Overall, I employ a multidisciplinary approach—integrating biology, paleontology, geochemistry, sedimentology, stratigraphy, and paleogeography—to study rocks and fossils from the critcal transitional Cambrian-Ediacaran interval (635 to 541 millions years ago) of Earth history. This interval--which notably witnessed the radiation of animal life--is marked by dramatic climatic and environmental changes. My work examines the evolutionary history of the earliest animals, and investigates the relationship between their radiation and the climatic/environmental changes that occurred during the Ediacaran-Cambrian interval. Supervisors: Shuhai Xiao Address: Department of Geological Sciences Jackson School of Geosciences The University of Texas at Austin 1 University Station C1100 Austin, Texas 78712 Office: JGB 3.314B
Konservat-Lagerstätten—deposits with exceptionally preserved fossils—vary in abundance across geo... more Konservat-Lagerstätten—deposits with exceptionally preserved fossils—vary in abundance across geographic and stratigraphic space due to paleoenvironmental heterogeneity. While oceanic anoxic events (OAEs) may have promoted preservation of marine lagerstätten, the environmental controls on their taphonomy remain unclear. Here, we provide new data on the mineralization of fossils in three Lower Jurassic Lagerstätten—Strawberry Bank (UK), Ya Ha Tinda (Canada), and Posidonia Shale (Germany) —and test the hypothesis that they were preserved under similar conditions. Biostratigraphy indicates that all three Lagerstätten were deposited during the Toarcian OAE (TOAE), and scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) show that each deposit contains a variety of taxa preserved as phosphatized skeletons and tissues. Thus, despite their geographic and paleoenvironmental differences, all of these Lagerstätten were deposited in settings conducive to phosphatiz...
Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138, USA Departme... more Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138, USA Department of Geological Sciences, University of Missouri, Columbia, MO 65211, USA Department of Chemical and Physical Sciences, University of Toronto Mississauga, Mississauga, ON L5L 1C6, Canada AECOM, Germantown, MD 20876, USA Department of Geological Sciences, Stanford University, Stanford, CA 94305, USA Geophysical Laboratory, Carnegie Institution for Science, Washington, DC 20015, USA Department of Geosciences, Virginia Tech, Blacksburg, VA 24061, USA School of Biological, Earth, and Environmental Sciences, University College Cork, Cork, T23 TK30, Ireland Department of Geological Sciences, SUNY Geneseo, Geneseo, NY 14454, USA Department of Geosciences, University of Montana, Missoula, MT 59812, USA
Because all organisms on Earth consist of complex organic biomolecules, any fossil may contain or... more Because all organisms on Earth consist of complex organic biomolecules, any fossil may contain organic matter. Indeed, organic matter represents one of the most common fossil materials, occurring within skeletal elements (shells and bones) and making up organically preserved microorganisms, plants, and exceptionally preserved eukaryotes (i.e., animals and algae with remains of non-biomineralized tissues).
For most of the history of science, data-driven discovery has been difficult and timeconsuming: a... more For most of the history of science, data-driven discovery has been difficult and timeconsuming: a lifetime of meticulous data collection and thoughtful synthesis was required to recognize previously hidden, higher-dimensional trends in multivariate data. Recognition of processes such as biological evolution by natural selection (1,2), continental evolution by plate tectonics (3,4), atmospheric and ocean oxygenation by photosynthesis (5,6), and climate change (7,8) required decades of integrated data synthesis preceding the discovery and acceptance of critical Earth phenomena. However, we stand at the precipice of a unique opportunity: to dramatically accelerate scientific discovery by coupling hard-won data resources with advanced analytical and visualization techniques (9,10). Today, Earth and life sciences are generating a multitude of data resources in numerous subdisciplines. Integration and synthesis of these diverse data resources will lead to an abductive, data-driven approac...
