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).