mya and ~53.6 mya, respectively), which have recently been documented in the strata of the Bighorn Basin of Wyoming. Such a stratigraphic framework is crucial for placing fossil localities into stratigraphic context, which can then aid in the interpretation of mammalian response to these warming events. Chapter II shows that a decrease in mammal body size accompanies the ETM2 warming event. Body size decreases are evident in three of the four taxonomic groups analyzed in this study, but they are most clearly observed in early equids (horses). During ETM2, the most extensively-sampled lineage of equids decreased in size by ~14%, as opposed to ~30% during the larger PETM. Thus, decrease in body size appears to be a common response for some early mammals during past global warming events, and the extent of dwarfing is related to the magnitude of the event.

Chapter III further investigates the observed early equid body size response to hyperthermal warming. The African Duiker (Philantomba) is used as a modern analog to investigate the relationship between body size and latitude, and to estimate early equid geographic range shift. Duikers were found to follow Bergmann’s rule, with a statistically significant positive correlation between body size and latitude. If early equids shared the body size-latitude relationship to the same extent that Duikers do today, they would require only a 10 degree and 4 degree latitudinal range shift to explain their apparent body size decrease across PETM and ETM2 stratigraphic records, respectively.

Community structure changes are a typical response to environmental stressors. Chapter IV attempts to better understand this relationship in the context of extreme climate change. Fossil genera co-occurrence patterns are investigated through both the PETM and ETM2 hyperthermal events. The main body of both events is associated with an increase in the number of significant pairs of taxa, with segregated pairs found to be more common. Such patterns reflect dynamic changes within these early Eocene communities in response to warming climate, and can serve as models for better understanding the impacts of modern-day warming on mammals and ecosystems.