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Browsing by Author "Xiao, Shuhai"

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    Are 'exceptionally' preserved skeletal fossils necessarily exceptional chemically and cytologically?
    Korneisel, Dana Elaine (Virginia Tech, 2019-09-19)
    At the macroscopic scale, vertebrate fossils are considered exceptional when non-biomineralized (soft) tissues are preserved. Histologically, high quality is defined by trueness to original shape of a bone, preservation of fine details (e.g. canaliculi), and presence or absence of matrix material in void spaces. Some fossils are hypothesized to preserve cells and durable organelles. Traditionally, cytological details and biomolecular remains have been sought in exceptional fossils. Durable cytological features such as melanosomes do appear to follow feather preservation, but traditionally exceptional fossils are not necessarily exceptional on a microscopic scale. Here, we analyze a feathered dinosaur specimen from the Jehol Lagerstätte to assess claims of blood cell preservation and the state of potential biomolecular preservation. Beipiaosaurus inexpectus is a fairly complete specimen with preserved feathers. Though crushed, fine details in thin section are prevalent. Using Raman spectroscopy, Energy Dispersive X-ray Spectrometry, and Time-of-Flight Secondary Ion Mass Spectroscopy we found no evidence of exceptional molecular preservation. Instead, we found evidence that the vasculature, once hypothesized to contain preserved red blood cells, is filled with clay minerals, with the purported cells chemically indistinguishable from materials of other shapes infilling the vessels. Despite yielding exceptional fossils, the preservational environment of the Jehol biota does not necessarily preserve exceptional details cytologically or biomolecularly. Consequently, we conclude that a systematic approach to biomolecular and cytological preservation studies should rely on traits other than classic exceptional preservation.
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    An authigenic response to Ediacaran surface oxidation: Remarkable micron-scale isotopic heterogeneity revealed by SIMS
    Cui, Huan; Kitajima, Kouki; Orland, Ian J.; Baele, Jean-Marc; Xiao, Shuhai; Kaufman, Alan J.; Denny, Adam; Spicuzza, Michael J.; Fournelle, John H.; Valley, John W. (Elsevier, 2022-08-01)
    The Ediacaran Shuram excursion (SE) records a global decrease in carbonate carbon isotope (δ13Ccarb) values from +6‰ down to ca. –10‰, representing the largest δ13Ccarb negative anomaly in Earth history. While the SE is widely recorded in the upper Doushantuo Formation of South China, it shows highly variable δ13Ccarb profiles among correlative sections. This inconsistent expression of the SE challenges the conventional view of a homogeneous marine dissolved inorganic carbon (DIC) reservoir. A potential process that could explain δ13Ccarb variability is local mineralization of isotopically distinct authigenic carbonates near the sediment–water interface during early diagenesis. However, a direct test of such authigenic carbonates is still limited. Here, following a recent study on the SE in an intra-shelf environment, we revisited an outer-shelf section, identified and analyzed μm-scale, syn-depositional authigenic calcite cements via integrated cathodoluminescence (CL), micro-X-ray fluorescence (μXRF), scanning electron microscope (SEM), energy-dispersive spectroscopy (EDS), and secondary ion mass spectrometry (SIMS). Our new SIMS results reveal remarkable micron-scale heterogeneity of δ13Ccarb in authigenic calcite cements, including extremely negative values down to –37.5‰ (VPDB). We interpret these calcite cements as methane-derived authigenic calcite (MDAC) resulting from microbial sulfate reduction (MSR) and anaerobic oxidation of methane (AOM) during early diagenesis. Based on the new results, we propose that the heterogeneous SE in South China — manifest on micrometer, centimeter, and basinal scales — was modulated by methane oxidation under variable local redox and water depth conditions. The SE, therefore, was coupled with different degrees of methane oxidation in individual basins, and globally triggered by enhanced seawater sulfate during an atmospheric oxygenation event. In light of this study, the potential role of redox variability in methane oxidation during the SE may have been underestimated. Our study demonstrates that integrated SIMS-SEM analysis can distinguish different generations of isotopically distinct carbonates otherwise undetected by conventional analysis, and is thus an effective approach to assess the origin and diagenetic history of δ13Ccarb anomalies in the sedimentary record.