The hyperthermal events of the Cenozoic, including the Paleocene-Eocene Thermal Maximum, provide ... more The hyperthermal events of the Cenozoic, including the Paleocene-Eocene Thermal Maximum, provide an opportunity to investigate the potential effects of climate warming on marine ecosystems. Here, we examine the shallow benthic marine communities preserved in the late Cretaceous to Eocene strata on the Gulf Coastal Plain (United States). In stark contrast to the ecological shifts following the end-Cretaceous mass extinction, our data show that the early Cenozoic hyperthermals did not have a long-term impact on the generic diversity nor composition of the Gulf Coastal Plain molluscan communities. We propose that these communities were resilient to climate change because molluscs are better adapted to high temperatures than other taxa, as demonstrated by their physiology and evolutionary history. In terms of resilience, these communities differ from other shallow-water carbonate ecosystems, such as reef communities, which record significant changes during the early Cenozoic hypertherma...
Abstract Major radiations of microscopic and macroscopic eukaryotes occurred respectively in the ... more Abstract Major radiations of microscopic and macroscopic eukaryotes occurred respectively in the early and middle Ediacaran Period. Various hypotheses have been proposed to attribute these evolutionary events to changes in ocean redox conditions. To date, published models of the Ediacaran ocean in South China have largely been focused on the Upper and Middle Yangtze blocks, as opposed to the slope and basin sections of the Lower Yangtze Block, where the oldest complex macrofossils of the Ediacaran (i.e. the ~600 Ma Lantian Biota) are preserved. To achieve a holistic understanding of the record of ocean chemistry in the entire Yangtze Block, we carried out carbon (δ13Ccarb and δ13Corg) and sulfur (δ34SCAS and δ34Spyr) isotopic investigations of four new sections (i.e., Wujialing, Meishuxia, Wangfu and Zhushuwu) in the Lower Yangtze Block, in addition to one previously reported drill core at Lantian. The Doushantuo and Lantian formations at these sections represent shelf margin, upper slope, lower slope, and deep basin environments. In conjunction with previous work on sections of the Upper and Middle Yangtze blocks, our study reconstructs the redox structure of the entire Ediacaran ocean in South China. The new results show profound δ13Ccarb negative excursions in the slope and deep basinal sections, which can be correlated to the EN3 and Shuram excursions at a global scale. Notably, the δ13Ccarb and δ34Spyr profiles from shelf to basinal sections show distinct spatial and temporal patterns, which can be explained by model of a redox stratified ocean during the early and middle Ediacaran Period. The finding of isotopically light (34S-depleted) pyrite from this time suggests that the oceanic chemocline extended to intermediate depth. Integrating this study with existing ecological and taphonomic data, it is likely that marine redox stratification in the early Ediacaran limited the spatial distribution of early metazoans in the Lower Yangtze Block.
Abstract The radiation of early animals involved dramatic turnover of both eukaryotic life and ma... more Abstract The radiation of early animals involved dramatic turnover of both eukaryotic life and marine environments, but the factors that drove origination and extinction of taxa remain controversial. Herein, we review current interpretations and uncertainties regarding changes in biodiversity through the Ediacaran Period. Overall, ocean oxygenation and/or ecological restructuring of ecosystems may have driven biologic turnover over time. To explore these possibilities, we provide a conceptual model, rooted in ecological theory, which holistically describes biodiversity change in terms of environmental disturbance frequency/intensity and essential resource availability. In light of this framework, we evaluate two potential drivers of turnover: evolution of mobile animals and the Shuram negative carbon isotope excursion event. Early trace-making bilaterians—acting as a control on substrate stability and microbial release of dissolved organic carbon (DOC; an essential resource for osmotrophic organisms with nutrient-dependent growth)—may have played key roles in both the diversification and disappearance of the Ediacara biota. We hypothesize that evolution of grazing bilaterians prior to 555 Ma caused perturbations in sediment stability and DOC concentrations, which limited competitive exclusion and allowed for diversification of Ediacara-type organisms, as exemplified by fossils in the taxonomically and ecologically rich localities of the White Sea assemblage. The ~ 550 Ma advent of behaviors, which were capable of mixing sediments, may have subsequently pushed environmental disturbance and resource availability to critical levels that depleted biodiversity, manifested in the depauperate Nama assemblage, and ultimately drove the Ediacara biota to extinction. Concurrently, changes in global redox budgets, sea level, or oceanic DOC reservoir size during the Shuram event may have contributed to biologic turnover by causing environmental disturbances, promoting metapopulation fragmentation, and/or limiting DOC availability for osmotrophic organisms. Such possibilities should be investigated in terms of our model, which makes predictions that can be tested with integrated paleontological, geochemical, and sedimentological data.
