4.2. Deposition of aluminium and silicon is an important mechanism of preservation of the soft-bodied organisms.
The ions released from the mineral matrices exhibit different preservation ability. Aluminium and silicon ions (released in the form of amphoteric oxides) provided the best preservation of the organic structures (Figure 6). We acknowledge that some other elements, which were absent or rare in our experimental context, may also enhance preservation (e.g., phosphorus).
Magnesium, iron and calcium were readily deposited on the organic tissues, but their abundance in the carcasses was not associated with better preservation. Nevertheless, owing to their fast deposition on the carcasses, they probably can create 2D-organo-mineral “portraits” in the sediment that are visually accentuated by the high concentration of the corresponding element (especially iron). In the presence of aluminium and/or silicon that slow down decay, carbonaceous films veneered by iron, calcium or magnesium may be formed on the surface of the carcasses. In the case of iron-rich sediments, a pyritized veneer can penetrate inside the tissues to produce a more or less pyritized fossil (Schiffbauer et al., 2014). It needs to be emphasized that the fast deposition of an element on the carcass and its subsequent accumulation in decaying tissues does not nesessarily mean that this element enhances preservation. We need to distinguish the two processes: the fast deposition of elements and their action as preservatives. This is especially important in the context of the current debates about the role of iron in preservational pathways (Schiffbauer et al., 2014; Newman et al., 2019).
In all experiments reported here, including the sediment-free control, calcium ions entered the body tissues very effectively (Figure 6, Table 5). Thus, calcification is expected to occur more frequently than other preservation pathways in aluminium- and silicon-containing sediments. However, without aluminium and silicon (or other putative preservation-enhancing ions), calcium does not appear to ensure good preservation. Interestingly, even highly calcified SBO fossils were found coated with a thin layer of aluminosilicate, e.g., in the Silurian Herefordshire Lagerstätte (Siveter et al., 2020).
In our experiments, good preservation was associated with strong adherence of sediment particles to decaying bodies; some specimens were wrapped in clay “envelopes” (Figure 2). As a result, surface body tissues became more rigid and the body retained its shape (the characteristic feature of the preservational groups 1 and 2).
Importantly, the chemical pathways of SBO preservation discussed here do not require any unusual environmental conditions and occur relatively quickly. Preservational processes take place whenever a sufficient amount of dead organic bodies is buried under a layer of fine-grained sediment. According to the mineral composition of the sediment and probably some other characteristics of the system, buried carcasses may become minerlized and fossilized in different modes and by different compounds. Aluminium and silicon are possibly required for SBO preservation, but they are among the most abundant elements in sedimentary rocks. Owing to diverse and ubiquitous contexts for SBO preservation, Konservat-Lagerstätten localities are expected to be quite numerous. The recent data seem to confirm that SBO fossil localities are not taphonomic rarities: during the last two decades, ten times more Lagerstätten have been described than during more than 100 years since their discovery in the 19th century: from 53 Lagerstätten occurrences in 1997 to 670 in 2017 (Muscente et al., 2017).