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    Beyond the stony veil: Reconstructing the Earth’s earliest large animal traces via computed tomography X-ray imaging
    Meyer, Mike; Polys, Nicholas F.; Yaqoob, Humza; Hinnov, Linda; Xiao, Shuhai (2017-09)
    Trace fossils are superb lines of evidence for examining the ancient biologic world because they offer an opportunity to infer behavioral ecology of organisms. However, traces can be difficult to parse from their matrix, which leads to the loss of important morphological and behavioral data. This is especially true for the earliest marine animal traces from the Ediacaran Period (635–541 Ma), which are usually small (<5 mm in diameter) and simple (mostly small horizontal trails and burrows), and are sometimes difficult to be distinguished from co-existing tubular body fossils. There is also evidence that the prevalence of microbial substrates in Ediacaran oceans may have influenced emerging trace makers in nonactualistic ways from a late Phanerozoic perspective (e.g., microbial mats may have facilitated a strong geochemical gradient across the sediment-water interface). Therefore, the discovery of the relatively large traces of Lamonte trevallis from the Ediacaran Shibantan Member of the Denying Formation (~551–541 Ma) in the Yangtze Gorges area of South China provides a unique opportunity to study early bioturbators. These trace fossils are large enough and have sufficient compositional contrast (relative to the matrix) for in situ analysis via X-ray computed tomography (CT) and microcomputed tomography (microCT). Each analytical method has its own advantages and disadvantages. CT scans can image larger specimens, but cannot adequately resolve small features of interest. MicroC scans can achieve higher resolution, but can only be used with small samples and may involve more post-processing than CT scans. As demonstrated in this study, X-ray CT and microCT in combination with other 3D imaging techniques and resources have the potential to resolve the 3D morphology of Ediacaran trace fossils. A new Volumetric Bioturbation Intensity (VBI) is also proposed, which quantifies whole rock bioturbation using 3D analysis of subsurface traces. Combined with the ability to examine trace fossils in situ, the VBI can enhance our view of ancient ecologies and life’s enduring relationship with sediments.
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    Biogeochemical Cycling and Paleoenvironmental Reconstructions of the Toarcian Oceanic Anoxic Event from Western North America
    Them II, Theodore Roland (Virginia Tech, 2016-08-02)
    The Toarcian Oceanic Anoxic Event (T-OAE; ~183 million years ago) represents an interval during the Mesozoic when the emplacement of the Karoo-Ferrar Large Igneous Province (LIP) is thought to have resulted in significant environmental change. Associated with this interval was the widespread deposition of organic-rich sediments, carbon cycle and seawater chemistry changes, global warming, the development of marine anoxia, and major extinction events. The majority of studies of this event that have documented these responses have come from the Boreal and Tethyan regions of Europe, thus casting some doubt to the regional versus global significance of the event. Thus my dissertation has sought to reconstruct biogeochemical and paleoenvironmental changes across the T-OAE from a sedimentary succession that was deposited on the margins of a different ocean basin away from the well-studied European successions. Specifically, I have studied the chemostratigraphy of the Fernie Formation of the Western Canada Sedimentary Basin (WCSB), which was deposited on the eastern margin of the Panthalassa Ocean. The Toarcian carbon isotope excursions (CIEs) in the WCSB confirm that these features are global phenomena. I have suggested a new driver for small-scale CIEs observed during the event: the release of wetland-derived methane during progressive global warming. The osmium isotope record and numerical modeling of the osmium cycle suggests that continental weathering rates increased during the T-OAE by 230 – 540%. Rhenium abundance data also suggests that the increased geographic extent of marine anoxia during the T-OAE caused a global drawdown in the seawater rhenium inventory. Iron speciation data are used to reconstruct redox conditions within the WCSB, which suggest ferruginous conditions developed in the more distal locations at the onset of the T-OAE before returning to euxinic (anoxic and sulfidic) conditions. This is likely related to enhanced pyrite burial on a global scale, which caused the drawdown of the seawater sulfate inventory, thus limiting pyrite formation in the distal locations. The proximal setting remained euxinic across the T-OAE, and in all locations the iron speciation data suggest anoxic conditions persistent well after the interval that has been traditionally called the end of the T-OAE.
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    Biomineralization by particle attachment in early animals
    Gilbert, Pupa U. P. A.; Porter, Susannah M.; Sun, Chang-Yu; Xiao, Shuhai; Gibson, Brandt M.; Shenkar, Noa; Knoll, Andrew H. (National Academy of Sciences, 2019-08-19)
    Crystallization by particle attachment (CPA) of amorphous precursors has been demonstrated in modern biomineralized skeletons across a broad phylogenetic range of animals. Precisely the same precursors, hydrated (ACC-H₂O) and anhydrous calcium carbonate (ACC), have been observed spectromicroscopically in echinoderms, mollusks, and cnidarians, phyla drawn from the 3 major clades of eumetazoans. Scanning electron microscopy (SEM) here also shows evidence of CPA in tunicate chordates. This is surprising, as species in these clades have no common ancestor that formed a mineralized skeleton and appear to have evolved carbonate biomineralization independently millions of years after their late Neoproterozoic divergence. Here we correlate the occurrence of CPA from ACC precursor particles with nanoparticulate fabric and then use the latter to investigate the antiquity of the former. SEM images of early biominerals from Ediacaran and Cambrian shelly fossils show that these early calcifiers used attachment of ACC particles to form their biominerals. The convergent evolution of biomineral CPA may have been dictated by the same thermodynamics and kinetics as we observe today.
