70 research outputs found
Paleobiological Perspectives on Early Eukaryotic Evolution
Eukaryotic organisms radiated in Proterozoic oceans with oxygenated surface waters, but, commonly, anoxia at depth. Exceptionally preserved fossils of red algae favor crown group emergence more than 1200 million years ago, but older (up to 1600–1800 million years) microfossils could record stem group eukaryotes. Major eukaryotic diversification ∼800 million years ago is documented by the increase in the taxonomic richness of complex, organic-walled microfossils, including simple coenocytic and multicellular forms, as well as widespread tests comparable to those of extant testate amoebae and simple foraminiferans and diverse scales comparable to organic and siliceous scales formed today by protists in several clades. Mid-Neoproterozoic establishment or expansion of eukaryophagy provides a possible mechanism for accelerating eukaryotic diversification long after the origin of the domain. Protists continued to diversify along with animals in the more pervasively oxygenated oceans of the Phanerozoic Eon.Earth and Planetary SciencesOrganismic and Evolutionary Biolog
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Ambient Pyrite in Precambrian Chert: New Evidence and a Theory
Ambient pyrites of two distinct types were described from middle Precambrian rocks of the Lake Superior area. A new class of this phenomenon is here described from middle Precambrian chert from western Australia. The newly found ambient pyrites are quite minute and characteristically occur in groups forming a "starburst" pattern. All three types of ambient pyrite may be explained in terms of pressure solution initiated by gas evolution from organic material attached to the pyrite. Thermal degradation of the kerogen produces the gases which, due to the impermeability of the encompassing chert, build up the pressures necessary to initiate solution. Pyrite appendages bear a striking resemblance to micro-organisms and, thus, constitute the smallest pseudofossils known.Organismic and Evolutionary Biolog
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Non-Skeletal Biomineralization by Eukaryotes: Matters of Moment and Gravity
Skeletal biomineralisation by microbial eukaryotes significantly affects the global biogeochemical cycles of carbon, silicon and calcium. Non-skeletal biomineralisation by eukaryotic cells, with precipitates retained within the cell interior, can duplicate some of the functions of skeletal minerals, e.g., increased cell density, but not the mechanical and antibiophage functions of extracellular biominerals. However, skeletal biomineralisation does not duplicate many of the functions of non-skeletal biominerals. These functions include magnetotaxis (magnetite), gravity sensing (intracellular barite, bassanite, celestite and gypsum), buffering and storage of elements in an osmotically inactive form (calcium as carbonate, oxalate, polyphosphate and sulfate; phosphate as polyphosphate) and acid-base regulation, disposing of excess hydroxyl ions via an osmotically inactive product (calcium carbonate, calcium oxalate). Although polyphosphate has a wide phylogenetic distribution among microbial eukaryotes, other non-skeletal minerals have more restricted distributions, and as yet there seems to be no definitive evidence that the alkaline earth components (Ba and Sr) of barite and celestite are essential for completion of the life cycle in organisms that produce these minerals.Organismic and Evolutionary Biolog
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Juvenile Chemical Sediments and the Long Term Persistence of Water at the Surface of Mars
Chemical sediments and the aqueous alteration products of volcanic rocks clearly indicate the presence of water, at least episodically, at the Martian surface. Compared to similar materials formed on the early Earth, however, Martian deposits are juvenile, or diagenetically under-developed. Here we examine the role of water in facilitating various diagenetic reactions and evaluate the predicted effects of time and temperature for aqueous diagenesis on Mars. Using kinetic formulations based on terrestrial sedimentary geology, we quantify the integrated effects of time and temperature for a range of possible burial and thermal histories of precipitated minerals on Mars. From this, we estimate thresholds beyond which these precipitates should have been converted to the point of non-detection in the presence of water. Surface water has been shown to be at least episodically present in recent times. Nonetheless, the integrated duration of aqueous activity recorded over geologically long intervals by hydrated amorphous silica, smectite clays and Fe-sulfate minerals suggests that where these minerals occur water did not persist much beyond their initial deposition. This geochemical conclusion converges with geomorphologic studies that suggest water limitation during the late Noachian–Hesperian peak of valley formation and a still more limited footprint of water since that time. In addition to documenting the presence of water and its chemical properties, a complete assessment of potentially habitable environments on Mars should address the timescales on which liquid water has persisted and the timing of aqueous episodes relative to major planetary events.Organismic and Evolutionary Biolog
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Systems paleobiology
