Quantification of Ichnological, Paleoecological, Paleohydrological, and Paleoclimatological Information from the Upper Jurassic Morrison Formation

Abstract

This dissertation takes a multifaceted approach to interpreting paleoenvironments and paleoclimate represented by the strata of the Morrison Formation (MF). The MF has been the subject of geological and paleontological investigations for well over a century, but a number of confounding factors have limited interpretations. One problem with interpreting MF ichnology is uncertainty about the sedimentary conditions under which deep dinosaur tracks were made. To solve this problem, I developed new methods to measure trace fossils, including footprints. I used multistripe laser triangulation scanning to create three-dimensional digital models of traces, from which I improved precision of traditional ichnological techniques. I also performed neoichnological experiments with elephants to collect empirical trackmaking data, to which I applied multiple regression to derive a quantitative relationship between physical trackmaking variables. Results showed that many deep sauropod tracks were created in near saturated conditions. Megafaunal track preservation was one factor taken into consideration when interpreting paleohydrology from moisture regimes represented by trace-fossil assemblages. I demonstrated the usefulness of ichnological moisture regimes by interpreting ichnocoenoses in MF avulsion deposits in the Bighorn Basin. I found a regular pattern of moisture profiles associated with crevassing that can be used to identify avulsion deposits in future ichnological studies. Ichnological moisture regimes were incorporated with other pedogenic features to develop a soil moisture index that was combined with measures of carbonate content, carbonate mineralogy, total organic carbon, and stable isotopes of carbonates and organic carbon to construct a detailed vertical profile through the MF in the Henry Mountains, Utah. This profile is useful for paleoclimatic and paleoenvironmental interpretations as well as for correlation to marine isotope records. I compared the vertical profile to paleoecological patterns determined from food-web network analyses and found a possible correlation between a global shift in organic carbon isotopes and an episode of biotic turnover. I also found that MF food webs were extremely stable, a factor that may have contributed to the success of dinosaur-dominated ecosystems during the Mesozoic.