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    Can NanoSIMS probe quantitatively the geochemical composition of ancient organic-walled microfossils? A case study from the early Neoproterozoic Liulaobei Formation
    Delarue, Frederic; Robert, Francois; Tartese, Romain; Sugitani, Kenichiro; Tang, Qing; Duhamel, Remi; Pont, Sylvain; Xiao, Shuhai (2018-07)
    Assessing the biogenicity of Precambrian putative remnants of life requires solid criteria. Among possible criteria, searching for evidence of pristine biological signatures and identifying various biological organic matter (OM) precursors in close association with microfossil morphology are of interest. Nano-scale Secondary Ion Mass Spectrometry (NanoSIMS) can provide a quantitative geochemical proxy at the scale of the individual microfossil but its use has remained limited because of potential analytical biases related to matrix effects and microtopography that may result in inaccurate NanoSIMS-derived measurement. No study so far has assessed whether these potential analytical biases were strong enough to preclude any identification of pristine OM degradation products and of organic precursors in ancient sediments. In this study, we characterized the geochemical composition of organic-walled microfossils from the early Neoproterozoic Liulaobei Formation in North China using NanoSIMS. The (CH-)-C-12/C-12(2)- ionic ratio allows us to distinguish filament from spheroid acritarchs, revealing the co-occurrence of two distinct pristine OM signatures that differ by their H and/or aliphatic contents. In addition, NanoSIMS data show that morphological degradation was tightly linked to a loss of H and/or hydrogenated organic compounds in spheroid acritarchs. In contrast, in situ N/C atomic ratios are homogeneous across all organic-walled microfossils studied. Although highly coherent with Proterozoic N/C atomic ratios from the literature, such homogeneity may alternatively reflect (i) a similar N content for different organic precursors or (ii) an extensive homogenization related to early degradation. Overall, these data obtained on microfossils from the Proterozoic Liulaobei Formation are the first to demonstrate that the quantitative capability of NanoSIMS can be used to track ancient OM precursors and to probe the effects of degradation on pristine OM. These findings open up tremendous perspectives and put forward new criteria for assessing the biogenicity of the putative early traces of life found in Archean metasediments.
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    Carbon cycle changes during the end-Marjuman (Cambrian) extinction in the Southern Appalachians
    Gerhardt, Angela Mae (Virginia Tech, 2014-05-16)
    The late Cambrian-early Ordovician transition contains several trilobite extinctions. The first of these extinctions (the end-Marjuman) is thought to coincide with the Steptoean Positive Carbon Isotope Excursion or SPICE, a large and rapid excursion in the marine carbon isotope record. This excursion, which is expressed in sedimentary successions globally, is thought to represent a large perturbation to the carbon cycle during this time. Additionally, a limited amount of carbon isotope data from the Deadwood Formation in the Black Hills of South Dakota suggests the possibility of a small negative ẟ¹³C excursion near the extinction and preceding the SPICE. Previous high-resolution biostratigraphy has identified an expanded record of extinction event within the Nolichucky Formation of the Southern Appalachians making it an excellent candidate for the study of the precise relationship between the extinction and changes in the carbon cycle. This investigation confirms the onset of the SPICE occurs at the extinction boundary however no negative ẟ¹³C excursion occurs at the extinction boundary. Further there is no systematic relationship between local facies changes and ẟ¹³C or the extinction interval across the basin, which suggests that global environmental changes were responsible for both the ẟ¹³C record and the extinction event.
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    Chuaria, Vendotaenia, and the taphonomy of the Carbonaceous Compression
    Anderson, Evan Pelzner (Virginia Tech, 2010-05-03)
    Carbonaceous Compressions are a widespread preservational style for fossils, yet their taphonomy remains poorly understood. Previous studies focusing on the taphonomy of carbonaceous compressions have primarily looked at exceptionally preserved faunas in plane view. The precious nature of these fossils leaves destructive techniques of analysis out of the question, but these techniques are necessary if the taphonomy of carbonaceous compressions is to be deciphered. This study analyzes Neoproterozoic carbonaceous compressions from the Yangtze Gorges area in order to address this issue. Chuaria fossils from the Jiulongwan, Sixi, and Sifangtan sections of the Doushantuo Formation and Vendotaenia fossils from the Wuhe and Miaohe sections of the Denying Formation are microchemically analyzed in both plane view and cross section in order to gain a greater understanding of the makeup of carbonaceous compressions. Results confirm and elaborate on previous studies. Likely clay coats are detected on some Chuaria specimens, while they are absent on less thermally mature specimens. Evidence for sulfate reduction in association with carbonaceous compressions is found. Sulfur enrichment, rather than clay coats, is found in association with Vendotaenia fossils. These observations lead to the hypothesis that while organic remains require a very precise set of taphonomic conditions in order to be preserved as carbonaceous compressions, there may be more than one set of conditions that allow for preservation. More studies of a greater taxonomic and taphonomic range of carbonaceous compressions are needed, however, if the mechanisms which control this preservational pathway are to be fully understood.
