Exceptional preservation within Pleistocene lacustrine sediments of Shiobara, Japan

A new conservation Lagerstätte is described from the middle Pleistocene Shiobara Group of central Japan. The biota includes mammals, birds, amphibians, fish, insects, arachnids, flowers, abundant leaves, fungi, and bacteria preserved within laminites that were deposited in a lacustrine environment. Comminuted plant material in medium-grained, massive sandstones was deposited from high-density flows. This fragmentation of plant material probably indicates that it had decayed prior to transport. Plant and animal remains are largely nonfragmented and were thus transported prior to decay. The laminites are composed of rhythmic, millimeter-scale alternations of clay-to-silt-grade clastics with siliceous, diatom-rich layers. 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 preserved 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 aggregated mats of diatoms that shrouded the biota on the lakebed. This stabilized the carcasses and prevented them from being disturbed. It also prevented the diffusion of both the incoming nutrients and outgoing metabolic by-products between carcasses and surrounding water and may thus have promoted soft-part preservation. Silica cementation also inhibited the destruction of fossils by the intense weathering in the humid Japanese climate.