Mass extinctions documented by the fossil record provide critical benchmarks for assessing change... more Mass extinctions documented by the fossil record provide critical benchmarks for assessing changes through time in biodiversity and ecology. Efforts to compare biotic crises of the past and present, however, encounter difficulty because taxonomic and ecological changes are decoupled, and although various metrics exist for describing taxonomic turnover, no methods have yet been proposed to quantify the ecological impacts of extinction events. To address this issue, we apply a network-based approach to exploring the evolution of marine animal communities over the Phanerozoic Eon. Network analysis of fossil co-occurrence data enables us to identify nonrandom associations of interrelated paleocommunities. These associations, or evolutionary paleocommunities, dominated total diversity during successive intervals of relative community stasis. Community turnover occurred largely during mass extinctions and radiations, when ecological reorganization resulted in the decline of one associatio...
Pyritization represents a major taphonomic pathway for exceptional preservation of soft tissues. ... more Pyritization represents a major taphonomic pathway for exceptional preservation of soft tissues. Although various Ediacaran Lagerstatten contain pyritized fossils, the controls on this taphonomic pathway prior to the Precambrian-Phanerozoic transition have only recently received significant attention, and no studies have yet thoroughly investigated environmental conditions facilitating exceptional preservation via pyritization in the early Ediacaran. Here, we investigate the preservational environments of macroscopic fossil Chuaria in the early Ediacaran Lantian Formation of South China using petrographic, electron microscopic, and geochemical data. Chuaria occur as pervasively pyritized (spheroid-shaped) globose and non-pervasively pyritized (disk-shaped) subglobose fossils in different stratigraphic intervals. Although these different stratigraphic intervals are similar in terms of total sulfur (TS, ~ 5%), subglobose Chuaria shales have greater total organic carbon (TOC, ~ 7.9%) contents than globose Chuaria shales (~ 3.6%). Additionally, petrographic observations and TOC-TS cross-plots suggest that, whereas globose fossils were preserved under suboxic bottom waters, the subglobose fossils were preserved in euxinic bottom waters. Overall, these results affirm that pyritization in the Precambrian was generally favored in organic-poor, reactive iron-rich, sulfate-rich environments probably with slow sedimentation rates. When organics were scarce and reactive iron was abundant, bacterial sulfate reduction (BSR) created hydrogen sulfide and reactive iron concentration gradients around fossils, which kinetically and thermodynamically favored rapid and pervasive pyritization, as observed in globose Chuaria. Conversely, when organics were abundant and reactive iron/sulfate availabilities were limited by widespread BSR, fossils were relatively rapidly buried beneath the BSR metabolic zone of sediment, where they were principally preserved as aluminosilicified carbonaceous fossils that were compacted into subglobose forms. Thus, preservational variations among pyritized fossils in the Ediacaran may reflect differences in sedimentary organic matter content and/or bottom water redox chemistry.
Morphology-based phylogenetic analyses support the monophyly of the Scalidophora (Kinorhyncha, Lo... more Morphology-based phylogenetic analyses support the monophyly of the Scalidophora (Kinorhyncha, Loricifera, Priapulida) and Nematoida (Nematoda, Nematomorpha), together constituting the monophyletic Cycloneuralia that is the sister group of the Panarthropoda. Kinorhynchs are unique among living cycloneuralians in having a segmented body with repeated cuticular plates, longitudinal muscles, dorsoventral muscles, and ganglia. Molecular clock estimates suggest that kinorhynchs may have diverged in the Ediacaran Period. Remarkably, no kinorhynch fossils have been discovered, in sharp contrast to priapulids and loriciferans that are represented by numerous Cambrian fossils. Here we describe several early Cambrian (~535 million years old) kinorhynch-like fossils, including the new species Eokinorhynchus rarus and two unnamed but related forms. E. rarus has characteristic scalidophoran features, including an introvert with pentaradially arranged hollow scalids. Its trunk bears at least 20 a...