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    Composition Systematics in the Exoskeleton of the American Lobster, Homarus americanus and Implications for Malacostraca
    Mergelsberg, Sebastian T.; Ulrich, Robert N.; Xiao, Shuhai; Dove, Patricia M. (Frontiers, 2019-04-05)
    Studies of biominerals from the exoskeletons of lobsters and other crustaceans report chemical heterogeneities across disparate body parts that have prevented the development of composition-based environmental proxy models. Anecdotal evidence, however, suggests underlying composition systematics may exist in the mineral component of this biocomposite material. We test this idea by designing a protocol to separately extract the mineral [amorphous calcium carbonate (ACC) plus calcite] and organic (chitin plus protein) fractions of the exoskeleton. The fractions were analyzed by ICP-OES and other wet chemistry methods to quantify Mg, Ca, and P contents of the bulk, mineral, and organic matrix. Applying this approach to the exoskeleton for seven body parts of the American lobster, Homarus americanus, we characterize the chemical composition of each fraction. The measurements confirm that Mg, P, and Ca concentrations in lobster exoskeletons are highly variable. However, the ratios of Mg/Ca and P/Ca in the mineral fraction are constant for all parts, except the chelae (claws), which are offset to higher values. By normalizing concentrations to obtain P/Ca and Mg/Ca, we show that all body parts conserve P/Mg to 1.27 +/- 0.30. The findings suggest lobsters hold promise as a novel class of animals that record composition systematics within their CaCO3 biominerals. Parallel structural analyses of the bulk samples confirm a large proportion of ACC relative to calcite in the mineral fractions for each body part using high-energy X-ray diffraction and PDF analysis. There is no evidence for a phosphate phase. Returning to compositions reported for other marine (crab, lobster, and marine shrimp) and terrestrial (pillbug) crustaceans, we find evidence for similar Mg/Ca and P/Ca patterns in these organisms. The relationships provide a basis for developing new proxies for environmental reconstructions using animals from the class Malacostraca and provide insights into how composition may be optimized to meet functional requirements of the mineral fraction in exoskeletons. Compositional variability, and hence differential solubility, suggests a thermodynamic basis for the taphonomic bias that is observed in the fossil record.
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    Compressed Sensing based Micro-CT Methods and Applications
    Sen Sharma, Kriti (Virginia Tech, 2013-06-12)
    High-resolution micro computed tomography (micro-CT) offers 3D image resolution of 1 um for non-destructive evaluation of various samples. However, the micro-CT performance is limited by several factors. Primarily, scan time is extremely long, and sample dimension is restricted by the x-ray beam and the detector size. The latter is the cause for the well-known interior problem. Recent advancement in image reconstruction, spurred by the advent of compressed sensing (CS) theory in 2006 and interior tomography theory since 2007, offers great reduction in the number of views and an increment in the volume of samples, while maintaining reconstruction accuracy. Yet, for a number of reasons, traditional filtered back-projection based reconstruction methods remain the de facto standard on all manufactured scanners. This work demonstrates that CS based global and interior reconstruction methods can enhance the imaging capability of micro-CT scanners. First, CS based few-view reconstruction methods have been developed for use with data from a real micro-CT scanner. By achieving high quality few-view reconstruction, the new approach is able to reduce micro-CT scan time to up to 1/8th of the time required by the conventional protocol. Next, two new reconstruction techniques have been developed that allow accurate interior reconstruction using just a limited number of global scout views as additional information. The techniques represent a significant progress relative to the previous methods that assume a fully sampled global scan. Of the two methods, the second method uses CS techniques and does not place any restrictions on scanning geometry. Finally, analytic and iterative reconstruction methods have been developed for enlargement of the field of view for the interior scan with a small detector. The idea is that truncated projections are acquired in an offset detector geometry, and the reconstruction procedure is performed through the use of a weighting function / weighted iteration updates, and projection completion. The CS based reconstruction yields the highest image quality in the numerical simulation. Yet, some limitations of the CS based techniques are observed in case of real data with various imperfect properties. In all the studies, physical micro-CT phantoms have been designed and utilized for performance analysis. Also, important guidelines are suggested for future improvements.
