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An investigation of radiolarian macroevolutionary dynamics in the tropical and polar Neogene oceans
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Radiolarians are a diverse group of microzooplankton that are ubiquitous in the world oceans and play a wide variety of vital roles in marine ecosystems and biogeochemical cycles. Polycystine radiolarians build siliceous skeletons, which readily preserve as fossils in the marine sedimentary record. Despite their importance and abundance of specimens, relatively little is known about the Late Neogene-Recent macroevolutionary dynamics of Radiolaria. So far, temporal trends in species richness, community structure, and extinction have only been exhaustively studied in the Southern Ocean (SO), leaving the geographic scale and underlying mechanisms of the observed signals largely unresolved. The purpose of this dissertation is to improve our knowledge of radiolarian macroevolutionary dynamics, by contributing a new biodiversity dataset from the tropical Pacific spanning the Late Miocene- Recent and a series of analyses utilizing data from both polar and tropical regions. The dataset generated for this study is comprised of comprehensive radiolarian species occurrence and abundance counts for 14 sediment samples from International Ocean Discovery Program Site U1337 in the eastern equatorial Pacific (EEP), dated 10.3 million years ago (Ma) to Recent. Approximately 5,000 specimens were enumerated per sample and identified to the most precise taxonomic level possible. These data are used to chart the history of radiolarian species richness and community structure across the study interval. Findings are compared to the SO dataset, and both are interpreted in the context of contrasting regional paleoclimate histories. Results indicate that relatively high magnitude temperature change in the SO is associated with a significant decline in radiolarian community evenness and species richness. By comparison, relatively low magnitude temperature change in the EEP had no discernable effect on any metric of biodiversity, all of which remained remarkably stable throughout the study interval. The majority of species that disappeared from the SO did not relocate to warmer regions as global temperatures dropped, and are thus interpreted to have gone extinct. This result suggests that radiolarians have a threshold response to temperature change and are not always able to track their preferred habitats on a global scale. Given ongoing anthropogenic change, we may expect an imminent radiolarian extinction interval at least as severe as that observed during the Late Neogene. Instances of extinction in the SO and EEP are further examined in the context of neutral ecological theory, which assumes a relationship between species abundance and extinction risk. Linear regression analyses are used to test whether abundance predicts longevity (time from origination to extinction) among radiolarians in the EEP and SO datasets. These analyses show only extremely weak or nonexistent associations between variables, suggesting that neutral processes are not primarily responsible for the extinction patterns of radiolarians, and other factors beyond abundance are likely more important. In addition, no connection was found between higher taxonomic identity (Collodaria, Nassellaria, and Spumellaria) and longevity in either dataset. This shows that more work is needed to understand the potential ecological, biological, and climatic mechanisms that contributed to past radiolarian extinction events, which also may help us mitigate their contemporary extinction risk. Included in this dissertation is also a review of Lophophaenidae, one of the most abundant, speciose, and under-documented families encountered in the Late Neogene EEP. Documentation of 101 lophophaenid taxa is provided, as well as new formal descriptions for 23 species, 1 subspecies, and 1 genus.