Konservat-Lagerstätten—deposits with exceptionally preserved fossils—vary in abundance across geo... more Konservat-Lagerstätten—deposits with exceptionally preserved fossils—vary in abundance across geographic and stratigraphic space due to paleoenvironmental heterogeneity. While oceanic anoxic events (OAEs) may have promoted preservation of marine lagerstätten, the environmental controls on their taphonomy remain unclear. Here, we provide new data on the mineralization of fossils in three Lower Jurassic Lagerstätten—Strawberry Bank (UK), Ya Ha Tinda (Canada), and Posidonia Shale (Germany) —and test the hypothesis that they were preserved under similar conditions. Biostratigraphy indicates that all three Lagerstätten were deposited during the Toarcian OAE (TOAE), and scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) show that each deposit contains a variety of taxa preserved as phosphatized skeletons and tissues. Thus, despite their geographic and paleoenvironmental differences, all of these Lagerstätten were deposited in settings conducive to phosphatiz...
Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138, USA Departme... more Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138, USA Department of Geological Sciences, University of Missouri, Columbia, MO 65211, USA Department of Chemical and Physical Sciences, University of Toronto Mississauga, Mississauga, ON L5L 1C6, Canada AECOM, Germantown, MD 20876, USA Department of Geological Sciences, Stanford University, Stanford, CA 94305, USA Geophysical Laboratory, Carnegie Institution for Science, Washington, DC 20015, USA Department of Geosciences, Virginia Tech, Blacksburg, VA 24061, USA School of Biological, Earth, and Environmental Sciences, University College Cork, Cork, T23 TK30, Ireland Department of Geological Sciences, SUNY Geneseo, Geneseo, NY 14454, USA Department of Geosciences, University of Montana, Missoula, MT 59812, USA
Because all organisms on Earth consist of complex organic biomolecules, any fossil may contain or... more Because all organisms on Earth consist of complex organic biomolecules, any fossil may contain organic matter. Indeed, organic matter represents one of the most common fossil materials, occurring within skeletal elements (shells and bones) and making up organically preserved microorganisms, plants, and exceptionally preserved eukaryotes (i.e., animals and algae with remains of non-biomineralized tissues).
For most of the history of science, data-driven discovery has been difficult and timeconsuming: a... more For most of the history of science, data-driven discovery has been difficult and timeconsuming: a lifetime of meticulous data collection and thoughtful synthesis was required to recognize previously hidden, higher-dimensional trends in multivariate data. Recognition of processes such as biological evolution by natural selection (1,2), continental evolution by plate tectonics (3,4), atmospheric and ocean oxygenation by photosynthesis (5,6), and climate change (7,8) required decades of integrated data synthesis preceding the discovery and acceptance of critical Earth phenomena. However, we stand at the precipice of a unique opportunity: to dramatically accelerate scientific discovery by coupling hard-won data resources with advanced analytical and visualization techniques (9,10). Today, Earth and life sciences are generating a multitude of data resources in numerous subdisciplines. Integration and synthesis of these diverse data resources will lead to an abductive, data-driven approac...
The hyperthermal events of the Cenozoic, including the Paleocene-Eocene Thermal Maximum, provide ... more The hyperthermal events of the Cenozoic, including the Paleocene-Eocene Thermal Maximum, provide an opportunity to investigate the potential effects of climate warming on marine ecosystems. Here, we examine the shallow benthic marine communities preserved in the late Cretaceous to Eocene strata on the Gulf Coastal Plain (United States). In stark contrast to the ecological shifts following the end-Cretaceous mass extinction, our data show that the early Cenozoic hyperthermals did not have a long-term impact on the generic diversity nor composition of the Gulf Coastal Plain molluscan communities. We propose that these communities were resilient to climate change because molluscs are better adapted to high temperatures than other taxa, as demonstrated by their physiology and evolutionary history. In terms of resilience, these communities differ from other shallow-water carbonate ecosystems, such as reef communities, which record significant changes during the early Cenozoic hypertherma...