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    Constraining the Ediacaran-Cambrian boundary in South China using acanthomorphic acritarchs and Palaeopascichnus fossils
    Odonnell, Kenneth H. (Virginia Tech, 2013-06-14)
    The Ediacaran-Cambrian boundary is arguably the most critical transition in Earth history. This boundary is currently defined by the GSSP (Global Stratotype Section and Point) at Fortune Head (Newfoundland, Canada) at a point that was once regarded as the first appearance of the branching trace fossil Treptichnus pedum. However, T. pedum has been subsequently found below the GSSP, and its distribution is largely restricted to sandstone facies where chemostratigraphic correlation tools are difficult to apply. Thus, the stratigraphic value of the Fortune Head GSSP has come under scrutiny, and there is a need to search for an alternative definition of this boundary using other biostratigraphic criteria. Investigations of acanthomorphic acritarchs in basal Cambrian strata of South China suggest that these microfossils may provide an appropriate biostratigraphic marker for the Ediacaran-Cambrian boundary, because of their wide distribution in chert-phosphorite layers intercalated with carbonates and shales, thus allowing their biostratigraphic occurrences to be calibrated with small shelly fossil (SSF) biostratigraphy and carbon isotope chemostratigraphy. Acanthomorphic acritarchs of the Asteridium-Heliosphaeridium-Comasphaeridium (AHC) assemblage zone have been identified at 11 localities in chert-phosphorite layers in the basal Cambrian Yanjiahe, Liuchapo, and Niutitang formations. These localities span a 300 km transect in South China, with depositional environments varying from a shallow carbonate shelf, to an outer shelf-slope transition and an open ocean slope-basin. The Ediacaran-Cambrian boundary can be bracketed between basal Cambrian AHC assemblage and the upper Ediacaran fossils, Horodyskia minor and Palaeopascichnus jiumenensis (HmPj assemblage zone), which occur in the lower Liuchapo Formation. There is no stratigraphic overlap between the AHC and HmPj assemblage zones. Available data show that the AHC assemblage zone is in close stratigraphic proximity with the basal Cambrian SSFs and a negative "13C excursion near the Ediacaran-Cambrian boundary. Thus, in South China, the first occurrences of AHC assemblage microfossils and last occurrences of HmPj fossils can effectively "bookend" the Ediacaran-Cambrian boundary to fine-scale resolution (down to 0.5 m in present study) in the Yanjiahe and Liuchapo formations. We propose that the AHC assemblage can be used to redefine the Ediacaran-Cambrian boundary and this proposal should be tested with detailed acanthomorph biostratigraphy beyond South China.
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    Contributions and New Methods in Paleontology: Geochemical, Ultrastructural, and Microstructural Characterization of Archean, Proterozoic, and Phanerozoic Fossils
    Schiffbauer, James Daniel (Virginia Tech, 2009-05-01)
    Over the past decade, the study of organismal or systematic paleobiology has been progressing into a new age of digital paleobiology, in which advanced instrumentation is utilized for primary data collection and analyses. Having been progressing throughout this field of study, advanced instruments–commonly electron- and ion- microbeam equipment–have been employed for numerous fossils over the entire range of geologic time, from microfossils to macrofossils and from the Archean (beginning at 3800 Ma) to the Cenozoic (ending at the recent). These techniques, predominantly used for geochemical, morphological, and ultra-/micro-structural analyses, have unlocked an incredible amount of detail contributing to our understanding of fossil organisms, their modes of life, and their biological affinities. But further, as these techniques continue to grow and become popularized in various fields of paleobiological study, they are certain to significantly progress our comprehension and knowledge of the evolution of life through time. While the chapters presented in this dissertation may not have a unifying theme in terms of a distinct fossil organism or specific time in Earth's history, furthering the use of electron- and ion- microbeam instrumentation and expanding the paleo-genres to which digital paleobiological approaches may be applied encompasses the fundamental intention of my research. Two of the chapters reported here focus on the geochemical, ultrastructural, and microstructural investigation of organic-walled microfossils, or acritarchs, from the Paleoproterozoic (2500–1600 Ma) and Mesoproterozoic (1600–1000 Ma), using a range of advanced instrumentation including field emission scanning electron microscopy, transmission electron microscopy, laser Raman spectroscopy, electron microprobe, secondary ion mass spectroscopy, and focused ion beam electron microscopy. Moving into the Neoproterozoic (1000–542 Ma), the third primary research chapter utilizes field emission scanning electron microscopy for high-resolution, high magnification imaging and quantitative evaluation of an entire fossil assemblage–from acritarchs and algal fossils to the earliest metazoan embryos. This study was conducted in an effort to examine and describe the phosphatization taphonomic window of the Doushantuo Formation of South China, which is a prime example of exceptional preservation. Finally, the fourth primary research chapter reported here uses field emission scanning electron microscopy and environmental scanning electron microscopy in a field of paleobiology in which advanced instrumentation has been highly underutilized – predatory-prey interactions. This research examines microstructural characteristics of predatory drill holes in both modern and fossil organisms in an attempt to mitigate the identification of predation traces in the fossil record.
