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Pyritized filamentous cyanobacteria have been discovered by scanning electron microscopy of Ordovician brachiopods from nineteenth century collections in the Natural History Museum, London. The cyanobacteria form mats on strophomenid brachiopods from the Cincinnatian Group (Upper Ordovician, Katian) near or in Cincinnati, Ohio. There is no additional stratigraphic or locality information. The cyanobacteria are found only on the concave exterior surfaces of dorsal valves. Most comprise uniserial, unbranching strands of cells that range from 5 to 9 micrometers in length and width. Some strands are up to 700 micrometers long and most are sinuous. There are no heterocysts or akinetes. The pyritic molds of some cells have a reticulate structure on their surfaces, with each unit a few hundred nanometers in diameter, interpreted as due to framboid microcrystal growth within an organic matrix. A few specimens show what may be extracellular matrix. These Ordovician microbes resemble cyanobacteria belonging to the extant Order Oscillatoriales (Subsection III). The cyanobacterial mats overgrew encrusting cyclostome bryozoans, including broken zooids and terminal diaphragms, sharing the same brachiopod substrates. Since the cyanobacteria are inferred to have been photosynthetic, the encrusted shells probably rested with their dorsal valve exteriors upwards in the photic zone. The mats likely developed shortly before burial of the brachiopod shells. That Cincinnatian brachiopod shells were colonized by cyanobacteria has been previously demonstrated by microborings, but this is the first direct evidence of microbial mats on the shell surfaces. The mats likely played a role in the paleoecology of sclerobiont communities, and they may have influenced preservation of the shell surfaces by the “death mask” effect.
Natural and anthropogenic eutrophication can increase food supply to basal consumers in aquatic food webs. All else being equal, increased food supply is expected to relax life history trade-offs between egg size and number, resulting in a reduction in egg size over time as individuals that produce more numerous, small eggs exhibit greater fitness. We tested this hypothesis by comparing the sizes of larval shells (PI) of the marine bivalve Nuculana acuta in living and death assemblages collected from surficial seafloor sediments on the Alabama continental shelf; PI size is positively correlated with egg size and can be measured from adult shells. We found that the mean PI size of living N. acuta was approximately three microns smaller than that of the associated death assemblage and that this difference was robust to potential taphonomic biases. This life history shift occurred relatively recently as no trend exists in PI size over the past 3100 years. The live-dead disagreement that we observed is consistent with the history of anthropogenic eutrophication in the Mississippi Bight. These data provide a baseline for comparison with other regions in the Gulf of Mexico that have more sustained histories of anthropogenic eutrophication. More broadly, live-dead comparisons of molluscan life history coupled with age dating of molluscan shells can complement community-level metrics when assessing the impacts of anthropogenic eutrophication on coastal ecosystems, and offer a unique study system for investigating life history adaptation in a field context.
A new experimental setup using a collapsible wooden tray, monopod, and digital video camera is used to observe and collect modern bird tracks. This setup is unique because it simultaneously captures tracemaker behavior, trace morphology, and media consistency (i.e., grain size and moisture content), and can be used in the laboratory and in natural environments. Here we provide examples produced by domestic chickens (Gallus gallus). Using this setup we determined that bird track morphology varies in a predictable manner with respect to sediment grain size and the percent of water present. The finer the sediment grain size, the more detail is likely to be preserved. If the sediment is completely dry, no track details will be preserved––digit impressions will be broad and will not taper at the tips; digit impression length will be longer than the actual digit lengths. If the sediment is wet (8.8%–6.7%), digit impressions will taper to points, will not be as wide as in dry sediment, and will not preserve pad impressions. If the sediment is variably moist (5.3%–3.2%), the detail level of pad or scale impressions, depending on the grain size, may be present. Within this study, we propose a sinuosity index that allows for quantification of sinuous avian trackways, and quantify the trackway parameters of behaviors, including start-stop walking, walking, running, takeoff, and landing. Both takeoff and landing traces are significantly deeper than the proceeding or following walking and running traces. Start-stop walking does not always result in side-by-side paired tracks, and often the bird will pause in midstride. Linking behavior and morphology of tracks can be used to better interpret ancient behavior and the depositional environment in which ancient tracks were produced.
Leaf adpression fossils vary in their organic content, relief, and quality of preservation. Some of the most enigmatic adpressions, known as leaf molds, retain fine morphological and anatomical details despite being found in coarse sandstones—a widespread phenomenon attributed to the presence of fine-grained minerals on the fossil surface. Previous taphonomic studies have demonstrated the importance of microbial biofilms in promoting mineralization and argued that authigenic iron oxides can serve as the preserving medium. Here, we propose that this role is played more commonly by biologically precipitated aluminosilicate phases (clays). To test this hypothesis, we conducted energy dispersive X-ray spectroscopy (EDS) analysis of thin sections through fossil leaves from five localities differing in age and depositional environment. Point spectra taken directly from the leaf-sediment interface revealed that cation-rich clays separate the leaf fossils from the matrix. Additional EDS analyses of biofilms on a fossil leaf and on modern oak leaves decaying in freshwater also revealed aluminosilicates, for which we infer a biofilm-mediated, authigenic origin. These results are the basis of a novel ‘Biofilm-Clay Template' taphonomic model, whereby microbially mediated clay authigenesis is commonly the first step in leaf adpression preservation.