Abstract Major radiations of microscopic and macroscopic eukaryotes occurred respectively in the ... more Abstract Major radiations of microscopic and macroscopic eukaryotes occurred respectively in the early and middle Ediacaran Period. Various hypotheses have been proposed to attribute these evolutionary events to changes in ocean redox conditions. To date, published models of the Ediacaran ocean in South China have largely been focused on the Upper and Middle Yangtze blocks, as opposed to the slope and basin sections of the Lower Yangtze Block, where the oldest complex macrofossils of the Ediacaran (i.e. the ~600 Ma Lantian Biota) are preserved. To achieve a holistic understanding of the record of ocean chemistry in the entire Yangtze Block, we carried out carbon (δ13Ccarb and δ13Corg) and sulfur (δ34SCAS and δ34Spyr) isotopic investigations of four new sections (i.e., Wujialing, Meishuxia, Wangfu and Zhushuwu) in the Lower Yangtze Block, in addition to one previously reported drill core at Lantian. The Doushantuo and Lantian formations at these sections represent shelf margin, upper slope, lower slope, and deep basin environments. In conjunction with previous work on sections of the Upper and Middle Yangtze blocks, our study reconstructs the redox structure of the entire Ediacaran ocean in South China. The new results show profound δ13Ccarb negative excursions in the slope and deep basinal sections, which can be correlated to the EN3 and Shuram excursions at a global scale. Notably, the δ13Ccarb and δ34Spyr profiles from shelf to basinal sections show distinct spatial and temporal patterns, which can be explained by model of a redox stratified ocean during the early and middle Ediacaran Period. The finding of isotopically light (34S-depleted) pyrite from this time suggests that the oceanic chemocline extended to intermediate depth. Integrating this study with existing ecological and taphonomic data, it is likely that marine redox stratification in the early Ediacaran limited the spatial distribution of early metazoans in the Lower Yangtze Block.
Abstract The radiation of early animals involved dramatic turnover of both eukaryotic life and ma... more Abstract The radiation of early animals involved dramatic turnover of both eukaryotic life and marine environments, but the factors that drove origination and extinction of taxa remain controversial. Herein, we review current interpretations and uncertainties regarding changes in biodiversity through the Ediacaran Period. Overall, ocean oxygenation and/or ecological restructuring of ecosystems may have driven biologic turnover over time. To explore these possibilities, we provide a conceptual model, rooted in ecological theory, which holistically describes biodiversity change in terms of environmental disturbance frequency/intensity and essential resource availability. In light of this framework, we evaluate two potential drivers of turnover: evolution of mobile animals and the Shuram negative carbon isotope excursion event. Early trace-making bilaterians—acting as a control on substrate stability and microbial release of dissolved organic carbon (DOC; an essential resource for osmotrophic organisms with nutrient-dependent growth)—may have played key roles in both the diversification and disappearance of the Ediacara biota. We hypothesize that evolution of grazing bilaterians prior to 555 Ma caused perturbations in sediment stability and DOC concentrations, which limited competitive exclusion and allowed for diversification of Ediacara-type organisms, as exemplified by fossils in the taxonomically and ecologically rich localities of the White Sea assemblage. The ~ 550 Ma advent of behaviors, which were capable of mixing sediments, may have subsequently pushed environmental disturbance and resource availability to critical levels that depleted biodiversity, manifested in the depauperate Nama assemblage, and ultimately drove the Ediacara biota to extinction. Concurrently, changes in global redox budgets, sea level, or oceanic DOC reservoir size during the Shuram event may have contributed to biologic turnover by causing environmental disturbances, promoting metapopulation fragmentation, and/or limiting DOC availability for osmotrophic organisms. Such possibilities should be investigated in terms of our model, which makes predictions that can be tested with integrated paleontological, geochemical, and sedimentological data.