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    Contributions to Exceptional Fossil Preservation
    Muscente, Anthony Drew (Virginia Tech, 2016-04-21)
    Exceptionally preserved fossils—or fossils preserved with remains of originally non-biomineralized (i.e. soft) tissues—constitute a key resource for investigating the history of the biosphere. In comparison to fossils of biomineralized skeletal elements, which represent the majority of the fossil record but only a fraction of the total diversity that existed in the past, exceptionally preserved fossils are comparatively rare because soft tissues are rapidly destroyed in typical depositional environments. Assemblages of such fossils, nonetheless, have received special attention among scientists in multiple fields of Earth and life sciences because they represent relatively 'complete' windows to past life. Through such windows, researchers are able to reconstruct original biological features (e.g. soft tissue anatomies) of extinct organisms and to describe the structures and compositions of ancient soft-bodied paleocommunities. To accomplish these goals, however, researchers must incorporate background information regarding the pre- and post-burial histories of exceptionally preserved fossils. In this context, my dissertation focuses on the environmental settings, diagenetic conditions, geomicrobiological activities, and weathering processes, which influence the conservation of original biological features within exceptionally preserved fossils and control their occurrences in time and space. An improved understanding of these critical factors involved in exceptional fossil preservation will ultimately our advance our knowledge regarding the history of the biosphere and the Earth system as a whole. Each chapter of original research in this dissertation includes an innovative and distinct approach for studying exceptional fossil preservation. The second chapter describes environmental and geologic overprints in the exceptional fossil record, as revealed by a comprehensive statistical meta-analysis of a global dataset of exceptionally preserved fossil assemblages. Moving from global to specimen-based perspectives, the second and third chapters focus on minerals (products of geomicrobioloigcal, diagenetic, and weathering processes) and carbonaceous materials replicating exceptionally preserved fossils. The third chapter examines the causes of preservational variations observed among organophosphatic tubular shelly Sphenothallus fossils in the lower Cambrian of South China using an experimental approach. (Although Sphenothallus is not an exceptionally preserved fossil sensu stricto, its conservation of original organic matrix tissues in South China provides key insights into the preservation of carbonaceous material within fossils.) Lastly, the fourth chapter presents data acquired using various in situ nanoscale analytical techniques to test the hypothesis that microstructures within exceptionally preserved microfossils of the Ediacaran Doushantuo Formation of South China are some of the oldest putative cylindrical siliceous demosponge spicules in the fossil record. Collectively, these chapters describe environmental, authigenic, diagenetic, and weathering processes that affect exceptional fossil preservation, and highlight innovative methods and approaches for testing major paleobiologic and geobiologic hypotheses regarding exceptionally preserved fossils.
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    Contributions to late Ediacaran geobiology in South China and southern Namibia
    Meyer, Michael B. (Virginia Tech, 2013-06-11)
    The Ediacaran, particularly the late Ediacaran-Cambrian (E-") boundary (551-541 Ma), has been noted as a major time of biological and biochemical transition, including: the rise of predators, the emergence of skeletons, the radiation of bioturbators, and large fluctuations in oceanic chemical conditions (e.g. anoxia/euxinia/ferruginia) with emerging data  suggesting that it is not until the late Ediacaran Period (551-542 Ma) when pervasive oxidation of the deep oceans occurred. In addition, the study of Ediacaran paleobiology involves numerous factors, including but not limited to, restricted amount of outcrops, taphonomic biases, metamorphism, enigmatic phylogenetic affinities, non-actualistic atmospheric and oceanic conditions, and unusual taphonomic windows, making it difficult to assign order to the myriad types of fossils (both body and trace) found during this time . It is with these problems in mind that my dissertation focuses on taphonomic questions in the late Ediacaran. We observe fossil through the taphonomic window and thereby colors all aspects of the study of that organism. By understanding how an organism was preserved we can learn about other factors affecting it, such as their taxonomic affinities, paleoecology, and morphology. All of these factors were examined through taphonomic investigations and this can be seen in the analytical path through my chapters. In chapter two I examined the preservation of trace fossils in the late Ediacaran. In chapter three I analyzed the taphonomy of an exceptionally preserved Ediacaran fossil to solve its affinity, tubular trace or tubular soft-bodied fossil. In chapter four, understanding the nature of exceptional preservation, I investigated the taphonomy of an Ediacara fossil in coarse grained sediments. In chapter five, I combined the findings of the previous chapters and applied that knowledge to analyze the in situ three dimensional morphology of an Ediacara fossil. Each of the projects presented pairs new technologic methodologies (SEM, BSE-Z, EDS, Raman, microCT) with traditional paleontological and petrologic investigations (field work, hand sample, and petrographic thin-section analysis). The result of this union of Old and New are findings that expand our understanding of these earliest multicellular organisms, their modes of life, and their biological affinities.
