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Symbiosis in the Fossil Record: Eocene Nummulites and Pleistocene Reefs of Egypt

  • Author(s): Casazza, Lorraine Rebecca
  • Advisor(s): Lipps, Jere H.
  • et al.
Abstract

Abstract

Symbiosis in the Fossil Record: Eocene Nummulites and Pleistocene Reefs of Egypt

by

Lorraine Rebecca Casazza

Doctor of Philosophy in Integrative Biology

University of California, Berkeley

Professor Emeritus Jere H. Lipps, Chair

Symbiosis is an intimate association between two or more different species encompassing a spectrum of interactions from parastism, through commenalism to mutualism. Although it was once thought to be rare and unusual, we now understand symbiosis to be remarkably common and it has emerged as a key factor in evolutionary processes and major evolutionary transitions. Relative to other biological disciplines paleontology has been somewhat limited in its contributions to understanding symbiosis, but the fossil record is uniquely suited to answer some research questions on the topic. The research projects of this dissertation use the fossil record to examine aspects of symbiosis in two types of calcifying, marine organisms: larger foraminifera and hermatypic corals. Geochemical methods can be used to detect ancient symbiosis in the fossils of some calcifying, marine organisms because the presence of photosynthesizing algal symbionts can alter the stable carbon and oxygen isotope signatures of calcium carbonate host shells. Detecting the presence and degree of diagenetic alteration of calcium carbonate tests is critical for the integrity of isotopic studies carried out on them. In order to establish that foraminiferal tests from an exceptionally preserved, rare sample from the Eocene of Egypt are pristine and will yield reliable isotopic results in a study of symbiosis, light microscopy, scanning electron microscopy (SEM), backscatter electron imaging and electron dispersive spectrometry (EDS) are used to detect and identify diagenetic minerals in the foraminifera N. gizehensis, Uvigerina spp. and Cibicides spp. Potential sources of contamination in this Eocene sample include dolomite, gypsum, and more rarely recrystalized test calcite, however all can be detected and avoided for isotopic analyses. Carbon and oxygen isotopic analyses of pristine test material, pure dolomite, and recrystalized calcite were carried out to calculate how much contamination a sample can have and still yield reliable results. General recommendations for establishing the degree of alteration in rare, calcium carbonate samples include an initial investigation using light microscopy, SEM, backscatter electron imaging, and EDS to establish diagenetic patterns in fossil taxa, after which visual inspection with a light microscope may be adequate to distinguish between altered and unaltered individuals. For rare or precious specimens, SEM and EDS yields sufficient qualitative results without carbon-coating so that sample integrity may be preserved. If the condition of the sample permits preliminary isotopic testing, then an allowable percentage of contamination can be calculated to ensure reliable isotopic results. The pristine preservation of these Eocene foraminifera makes them appropriate for stable isotope analysis to detect ancient symbiosis. It is widely accepted in the literature that extinct species of larger foraminifera hosted algal symbionts, and that the evolution of larger foraminifera has been driven by algal symbiosis. However, there is no definitive evidence for symbiosis in fossil larger foraminifera. The known characteristics of stable isotope chemistry in extant symbiotic rotaliid foraminifera are used to test the hypothesis that the extinct rotaliid, Nummulites gizehensis, hosted photosynthesizing endosymbionts. The carbon isotopic signatures of symbiotic rotaliid foraminifera are 3-5 / lower in δ13C than calcite in equilibrium with ambient seawater, and exhibit a less well-supported trend of increasing δ13C values over ontogeny. The micro- and megalospheric Nummulites gizehensis tested show a mean negative shift of 3.75 / relative to contemporary fossil equilibrium calcite as calculated from co-occurring Uvigerina spp. and Cibicides spp. Seven out of nine microspheric individuals show the expected ontogenetic trend in δ13C over ontogeny, however all of the megalospheric forms show the opposite trend. Like their larger extant relatives, Nummulites gizehensis appear to have hosted photosynthesizing symbionts. Calcifying, symbiotic, marine organisms also provide one of the best case studies for understanding symbiosis in relation to environmental conditions. Reef building corals, taxa known to host photosynthesizing algal symbionts today and also confirmed as an ancient symbiosis, flourish in times of warm, nutrient-poor ocean conditions and all but disappear during periods of cooler, nutrient-rich conditions. Coral reefs are endangered globally, and predicting their response to changing environmental conditions is a priority for researchers and managers. The fossil record of Red Sea fringing reefs provides a unique opportunity to study the history of coral survival and recovery in the context of environmental catastrophe. Coral assemblage data from eight emerged fossil reef terraces on the Egyptian coast are described and used to determine if and how coral assemblages change from Middle to Late Pleistocene and Late Pleistocene to modern reefs. The first Red Sea occurrence of Favites micropentagona is reported from the Middle Pleistocene. Coral taxa are constant over the studied time period, as are coral assemblages, despite likely extinctions of coral species over two-thirds of the Red Sea basin during glacial low-stands. A less saline but still hostile southern Red Sea may have acted as a refuge for small communities of salinity-tolerant taxa during these extinction events, and provided the early settlers for subsequent recolonization. If populations of corals can be sustained, even in marginal and geographically limited habitat, they retain the potential to re-establish themselves when warm, nutrient-poor conditions appropriate for coral symbiosis return.

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