as found in Ediacara, the fossils are preserved on the underside of usually coarse grained sandstone beds; Fermeuse-style: A subset of Flinders-style, known from deep water only. Only trace fossils and imprints of holdfast structures are preserved; the soft tissue itself has decayed (although the sediment within the holdfasts remains); Conception-style: The fossils are preserved as an impression in an overlying bed of freshly fallen volcanic ash, often in exquisitely fine detail; Nama-style: Fossils are preserved in three dimensions, within fine-grained beds that were deposited in single storm or mudflow events. Hypotheses for Flinders-style preservation Microbial mats The presence of widespread microbial mats probably aided preservation by stabilising their impressions in the sediment below, in combination with the formation of iron sulfides and pyrite to form a "death mask" mantling the organisms. Rapid cementation of overlying sediment Many models suggest that overlying sediment mineralized before the underlying organism decayed, causing the un-mineralized underlying sediment to fill the void after decay. One mode of early sediment mineralization, which accounts for the occurrence of this preservational mode into the Cambrian and its increasing scarcity thereafter, is silicification: this links the preservation of the fossils to the higher silica content of oceans before sponges, diatoms and other silica sinks became widespread. This hypothesis struggles to account for a number of observations, particularly in the Flinders and White Sea deposits; it is therefore difficult to argue that it formed a necessary component of Ediacara type preservation. Sediment rheology In Flinders-style preservation, the overlying sediment always has a larger grain size than the sediment layer beneath. Because sediments with smaller grain sizes are more fluid, these can be squeezed up into a void that forms as organic material begins to decay. This model, proposed by Mary Wade in 1969, has found experimental support from cardboard-containing Death Star ice cubes. What is preserved? The rate of cementation of the overlying substrate, relative to the rate of decomposition of the organism, determines whether the top or bottom surface of an organism is preserved. Most disc-shaped fossils decomposed before the overlying sediment was cemented, and the ash or sand slumped in to fill the void, leaving a cast of the underside of the organism. Conversely, quilted fossils tend to decompose after the cementation of the overlying sediment; hence their upper surfaces are preserved. Their more resistant nature is reflected in the fact that in rare occasions, quilted fossils