N. MacLeod, 1990. "Effects of late Eocene impacts on planktic foraminifera", Global Catastrophes in Earth History; An Interdisciplinary Conference on Impacts, Volcanism, and Mass Mortality, Virgil L. Sharpton, Peter D. Ward
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A series of iridium anomalies and microtektite-bearing layers indicative of extraterrestrial impact events have been found in late Eocene sediments in the deep sea. Previously, these deposits had been reported to be approximately synchronous with major extinction events in groups of both terrestrial and marine organisms. High-resolution biostratigraphic studies, though, have failed to find any direct evidence for association between impact-derived materials and species extinctions in individual stratigraphic sequences. In addition, stable isotopic studies of the deep-sea record reveal abundant evidence for an episode of major climatic change beginning in the middle Eocene and continuing through the Oligocene. A new analysis of patterns of taxic richness, extinction rates, and origination rates in 17 Eocene through Oligocene planktic foraminiferal biozones indicates that relatively high numbers of planktic foraminiferal taxic extinctions are not confined to the Eocene/Oligocene boundary, or to biozones containing impact ejecta (e.g., microtektite layers, Ir anomalies). Instead, virtually all middle Eocene through middle Oligocene biozones are characterized by broadly comparable numbers of taxic extinctions. However, analysis of relative abundances of individual species and patterns of morphometric variation in middle and late Eocene populations of planktic foraminifera suggest that impacts may have had substantial and long-lasting effects on the dynamics of local populations within areas of direct environmental perturbation. In order to understand the local biotic effects and potential evolutionary role of events such as extraterrestrial impacts, it is important that detailed analyses of species- and population-level patterns of morphological variation and faunal turnover proceed in concert with coarser grained investigations so that patterns of variation can be compared on a wide variety of taxonomic and stratigraphic scales (e.g., species-level versus family-level taxonomic resolution, biozone-level versus stage-level stratigraphic resolution).