Graduate Studies, UNL

 

Dissertations and Doctoral Documents from University of Nebraska-Lincoln, 2023–

First Advisor

Peter Wagner

Degree Name

Doctor of Philosophy (Ph.D.)

Committee Members

Kate Lyons, Sherilyn Fritz, Tracy Frank

Department

Geosciences

Date of this Version

2025

Document Type

Dissertation

Citation

A dissertation presented to the faculty of the Graduate College of the University of Nebraska in partial fulfillment of requirements for the degree Doctor of Philosophy (Ph.D.)

Major: Geosciences

Under the supervision of Professor

Lincoln, Nebraska, December 2025

Comments

Copyright 2025, the author. Used by permission

Abstract

The Miocene of North America is characterized by dynamic changes in herbivore communities, habitat composition, and global climate. Long-term cooling and drying trends influenced large changes in floral and faunal communities. The Great Plains, North America shifted from closed-canopy biomes to open-canopy biomes with C3-grasses to biomes with C4-grasses. This shift affected the herbivore communities as browsing-adapted ungulates were replaced by grazing-adapted ungulates. Much focus has centered on the Great Plains, so it is not well understood how the changes in biomes compare to other regions of North America. The models used for vegetation density indicate open-canopy biomes since at least the early Miocene, but open canopy biomes include a range of modern analogs and other environmental interactions are not well understood. Also, the early investigations of herbivore community changes were performed with proxies that are now known to not be true representations of diet.

First, I compile a large dataset of stable carbon isotopes from Miocene herbivores across North America and apply a predictive vegetation density model to examine how biomes in different regions change throughout the Miocene. The shift to open-canopy biomes across North America was asynchronous and each region experienced the incorporation of C4-grasses into the environment and herbivore diets differently due to regional climatic differences.

Second, I reexamine the predictive vegetation density model by comparing bulk stable carbon isotopes of various global biomes to remotely sensed, satellite-derived vegetation indices. Bulk stable carbon isotopes follow the same trends for sunlight and water availability as do vegetation indices used as proxies for vegetation density, productivity, etc. and can be used to predict biome characteristics in more detail than previously thought.

Lastly, I revisit herbivore community changes through the Miocene of Nebraska, USA by investigating dietary guild ratios assigned by a combined mesowear-score and hypsodonty-index method. Hypsodonty is still a useful proxy in understanding community dietary changes and browsing taxa in the Miocene may have been more flexible in their diets than previously thought possible. Browse-rich habitats were still abundant despite open-canopy biomes dominating the Miocene of Nebraska.

Advisor: Peter Wagner

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