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Exceptional Preservation Within Pleistocene Lacustrine Sediments of Shiobara, Japan
Abstract and Figures
A new conservation Lagerstätte is described from the middle Pleis-tocene Shiobara Group of central Japan. The biota includes mam-mals, birds, amphibians, fish, insects, arachnids, flowers, abundant leaves, fungi, and bacteria preserved within laminites that were de-posited in a lacustrine environment. Comminuted plant material in medium-grained, massive sandstones was deposited from high-density flows. This fragmentation of plant material probably indi-cates that it had decayed prior to transport. Plant and animal re-mains are largely nonfragmented and were thus transported prior to decay. The laminites are composed of rhythmic, millimeter-scale al-ternations of clay-to-silt-grade clastics with siliceous, diatom-rich lay-ers. In the western part of the basin the diatoms are preserved as opal-A, but in the eastern part, where soft-part preservation is most common, they have been altered to opal-CT and form thin, white, porcelaneous layers with a lepispheric texture. Soft parts are pre-served as carbon residues and microbial films, and although siliceous laminae enclose the fossils, permineralization of tissues is infrequent. Soft-part preservation was promoted by the self-sedimentation of ag-gregated mats of diatoms that shrouded the biota on the lakebed. This stabilized the carcasses and prevented them from being dis-turbed. It also prevented the diffusion of both the incoming nutrients and outgoing metabolic by-products between carcasses and sur-rounding water and may thus have promoted soft-part preservation. Silica cementation also inhibited the destruction of fossils by the in-tense weathering in the humid Japanese climate.
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