Stratigraphic Distribution, Taphonomy, and Isotope Paleoecology of the Dinosaurian Fauna in the latest Campanian lower Horseshoe Canyon Formation, Alberta, Canada

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Title: Stratigraphic Distribution, Taphonomy, and Isotope Paleoecology of the Dinosaurian Fauna in the latest Campanian lower Horseshoe Canyon Formation, Alberta, Canada
Author: Straight, William Herbert
Advisors: Elana Leithold, Committee Co-Chair
Dale A. Russell, Committee Chair
Reese E. Barrick, Committee Member
Elisabeth Wheeler, Committee Member
David Eberth, Committee Member
Abstract: Vertebrate fossils in the lower Horseshoe Canyon Formation are remains of dinosaurs, crocodilians, champsosaurs, turtles, and fish supported during the last ~2 m.y. of the Campanian by a coastal lake-strewn wetland occupying what is now south-central Alberta, Canada. Bones accumulated on the floodplain through attritional mortality and are preserved unweathered except for surface polish, scratches, and mottling characteristic of bioturbation during rapid burial in fine-grained sediment. Fossil-bearing sites cluster stratigraphically in laterally extensive horizons between thicker less fossil-rich intervals of similar fluvial strata. These horizons, formed by a long-term balance between bone supply, accommodation, and depositional rate, result from a newly recognized 'floodplain fill' mode of preservation for vertebrate fossils and are analogous to marine condensed sections. Like condensed sections, these fossiliferous horizons lie adjacent to lithostratigraphic surfaces created by stillstands in base-level. Together, hiatal surfaces and fossiliferous horizons reveal repeating rhythms in the facies distribution and fluvial architecture. These rhythms, 'packages' of strata bounded by hiatal surfaces, arose through two scales of variation in base-level: a grand-scale base-level cycle reflecting tectonic control during the construction of the clastic wedge, and a smaller 'package'-scale cycle reflecting Milankovitch control over local climate and precipitation. Both the fluvial architecture and the accumulations of fossils are a consequence of this change in accommodation and sediment supply through time. Fossil evidence does not indicate a faunal change through time, but changes in climate through time resulted in a reduction in organic-rich mudrocks and coal, an increase in soil development, and changes in the dominant configuration for fossil preservation from sparse bonebeds to microsites. Climate change was also investigated through stable oxygen isotopes in tyrannosaur tooth enamel phosphate, which daily recorded the response of surface (drinking) water to changes in humidity and temperature. The enamel isotopic record shows a transition from highly variable, seasonal climate to relatively constant conditions, consistent with the interpretation of change in the stratigraphy and taphonomy. This combined application of architectural stratigraphy, vertebrate taphonomy, and stable isotope paleoecology represents a new approach for paleontologists interested in evaluating changes through geologic time in paleoenvironment and animal communities in a fluvial succession.
Date: 2003-06-02
Degree: PhD
Discipline: Marine, Earth and Atmospheric Sciences

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