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    Contributions to the Neoproterozoic Geobiology
    Shen, Bing (Virginia Tech, 2007-11-29)
    This thesis makes several contributions to improve our understanding of the Neoproterozoic Paleobiology. In chapter 1, a comprehensive quantitative analysis of the Ediacara fossils indicates that the oldest Ediacara assemblage "the Avalon assemblage" already encompassed the full range of Ediacara morphospace. A comparable morphospace range was occupied by the subsequent White Sea and Nama assemblages, although it was populated differently. In contrast, taxonomic richness increased in the White Sea assemblage and declined in the Nama assemblage. The Avalon morphospace expansion mirrors the Cambrian explosion, and both may reflect similar underlying mechanisms. Chapter 2 describes problematic macrofossils collected from the Neoproterozoic slate of the upper Zhengmuguan Formation in North China and sandstone of the Zhoujieshan Formation in Chaidam. Some of these fossils were previously interpreted as animal traces. Our study of these fossils recognizes four genera and five species. None of these taxa can be interpreted as animal traces. Instead, they are problematic body fossils of unresolved phylogenetic affinities. Chapter 3 reports stable isotopes of the Zhamoketi cap dolostone atop the Tereeken diamictite in the Quruqtagh area, eastern Chinese Tianshan. Our new data indicate that carbonate associated sulfate (CAS) abundance decreases rapidly in the basal cap dolostone and δ34SCAS composition varies between +9â ° and +15â ° in the lower 2.5 m. In the overlying interval, CAS abundance remains low while δ34SCAS rises ~5â ° and varies more widely between +10â ° and +21â °. δ34Spy is typically greater than δ34SCAS measured from the same samples. We propose that CAS and pyrite were derived from two isotopically distinct reservoirs in a chemically stratified basin. Chapter 4 studies δ13C, δ18O, δ34SCAS, and δ34Spy of the Zhoujieshan cap carbonate that overlies the Ediacaran Hongtiegou glaciation. The Zhoujieshan cap dolostone shows positive δ13C values (0 â 1.7â °). δ34SCAS shows rapid stratigraphic variations from +13.9 to +24.1â °, probably due to relatively low oceanic sulfate concentrations. δ34Spy shows a steady stratigraphic trend. Thus, the δ34SCAS and δ34Spy trends are decoupled from each other. The decoupling of δ34SCAS and δ34Spy trends suggests that CAS and pyrite were derived from different sulfur pools, which were probably due to the postglacial basin stratification.
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    Contributions to the Proterozoic and Cambrian Evolution of Eukaryotes
    Dong, Lin (Virginia Tech, 2007-03-27)
    This thesis makes several contributions to improve our understanding of Proterozoic-Cambrian evolution of eukaryote life. Chapter 1 provides, for the first time, a quantitative characterization of the evolutionary trends of Proterozoic macroalgae. The analysis reveals that morphological disparity of Paleoproterozoic macroalgae was low but increased in the Mesoproterozoic and Ediacaran, with a plateau in between. There was also a significant increase in thallus surface/volume ratio and maximum canopy height of the Ediacaran macroalgal communities. The prolonged plateau between the Mesoproterozoic and Ediacaran may be related to either nutrient stress or the absence of animal grazing pressure. The Ediacaran increase in surface/volume ratio and morphological complexity may have been driven by decreasing pCO₂ levels and increasing animal grazing pressure. Chapter 2 presents a systematic re-examination of the carbonaceous compression fossils Protoarenicola baiguashanensis Wang, 1982, Pararenicola huaiyuanensis Wang, 1982, and Sinosabellidites huainanensis Zheng, 1980, from the early Neoproterozoic Liulaobei and Jiuliqiao formations in northern Anhui, North China. These fossils were previously interpreted as worm-like metazoans. Our study reveals new morphological features that weaken the metazoan interpretation. Instead, the new data indicate that these fossils can be alternatively interpreted as erect epibenthic organisms, possibly coenocytic algae. Chapter 3 examines two important eukaryote fossils: Horodyskia Yochelson and Fedonkin, 2000, and Palaeopascichnus Palij, 1976, from the upper Ediacaran chert of the Liuchapo Formation in central Guizhou, South China. These exceptionally preserved fossils offer us a unique opportunity to investigate their body constructions and affinities. The morphologies of Horodyskia and Palaeopascichnus support a phylogenetic relationship with agglutinated foraminifers, shedding new light on the divergence of bikont eukaryotes, the rise of rhizarians, and the ecological importance of heterotrophic eukaryotes in Proterozoic ecosystems. Chapter 4 focuses on Cambrian microfossils that represent the primary producers' cyanobacteria and eukaryotic phytoplankton (acritarchs). Careful investigation of the basal Cambrian Yanjiahe Formation in the Yangtze Gorges area and the Yurtus Formation in the Aksu area revealed abundant acanthomorphic acritarchs, clustered coccoidal microfossils, filamentous cyanobacteria, and tubular microfossils. This study confirms previous stratigraphic correlation between the Yanjiahe and Yurtus formations and suggests that animals and phytoplankton radiated in tandem during the Cambrian explosion.
