Person: Cohen, Phoebe A
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Cohen
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Phoebe A
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Cohen, Phoebe A
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Publication Early Neoproterozoic scale microfossils in the Lower Tindir Group of Alaska and the Yukon Territory(Geological Society of America, 2010) Macdonald, Francis; Cohen, Phoebe A; Dudas, Francis Ő.; Schrag, DanielThe Tindir Group is a <4-km-thick Neoproterozoic succession exposed in the Tatonduk inlier of east-central Alaska and the western Yukon Territory. The Tindir Group is informally divided into the Lower Tindir Group, which consists of <2 km of mixed carbonate and clastic rocks, and the overlying Upper Tindir Group, which contains two Cryogenian glacial deposits and an additional Ediacaran succession of mixed carbonate and clastic strata. Unique mineralized scale microfossils have been recovered from sections previously correlated with the Upper Tindir Group, and interpreted variously as Cryogenian to early Cambrian in age. Our remapping of the area indicates that these sections are stratigraphically below an early Cryogenian glacial diamictite, unit 2 of the Upper Tindir Group, and are actually part of the Lower Tindir Group. Carbon and strontium isotope correlations further suggest that the fossiliferous Lower Tindir Group is correlative with early Neoproterozoic strata of the northwestern Canadian Cordillera. This new age model is consistent with the accompanying microfossil assemblage and indicates that the diverse microfossils in the Lower Tindir Group can be added to the early Neoproterozoic record of eukaryotic evolution.Publication Calibrating the Cryogenian(American Association for the Advancement of Science (AAAS), 2010) Macdonald, Francis; Schmitz, Mark D.; Crowley, James L.; Roots, Charles F.; Jones, David S.; Maloof, Adam C.; Strauss, Justin Vincent; Cohen, Phoebe A; Johnston, David; Schrag, DanielThe Neoproterozoic was an era of great environmental and biological change, but a paucity of direct and precise age constraints on strata from this time has prevented the complete integration of these records. We present four high-precision U-Pb ages for Neoproterozoic rocks in northwestern Canada that constrain large perturbations in the carbon cycle, a major diversification and depletion in the microfossil record, and the onset of the Sturtian glaciation. A volcanic tuff interbedded with Sturtian glacial deposits, dated at 716.5 million years ago, is synchronous with the age of the Franklin large igneous province and paleomagnetic poles that pin Laurentia to an equatorial position. Ice was therefore grounded below sea level at very low paleolatitudes, which implies that the Sturtian glaciation was global in extent.Publication Eukaryotic Organisms in Proterozoic Oceans(Royal Society, The, 2006) Knoll, Andrew; Javaux, Emmanuelle J.; Hewitt, David; Cohen, Phoebe AThe geological record of protists begins well before the Ediacaran and Cambrian diversification of animals, but the antiquity of that history, its reliability as a chronicle of evolution and the causal inferences that can be drawn from it remain subjects of debate. Well-preserved protists are known from a relatively small number of Proterozoic formations, but taphonomic considerations suggest that they capture at least broad aspects of early eukaryotic evolution. A modest diversity of problematic, possibly stem group protists occurs in ca 1800–1300 Myr old rocks. 1300–720 Myr fossils document the divergence of major eukaryotic clades, but only with the Ediacaran–Cambrian radiation of animals did diversity increase within most clades with fossilizable members. While taxonomic placement of many Proterozoic eukaryotes may be arguable, the presence of characters used for that placement is not. Focus on character evolution permits inferences about the innovations in cell biology and development that underpin the taxonomic and morphological diversification of eukaryotic organisms.