Miocene Ichnofaunas and Mammalian Communities in the Great Basin Region, Nevada and California

The purpose of this dissertation is to document and interpret three new ichnofaunas and two fossil assemblages from the Neogene of Nevada and California. Ichnofossils are important for understanding behavior and geographical range of animals in the distant past and are key to locating and preserving sensitive material for future studies. I examined ichnofauna at three localities: two within the Mio-Pliocene Muddy Creek Formation of Nevada, and one within the Miocene China Ranch Beds of California. My research focuses primarily on (1) camelid and ursid ichnotaxa and how these new localities help in understanding the evolutionary history of these taxa within the southwestern United States, and (2) the composition and structure of mammalian communities in the Great Basin region during the Mid-Miocene Climatic Optimum. Chapter 1 documents a new camelid trackway locality in the Muddy Creek Formation, exposed near Flat Top Mesa in Mesquite, Nevada. Using a combination of field and photogrammetric analysis, I document the presence of 40 prints within a thin layer of limestone. The average length of these prints is 135 mm. I refer these camelid tracks to the ichnotaxon Lamaichnum isp. In comparison to older camelid tracks, the Flat Top Mesa tracks are larger on average, which reinforces a previously recognized trend of increasing camelid body size through the Neogene. In Chapter 2, I document and interpret a more diverse ichnofauna within the Muddy Creek Formation at the informally named Muddy Creek Canyon locality near Overton, Nevada. This ichnofauna consists of camelid, ursid, felid, equid, and avian tracks. While most of the tracks are isolated in float or on the undersides of overhanging ledges, two camelid-trackway bedding-plane exposures are documented and analyzed. The camelid tracks at this locality exhibit extramorphological variation as Lamaichnum isp. and are smaller on average than the camelid tracks at the Flat Top Mesa locality. When combining the Muddy Creek Canyon data with the Flat Top Mesa camelid track length data, the average length is 120 mm for the entirety of the Muddy Creek Formation. The depositional environment is interpreted to be the shoreline of an ephemeral lake. Chapter 3 documents an ichnofauna consisting of camelid and ursid tracks within the Miocene China Ranch Beds of southeastern California. While camelid tracks are known from these beds, the ursid tracks are new and are possibly some of the oldest ursid tracks within the United States. They document the first known occurrence of a galloping gait in ursids. The depositional environment is interpreted to be similar to that of the Muddy Creek Formation at the shoreline of an ephemeral lake. The Muddy Creek Canyon ichnofauna diversity is typical for Miocene ichnofaunas and is higher than the China Ranch Beds ichnofauna diversity. Therefore, it is possible new tracks in the China Ranch Beds may be exposed in the future and monitoring of this site is suggested. Chapter 4 analyzes and compares two mammalian assemblages that document species richness through the Mid-Miocene Climatic Optimum. A combination of changing climate and topography influenced the transition from warmer wooded ecosystems to cooler grasslands. In general, mammalian species richness declined after the MMCO. Carnivore species richness was surprisingly high in both faunas while herbivore species richness was not