Systems paleobiology seeks to interpret the history of life within the framework of Earth’s environmental history, using physiology as the conceptual bridge between paleontological and geochemical data sets. In some cases, physiological performance can be estimated directly and quantitatively from fossils—this is commonly the case for vascular plant remains. In other instances, statistical inferences about physiology can be made on the basis of phylogenetic relationships. Examples from research in paleobotany, marine micropaleontology, and invertebrate paleontology illustrate how physiological observations, experiments, and models can link biological radiations and extinctions to both long-term environmental trajectories and transient perturbations to the Earth system. The systems approach also provides a template for evaluating the habitability of other planets, not least of which is the ancient surface of Mars. Expanding physiological research motivated by concerns about our environmental future provides an increasing diversity of tools for understanding the relationship between Earth and life through time. The geologic record, in turn, provides critical input to research on contemporary global change.Organismic and Evolutionary Biolog
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The Deep History of Life
The conventional fossil record is built of hard parts—bones, shells, and decay-resistant organic tissues buried in the sediments that accumulate on floodplains, in lakes, and on the seafloor. In the 1950s, geologists first began the routine application of radioactive decay to problems of geologic age. The geologic record of microbial life is preserved in four distinct ways. First, bacteria and protists leave what we can consider an extension of the conventional fossil record: cell walls and extracellular envelopes preserved directly in sedimentary rocks. Second, microorganisms also leave molecular fossils that complement the record of morphology. Sediments transported across the seafloor interact physically with microbial mat communities, providing a third and distinctly different biological signature in sedimentary rocks. Finally, microbial populations can actually influence the composition of seawater, providing a distinct chemical signature in minerals precipitated from ancient oceans. One can hazard only broad guesses about the biological properties of early microorganisms, but one can make one key statement with confidence: early cells lived without oxygen. The plants and animals so conspicuous in our own world are evolutionary latecomers, intercalated into ecosystems that were already 3 billion years old when sponges first gained a foothold on the seafloor. The author suspects that the correct explanation will not point to physical or biological processes acting alone but, rather, will emphasize the interactions between Earth and life.Earth and Planetary Science
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Did sulfate availability facilitate the evolutionary expansion of chlorophyll a+c phytoplankton in the oceans?
During the Mesozoic Era, dinoflagellates, coccolithophorids and diatoms became prominent primary producers in the oceans, succeeding an earlier biota in which green algae and cyanobacteria had been proportionally more abundant. This transition occurred during an interval marked by increased sulfate concentration in seawater. To test whether increasing sulfate availability facilitated the evolutionary transition in marine phytoplankton, the cyanobacterium Synechococcus sp., the green alga Tetraselmis suecica and three algae containing chlorophyll a+c (the diatom Thalassiosira weissflogii, the dinoflagellate Protoceratium reticulatum and the coccolithophorid Emiliania huxleyi) were grown in media containing 1, 5, 10, 20, or 30 mm SO42−. The cyanobacterium and the green alga showed no growth response to varying [SO42−]. By contrast, the three chlorophyll a+c algae showed improved growth with higher [SO42−], but only up to 10 mm. The chlorophyll a+c algae, but not the green alga or cyanobacterium, also showed lower C:S with higher [SO42−]. When the same experiment was repeated in the presence of a ciliate predator (Euplotes sp.), T. suecica and T. weissflogii increased their specific growth rate in most treatments, whereas the growth rate of Synechococcus sp. was not affected or decreased in the presence of grazers. In a third experiment, T. suecica, T. weissflogii, P. reticulatum and Synechococcus sp. were grown in conditions approximating modern, earlier Paleozoic and Proterozoic seawater. In these treatments, sulfate availability, nitrogen source, metal availability and Pco2 varied. Monospecific cultures exhibited their highest growth rates in the Proterozoic treatment. In mixed culture, T. weissflogii outgrew other species in modern seawater and T. suecica outgrew the others in Paleozoic water. Synechococcus sp. grew best in Proterozoic seawater, but did not outgrow eukaryotic species in any treatment. Collectively, our results suggest that secular increase in seawater [SO42−] may have facilitated the evolutionary expansion of chlorophyll a+c phytoplankton, but probably not to the exclusion of other biological and environmental factors.Earth and Planetary Science
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Eumetazoan Fossils in Terminal Proterozoic Phosphorites?
