The diversity, ecology, climate, and preservation of marine communities in the twenty million years following the end cretaceous mass extinction

Although mass extinctions are typically brief geological events, they disproportionately affect the tempo and mode of the history of life. Mass extinction is considered a governing influence on the diversification, migration, and ecological innovation of biota. Diverse and widespread clades are often obliterated at extinction boundaries and previously obscure taxa sometimes rise to prominence in the wake of extinction. While much is known about the proximate causes and consequences of mass extinction, relatively less study has focused on subsequent recovery periods, especially on timescales of tens of millions of years. Climate change is also considered a fundamental force that shaped the origination and distribution of taxa, however relatively few studies have tested this correlation in deep time. The Upper Cretaceous through lower Paleogene (84--46 Ma) Gulf Coastal Plain of the United States (GCP; Alabama, Mississippi, Georgia, and Texas) contain the Cretaceous-Paleogene (K-Pg) mass extinction and a global warming event at the Paleocene-Eocene boundary known to have affected marine organisms. I analyzed molluscan-dominated assemblages to better understand how these forces shaped the diversity and ecological characteristics of assemblages at the local and regional level. I also derived temperature estimates from mollusk shells to assess how climate change affected these organisms. Because of vagaries in the architecture of the fossil record, biases must first be accounted for to remove the spurious effects of preservation and unequal sampling. By doing so, this dissertation represents a synoptic treatment of the diversity and ecological trends of GCP marine assemblages in the 20 million years following the K-Pg extinction.;Abundance data were collected from bulk samples, museum collections, and the literature, resulting in a data matrix of 153 samples, 1494 taxa, and 52,585 individuals spanning ∼14 fossiliferous horizons in the GCP during the Paleocene to middle Eocene (65.5--46.6 Ma). These faunas are dominated by molluscs (bivalves and gastropods) with less common bryozoans, corals, echinoderms, and brachiopods. A subset of these data contains body size measurements. Geochemical data were analyzed from two late Paleocene units and one early Eocene unit.;Chapter 1, published in Geology (Sessa et al., 2009), examines the effect of temporally heterogeneous lithification on recovery patterns following the K-Pg extinction. Lithification has been advanced as a potential bias on diversity patterns, and this is one of the first studies to quantify the magnitude of this bias. Lithified units are significantly less diverse than their unlithified counterparts, primarily because small taxa are obscured in lithified rocks. In the GCP, this bias results in the artificial protraction of the K-Pg recovery interval by 7 Myr.;Chapter 2 explores the climatic and water mass properties of the GCP during the late Paleocene through early Eocene, a period of extreme global warmth. Temperature estimates from the oxygen isotopes of bivalve shells show a 3°C increase from the late Paleocene through early Eocene. Carbon isotopic profiles from bivalve shells suggest that the GCP water mass became progressively more stratified through this interval. While climatic changes had little effect on the diversity and ecology of assemblages (Chapter 3), the results of Chapter 2 are significant for the calibration and validation of models that reconstruct past greenhouse climates.;Chapter 3 analyzes the diversity, ecological structure, and turnover patterns of Late Cretaceous and early Paleogene assemblages. After accounting for unequal sampling intensity, bin duration, and preservation, I found that regional GCP diversity recovered to pre-extinction values in ∼2.7 Myr, nearly 20 Myr sooner than suggested by unstandardized estimates. The ecological composition of assemblages was also changed by the K-Pg extinction and was tightly correlated to periods of diversity recovery, expansion, and subsequent equilibrium.