Author ORCID Identifier:

https://orcid.org/0000-0002-4743-3386

Date of Graduation

7-2026

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Geosciences (PhD)

Degree Level

Graduate

Department

Geosciences

Advisor/Mentor

Suarez, Celina

Committee Member

Villasenor, Amelia

Second Committee Member

Sharman, Glenn

Third Committee Member

Suarez, Marina

Fourth Committee Member

Cullen, Thomas

Keywords

carbon isotopes; Cretaceous; dinosaurs; North America; oxygen isotopes; paleoclimate

Abstract

The relationships between the environment and an organism can be traced through stable isotope and trace element geochemistry. This thesis explores the stable oxygen and carbon isotopic composition of fossil vertebrate assemblages in pursuit of revealing the relationships between dinosaurs and their archosaur relatives with the environment in which they lived during the Cretaceous. The Cretaceous Period of North America experienced some of the hottest intervals in the Phanerozoic Eon and the continental vertebrate taxa that existed during this time recorded these periods within the stable isotopic composition of their skeletal bioapatite. Oxygen isotopes in the bioapatite of vertebrates reflect their dietary source of water. The hydroclimate of Earth in the past can be described through the oxygen isotopic composition of water proxies such as pedogenic minerals, phyllosilicate clays, and biologic hydroxyapatite. The geographic spatial distribution of oxygen isotope compositions reflects the dominant global climate state of icehouse versus greenhouse. Since, temperature varies with latitude so does the oxygen isotopic composition of rainfall. During greenhouse climates, temperatures gradients are shallower than gradients during icehouse climates and this can be reflected by oxygen isotopes of preserved minerals. A polynomial equation of the gradient between latitude and oxygen isotopic composition of meteoric water is described in Chapter 2. Carbon isotopes preserved in the carbonate fraction of tooth enamel are incorporated by an organism from their diet. For pre-Cenozoic ecosystems, this reflects the average metabolic offset from the background value of the environment based on a diet of C3 plants and is expressed in enamel deposition. The carbon isotope composition of marine macroalgae is much higher than the typical C3 plant and, if incorporated into a vertebrate’s diet, influences the carbon isotope composition of the carbonate in skeletal materials. Thus, gradients of carbon isotope composition exist in the average of animal bioapatite from inland to shoreline. The closer to the shoreline, the more likely an organism has a bioapatite signature influenced by marine carbon. This phenomenon, not previously observed in paleontological-aged ecosystems, may explain higher carbon isotope compositions of some coastal Mesozoic vertebrates. Chapter 3 contains more details about dietary marine subsidization. The trophic level of an organism is also reflected in the trace element composition of skeletal materials and can be relatively estimated by analysis of Sr/Ca and Ba/Ca ratios of vertebrate bioapatite. Niche partitioning, marine influence, geologic baselines, and trophic position dictate the values of Sr/Ca and Ba/Ca in vertebrate bioapatite and, hence, provide additional information to complement the isotopic compositions of those same organisms. By reconstructing trophic structure of ecosystems across time and space, paleontologists can understand how instances of rapid climate change and extinction can affect food-webs. Sr/Ca and Ba/Ca values of Cretaceous vertebrate bioapatite is explored in Chapter 4. The combination of oxygen isotopes, carbon isotopes, and Sr/Ca and Ba/Ca values provide a near complete picture of a fossil organism, their environment, and trophic interactions. These provide a glimpse of ancient ecosystems that can be used for comparisons for today’s rapidly warming ecosystems. Supplementary data is included.

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