Phosphatic sedimentary rocks preserve a record of early animal life different from and complementary to that provided by Ediacaran fossils in terminal Proterozoic sandstones and shales. Phosphorites of the Doushantuo Formation. South China. contain eggs, egg cases, and stereoblastulae that document animals of unspecified phylogenetic position; small fossils containing putative spicules may specifically record the presence of sponges. Microfossils recently interpreted as the preserved gastrulae of cnidarian and bilaterian metazoans can alternatively be interpreted as conventional algal cysts and/or egg Eases modified by diagenetic processes known to have had a pervasive influence on Doushantuo phosphorites. Regardless of this interpretation, evidence for Doushantuo eumetazoans is provided by millimeter-scale tubes that display tabulation and apical budding characteristic of some Cnidaria, especially the extinct tabulates. Like some Ediacaran remains, these small, benthic, colonial fossils may represent stemgroup eumetazoans or stem-group cnidarians that lived in the late Proterozoic ocean.Organismic and Evolutionary Biolog
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Covariance of Microfossil Assemblages and Microbialite Textures Across an Upper Mesoproterozoic Carbonate Platform
Early diagenetic chert nodules and beds in the upper Mesoproterozoic Angmaat (formerly Society Cliffs) Formation, Baffin and Bylot islands, preserve microfossils and primary petrofabrics that record microbial mat deposition and lithification across a range of peritidal carbonate environments. Five distinct microfossil assemblages document the distribution of mat-building and mat-dwelling populations across a gradient from restricted, frequently exposed flats to more persistently subaqueous environments. Mats built primarily by thin filamentous or coccoidal cyanobacteria give way to a series of more robust forms that show increasing assemblage diversity with decreasing evidence of subaerial exposure. Distinct fabric elements are associated with each microbial assemblage, and aspects of these petrofabrics are recognizably preserved within unsilicified carbonate in the same beds. These include some features that are distinctly geologic in nature (e.g., seafloor cements) and others that reflect microbial growth and decomposition (e.g., tufted microbialites). A particularly distinctive, micronodular fabric is here interpreted as carbonate infilling of primary voids within microbial mat structures. Such structures mark the co-occurrence of cyanobacterial photosynthesis that produced oxygen gas, filamentous mat builders that imparted the coherence necessary to trap gas bubbles, elevated carbonate saturation required to preserve void fabrics via penecontemporaneous cementation, and a relative paucity of detrital sediment that would have inhibited mat growth. Petrofabrics preserved in Angmaat samples are widespread in upper Paleoproterozoic and Mesoproterozoic carbonate successions but are rare thereafter, perhaps recording, at least in part, the declining carbonate saturation state of seawater. Covariation of microfossil assemblages with petrofabrics in both silicified and unsilicified portions of carbonate beds supports hypotheses that link stromatolite microstructure to the composition and diversity of mat communities.Earth and Planetary SciencesOrganismic and Evolutionary Biolog
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