Mass extinctions documented by the fossil record provide critical benchmarks for assessing change... more Mass extinctions documented by the fossil record provide critical benchmarks for assessing changes through time in biodiversity and ecology. Efforts to compare biotic crises of the past and present, however, encounter difficulty because taxonomic and ecological changes are decoupled, and although various metrics exist for describing taxonomic turnover, no methods have yet been proposed to quantify the ecological impacts of extinction events. To address this issue, we apply a network-based approach to exploring the evolution of marine animal communities over the Phanerozoic Eon. Network analysis of fossil co-occurrence data enables us to identify nonrandom associations of interrelated paleocommunities. These associations, or evolutionary paleocommunities, dominated total diversity during successive intervals of relative community stasis. Community turnover occurred largely during mass extinctions and radiations, when ecological reorganization resulted in the decline of one associatio...
Pyritization represents a major taphonomic pathway for exceptional preservation of soft tissues. ... more Pyritization represents a major taphonomic pathway for exceptional preservation of soft tissues. Although various Ediacaran Lagerstatten contain pyritized fossils, the controls on this taphonomic pathway prior to the Precambrian-Phanerozoic transition have only recently received significant attention, and no studies have yet thoroughly investigated environmental conditions facilitating exceptional preservation via pyritization in the early Ediacaran. Here, we investigate the preservational environments of macroscopic fossil Chuaria in the early Ediacaran Lantian Formation of South China using petrographic, electron microscopic, and geochemical data. Chuaria occur as pervasively pyritized (spheroid-shaped) globose and non-pervasively pyritized (disk-shaped) subglobose fossils in different stratigraphic intervals. Although these different stratigraphic intervals are similar in terms of total sulfur (TS, ~ 5%), subglobose Chuaria shales have greater total organic carbon (TOC, ~ 7.9%) contents than globose Chuaria shales (~ 3.6%). Additionally, petrographic observations and TOC-TS cross-plots suggest that, whereas globose fossils were preserved under suboxic bottom waters, the subglobose fossils were preserved in euxinic bottom waters. Overall, these results affirm that pyritization in the Precambrian was generally favored in organic-poor, reactive iron-rich, sulfate-rich environments probably with slow sedimentation rates. When organics were scarce and reactive iron was abundant, bacterial sulfate reduction (BSR) created hydrogen sulfide and reactive iron concentration gradients around fossils, which kinetically and thermodynamically favored rapid and pervasive pyritization, as observed in globose Chuaria. Conversely, when organics were abundant and reactive iron/sulfate availabilities were limited by widespread BSR, fossils were relatively rapidly buried beneath the BSR metabolic zone of sediment, where they were principally preserved as aluminosilicified carbonaceous fossils that were compacted into subglobose forms. Thus, preservational variations among pyritized fossils in the Ediacaran may reflect differences in sedimentary organic matter content and/or bottom water redox chemistry.
Morphology-based phylogenetic analyses support the monophyly of the Scalidophora (Kinorhyncha, Lo... more Morphology-based phylogenetic analyses support the monophyly of the Scalidophora (Kinorhyncha, Loricifera, Priapulida) and Nematoida (Nematoda, Nematomorpha), together constituting the monophyletic Cycloneuralia that is the sister group of the Panarthropoda. Kinorhynchs are unique among living cycloneuralians in having a segmented body with repeated cuticular plates, longitudinal muscles, dorsoventral muscles, and ganglia. Molecular clock estimates suggest that kinorhynchs may have diverged in the Ediacaran Period. Remarkably, no kinorhynch fossils have been discovered, in sharp contrast to priapulids and loriciferans that are represented by numerous Cambrian fossils. Here we describe several early Cambrian (~535 million years old) kinorhynch-like fossils, including the new species Eokinorhynchus rarus and two unnamed but related forms. E. rarus has characteristic scalidophoran features, including an introvert with pentaradially arranged hollow scalids. Its trunk bears at least 20 a...
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Papers by A. D. Muscente