Enzymatic phosphatization of fish scales-a pathway for fish fossilization.

Phosphatized fish fossils occur in various locations worldwide. Although these fossils have been intensively studied over the past decades they remain a matter of ongoing research. The mechanism of the permineralization reaction itself remains still debated in the community. The mineralization in apatite of a whole fish requires a substantial amount of phosphate which is scarce in seawater, so the origin of the excess is unknown. Previous research has shown that alkaline phosphatase, a ubiquitous enzyme, can increase the phosphate content in vitro in a medium to the degree of saturation concerning apatite. We applied this principle to an experimental setup where fish scales were exposed to commercial bovine alkaline phosphatase. We analyzed the samples with SEM and TEM and found that apatite crystals had formed on the remaining soft tissue. A comparison of these newly formed apatite crystals with fish fossils from the Solnhofen and Santana fossil deposits showed striking similarities. Both are made up of almost identically sized and shaped nano-apatites. This suggests a common formation process: the spontaneous precipitation from an oversaturated solution. The excess activity of alkaline phosphatase could explain that effect. Therefore, our findings could provide insight into the formation of well-preserved fossils.

Rheological properties of calcite oozes: Implications for the fossilisation in the plattenkalks of the Solnhofen-Eichstätt lagoons in the Franconian Alb, Germany.

We report on an experimental study to investigate the sedimentation behaviour and rheological properties of extremely fine-grained calcite oozes. The experiments are aimed at clarifying if thixotropic behaviour may have played a role in the preservation of marine biota in plattenkalks of the Solnhofen lagoons of the Franconian Alb. Calcite particles with grain sizes from 2.2 to 4.4 µm were sedimented from water, seawater proxies, and hypersaline brines with up to 14 wt.% NaCl, for 170 days. High salinities as envisioned for the bottom waters of some Solnhofen lagoons slow down settling rates of calcite and may produce plattenkalks more porous and more friable than plattenkalks elsewhere in the Solnhofen archipelago. Rheological properties of calcite suspensions were measured with an oscillation rheometer. Calcite oozes with 40 vol.% calcite in suspension behave thixotropically regardless of the salinity of the pore solutions. Thixotropic behaviour may have the potential to promote the fossilisation of marine biota. Even if the sediment cover is thin, i.e. a few millimeters, a carcass covered by a thixotropic sediment would be largely isolated from the overlying water column because pore solutions in thixotropic media hardly communicate with the overlying water column. A fish carcass covered by a thixotropic sediment could impose local-scale physicochemical conditions on its direct sedimentary envelope favourable for preservation and the replacement of organic material by inorganic materials.

Experimental taphonomy of fish - role of elevated pressure, salinity and pH.

Experiments are reported to reconstruct the taphonomic pathways of fish toward fossilisation. Acrylic glass autoclaves were designed that allow experiments to be carried out at elevated pressure up to 11 bar, corresponding to water depths of 110 m. Parameters controlled or monitored during decay reactions are pressure, salinity, proton activities (pH), electrochemical potentials (Eh), and bacterial populations. The most effective environmental parameters to delay or prevent putrefaction before a fish carcass is embedded in sediment are (1) a hydrostatic pressure in the water column high enough that a fish carcass may sink to the bottom sediment, (2) hypersaline conditions well above seawater salinity, and (3) a high pH to suppress the reproduction rate of bacteria. Anoxia, commonly assumed to be the key parameter for excellent preservation, is important in keeping the bottom sediment clear of scavengers but it does not seem to slow down or prevent putrefaction. We apply our results to the world-famous Konservat-Lagerstätten Eichstätt-Solnhofen, Green River, and Messel where fish are prominent fossils, and reconstruct from the sedimentary records the environmental conditions that may have promoted preservation. For Eichstätt-Solnhofen an essential factor may have been hypersaline conditions. Waters of the Green River lakes were at times highly alkaline and hypersaline because the lake stratigraphy includes horizons rich in sodium carbonate and halite. In the Messel lake sediments some fossiliferous horizons are rich in FeCO3 siderite, a mineral indicating highly reduced conditions and a high pH.