Current Research Projects
The Paleoecology of the Diamond Valley Lake Local Fauna
The Diamond Valley Lake Local Fauna (DVLLF) is an assemblage of Late Pleistocene mammals that is extraordinarily diverse and abundant in large ungulates (including Bison, Equus and Camelops), proboscideans (Mammut and Mammuthus) and all 3 species of giant ground sloth. The assemblage contains an older (46 - 49 ka) and younger (16 - 19 ka) assemblage of large mammals, and is also known as the "Valley of the Mastodons" for its unusually abundant Mammut findings ( > 50 individuals recovered). The DVLLF is a remarkable find and a window into the final days of these large herbivores; understanding the paleoecology of these organisms may be key to understanding why they went extinct and whether similar pressures threaten modern horse, bison, and camels.
Image Credit: Brian Engh
Gomphothere Paleocology in North America and the Competitive Exclusion Principle
North America played host to two families of proboscideans for at least 14 million years - the mastodons (Mammutidae) and the gomphotheres (Gomphotheriidae). Beginning in the early Pleistocene (ca. 1.8 Ma), the arrival of mammoths (Elephantidae) in North America coincided with the drastic drop in both the abundance and diversity of the gomphotheres. An often-cited reason for this drop in abundance is competition from two end-member dietary specialists - the browsing mastodons and the grazing mammoths. However, this assertion has yet to be directly tested using available data. The bulk of my dissertation work has been on testing this hypothesis using two paleoecological dietary proxies - 1) stable isotopes preserved in mammalian enamel, and 2) dental microwear textures. Understanding why gomphotheres went extinct in North America before mammoths and mastodons, as well as why gomphotheres were the only proboscideans to colonize South America, are my primary research goals.
Past Research Projects
The Dietary Niches of Mammoths and Mastodons inferred from Dental Microwear Textures
Dental microwear are the microscopic features left on the wear facets of mammalian enamel as a result of food processing. Depending on the textural properties of foods consumed, these features can vary considerably. For example, food that is very hard (woody browse, seeds, or fruit pits in herbivores; bones in carnivores) tends to leave pits or gauges on tooth surfaces. On the other hand, structurally tough foods (leaves or grass in herbivores; meat in carnivores) take quite a bit of effort to process and the repeated chewing of these foods tends to leave obvious scratches on the tooth surface. The relative proportion of these features (e.g., scratches:pits) and their size or volume can therefore be informative as to what was the nature of food consumed by an extinct organism. These diets can then be used to infer paleoenvironment - did the organism occupy a closed forest rich with woody browse, or was it better characterized by an open grassland with abundant graze? Much of my work focuses on quantifying these features using dental microwear texture analysis (DMTA), a 3D and repeatable process that can best characterize dietary differences between taxa. Further, my work is some of the first to focus on extinct proboscideans, massive ecosystem engineers that are capable of widespread vegetation change and habitat alteration. The extent to which these textures are recorded, as well as what they mean for habitat flexibility for these large organisms, is an area of active research for understanding the ecological habits of extinct megaherbivores.
Smith and Desantis, 2018
How dental wear impedes taxonomic identification in North American mammoths
Traditional taxonomic identification of mammoths has utilized the width and relative spacing of enamel lophs to separate species. However, it has been well-recognized amongst proboscidean paleontologists that there are many problems plaguing the effective separation of species. Mammoth molars are dynamic systems and constantly wear throughout life to allow for repeated processing of structurally tough foods. As a result, the relative proportions of enamel, dentine and cementum that comprise any given molar change with wear. My Masters' research focused heavily on attempting to quantify this change using computed tomography - the 3-dimensional imaging of materials, including enamel (hydroxyapatite) and dentin. Additional work has been carried out by other researchers, and the best method for separating species has yet to be determined. Computed tomography is a promising avenue for study because it promises to quantify features that might otherwise require destructive methods to visualize and measure.
Smith and Graham, 2015