Stable Isotope Paleoecology in the Bridgerian (Early Eocene): Investigation of the Bridgerian Crash and the Canopy Effect

Authors

Aaron M. English, University of Nebraska-LincolnFollow

First Advisor

Ross Secord

Committee Members

R. Matthew Joeckel, Richard Kettler

Date of this Version

11-2025

Document Type

Thesis

Citation

A thesis presented to the faculty of the Graduate College at the University of Nebraska in partial fulfillment of requirements for the degree of Master of Science

Major: Geosciences

Under the supervision of Professor Ross Secord

Lincoln, Nebraska, November 2025

Comments

Copyright 2025, Aaron M. English. Used by permisison

Abstract

The Early Eocene Climatic Optimum (EECO) was the warmest sustained interval of the Cenozoic. Its waning stages coincided with a marked decline in mammalian biodiversity known as the Bridgerian Crash, which was likely driven by increasingly cooler and drier conditions. Paleobotanical evidence suggests the early Eocene supported wet, subtropical to tropical forests at mid-latitudes, however the structure of these forests at the close of the EECO remains poorly understood. In this study, I measure carbon isotope values (d¹³C_E) from the enamel of early Bridgerian (~52.2 – 48.3 Ma) herbivorous mammals from the Greater Green River Basin, Wyoming, to assess whether this ecological transition is reflected in the faunal isotope record of interior western North America. I also apply a predictive model based on d¹³C values from modern habitats to interpret Bridgerian environments in the study area.

A significant positive shift in d¹³C_E values occurs at the onset of the Bridgerian Crash, suggesting a transition toward drier, more open vegetation communities. Existing models reconstruct open-canopy C₃ habitats throughout the early Bridgerian with some individual mammalian enamel values plot in the range for mixed C₃/C₄ diets. Mean faunal values plot in the range expected for semi-xeric, or xeric, environments. This interpretation contrasts sharply with other evidence, such as the commonness of Bridgerian arboreal primates in the study area throughout this time interval. Additionally, the wide, negatively skewed distribution of individual d¹³C_E values seen in one of this studies time intervals is consistent with the presence of a forest understory. These observations suggest a substantial systematic offset in model results, likely originating from an underestimation of δ¹³C values in atmospheric CO₂. Nevertheless, relative shifts in mean d¹³C_E values and the distribution of individual measurements suggest that a transition to drier conditions occurred in concert with biodiversity loss in a forest that maintained either an understory or locally wet habitats.

Advisor: Ross Secord