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A First Account of Organelle Preservation in Eocene Nonmarine Diatoms: Observations and Paleobiological Implications

¹ Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, T6G 2E3, Canada; awolfe{at}ualberta.ca
² St. Croix Watershed Research Station, Science Museum of Minnesota, Marine on St. Croix, MN, 55047
³ Geological Survey of Canada, 3303-33rd St. NW, Calgary, Alberta, V1X 4L1, Canada
⁴ Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, T6G 2E3, Canada

Post-eruptive lacustrine sediments that infill a Middle Eocene kimberlitic diatreme near Lac de Gras (Northwest Territories, Canada) have spectacular preservation of diatom microfossils. A single undiagnosed species belonging to the genus Aulacoseira is abundant in this material and reveals, in addition to the full complement of siliceous structures comprising the frustule, an array of soft parts that hitherto have been unavailable for study in diatoms of this age. These features include the velar complex, which lines the interior of valves, layered internal membranes, lamellate plastid fragments with pyrenoid-like inclusions, and extracellular mucilagenous bodies. Both ultrastructural and cytoplasmic characters have pronounced affinities with extant congeneric taxa, with an especially close resemblance to the resting cells observed in living Aulacoseira. These observations suggest that modern cellular organization and associated ecological adaptations were present early in the history of nonmarine aulacoseiroid diatoms, implying that prolonged evolutionary stasis has characterized the ecologically important genus Aulacoseira.

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