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    Cracking the superheavy pyrite enigma: possible roles of volatile organosulfur compound emission
    Lang, Xianguo; Zhao, Zhouqiao; Ma, Haoran; Huang, Kangjun; Li, Songzhuo; Zhou, Chuanming; Xiao, Shuhai; Peng, Yongbo; Liu, Yonggang; Tang, Wenbo; Shen, Bing (2021-10)
    The global deposition of superheavy pyrite (pyrite isotopically heavier than coeval seawater sulfate in the Neoproterozoic Era and particularly in the Cryogenian Period) defies explanation using the canonical marine sulfur cycle system. Here we report petrographic and sulfur isotopic data (delta S-34(py)) of superheavy pyrite from the Cryogenian Datangpo Formation (660-650 Ma) in South China. Our data indicate a syndepositional/early diagenetic origin of the Datangpo superheavy pyrite, with S-34-enriched H2S supplied from sulfidic (H2S rich) seawater. Instructed by a novel sulfur-cycling model, we propose that the emission of S-34-depleted volatile organosulfur compounds (VOSC) that were generated via sulfide methylation may have contributed to the formation of S-34-enriched sulfidic seawater and superheavy pyrite. The global emission of VOSC may be attributed to enhanced organic matter production after the Sturtian glaciation in the context of widespread sulfidic conditions. These findings demonstrate that VOSC cycling is an important component of the sulfur cycle in Proterozoic oceans.
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    Cryptic terrestrial fungus-like fossils of the early Ediacaran Period
    Gan, Tian; Luo, Taiyi; Pang, Ke; Zhou, Chuanming; Zhou, Guanghong; Wan, Bin; Li, Gang; Yi, Quiru; Czaja, Andrew D.; Xiao, Shuhai (Nature Research, 2021)
    The colonization of land by fungi had a significant impact on the terrestrial ecosystem and biogeochemical cycles on Earth surface systems. Although fungi may have diverged ~1500–900 million years ago (Ma) or even as early as 2400 Ma, it is uncertain when fungi first colonized the land. Here we report pyritized fungus-like microfossils preserved in the basal Ediacaran Doushantuo Formation (~635 Ma) in South China. These micro-organisms colonized and were preserved in cryptic karstic cavities formed via meteoric water dissolution related to deglacial isostatic rebound after the terminal Cryogenian snowball Earth event. They are interpreted as eukaryotes and probable fungi, thus providing direct fossil evidence for the colonization of land by fungi and offering a key constraint on fungal terrestrialization.
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    Deposition or diagenesis? Probing the Ediacaran Shuram excursion in South China by SIMS
    Cui, Huan; Kitajima, Kouki; Orland, Ian J.; Xiao, Shuhai; Baele, Jean-Marc; Kaufman, Alan J.; Denny, Adam; Zhou, Chuanming; Spicuzza, Michael J.; Fournelle, John H.; Valley, John W. (Elsevier, 2021-11)
    The Ediacaran Period (ca. 635–541 Ma) witnessed the earliest paleontological evidence for macroscopic animals (i.e., Ediacara biota) and geochemical observations of the largest carbon cycle anomaly in Earth history (i.e., Shuram Excursion, SE). Numerous hypotheses have been proposed for the origins of the SE, ranging from primary seawater anomaly to syn- or post-depositional diagenesis. Despite intensive geochemical and theoretical work published in the past decade, empirical evidence that is strictly based on fundamental petrographic results at the micrometer scale is still limited. To evaluate depositional compositions and diagenetic effects on samples from the SE, we investigated the EN3 interval in the Doushantuo Formation of South China via integrated cathodoluminescence (CL), secondary ion mass spectrometry (SIMS), and scanning electron microscope (SEM). Detailed petrographic observations reveal that the EN3 limestone is dominated by calcite microspar, with minor but variable amounts of disseminated zoned dolomite crystals. The former likely formed via neomorphism of depositional micrite, while the latter was the result of progressive post-depositional dolomitization. The mean values of paired SIMS δ13Ccalcite and δ13Cdolomite compositions are indistinguishable in each sample and consistent with published micro-drilled bulk-powder δ13C values, which we interpret to represent depositional “background” signals of seawater dissolved inorganic carbon (DIC). We also observed μm-scale variability of SIMS δ13C data that may reflect a variable diagenetic overprint after deposition. Our integrated petrographic and geochemical results are consistent with a depositional origin of the SE and provide little evidence for the hypothesized isotope alteration by meteoric and mixing-zone diagenesis or late burial diagenesis. In light of this study, we propose that the SE indeed represents a marine carbon cycle anomaly that bears a close temporal link to the Ediacaran surface environment.