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Abstract
Water depth is the most common environmental gradient correlated with community composition of marine benthic invertebrates in the modern and fossil record. It is a complex gradient comprising of other environmental gradients such as temperature, substrate, water pressure, shear stress, salinity, light penetration, nutrients, and oxygen. However, the frequency of each covarying gradient is currently unknown for the fossil record, as well as the frequencies of gradients not correlated with water depth. This dissertation conducts an extensive literature review of modern and ancient studies and fieldwork in modern and ancient carbonate environments to accomplish two goals. First, it quantifies the frequency of environmental gradients identified in ancient settings and compares those frequencies and amounts of explained variance to gradients in the modern. Second, it combines detailed faunal and lithological data in the modern and ancient to test for gradients that covary with water depth as well as any additional gradients not correlated with water depth. The literature review focuses on studies that use multivariate ordinations to interpret environmental gradients in benthic invertebrate assemblages and compares frequencies among ordination axes to interpret each gradient’s relative impact on assemblage variation in the modern and ancient. The modern study tests for environmental gradients controlling the molluscan death assemblage of the shallow subtidal of San Salvador Island, The Bahamas. Sampling is restricted to a 4.2 m depth range and vegetation density, grainsize distribution, and physical and chemical oceanographic data were collected for each assemblage. The ancient study tests for environmental gradients controlling the Mississippian benthic invertebrate assemblage of the lower Madison Group in Montana. Sampling collected lithological specimens in conjunction with faunal counts, providing quantitative lithologic data for every sample. The results of this dissertation underline the predominance of the water-depth gradient, but it also identifies substrate and wave energy as the covarying gradients that commonly impact assemblage compositions. Additionally, it demonstrates that substrate gradients that are not correlated with water depth are equally important to assemblage variation. Furthermore, it emphasizes how closely spaced, replicate sampling of lithological data in conjunction with faunal data can highlight sources of environmental variation controlling fossil community compositions.