1. INTRODUCTION
How can a soft-bodied organism escape decomposition and turn into a
fossil? This question is crucial for understanding the patterns in the
fossil record, such as the fast growth of the observed diversity of
metazoans during the Cambrian including soft bodied organisms found in
Lagerstätten. Many hypotheses have been suggested to explain the
fossilization of soft-bodied organisms (SBO), as well as different modes
of their preservation, by various abiotic and biotic factors. Abiotic
factors that presumably enhance SBO preservation include low oxygenation
(Allison, 1988; McCoy et al., 2015a, b; Naimark et al., 2016a, b), low
or high acidity induced by decay (Berner, 1968; Allison, 1988; Briggs,
Kear, 1997; Sagemann et al., 1999; Wilson, Butterfield, 2014; Naimark et
al., 2016b), deposition of iron (Petrovich, 2001; Schiffbauer et al.,
2014), phosphorus (Raff et al., 2008), calcium (Butterfield, 2003),
silicon (Strang et al., 2016), or ”tanning” by aluminium (Wilson,
Butterfield, 2014; Naimark et al., 2016a, b, 2018a). Biotic factors
include the lack of bioturbation and detritus consumers (Garson et al.,
2011; but see: Pratt, Kimmig, 2019), the slow proliferation of sulfate
reducers due to the depletion of sulfur and iron caused by early surface
cementation (Hammarlund et al., 2011; Gaines et al., 2012; McCoy et al.,
2015b), inhibition of bacterial growth and absorption of bacterial lytic
enzymes by some clays (Butterfield, 1995; Kompantseva et al., 2011;
McMahon et al., 2016), or otherwise, increased activity of cyanobacteria
(or other bacteria) producing a mineral “death mask” (Gehling, 1999;
Martin et al., 2004; Darroch et al., 2012; Raff et al., 2013). Such a
wide variety of hypotheses reflects the fact that SBO can be fossilized
in many different paleoenvironments, and many environmental factors can
possibly affect the fossilization potential of SBO.
We asked (1) if there are some core processes among this wide variety
that underpin the preservation of SBO, and (2) whether these processes
differ between uni- and multicellular organisms. The latter question
stems from the extreme rarity of soft-bodied unicellular eukaryotic
fossils, whereas the Ediacaran and Phanerozoic fossil record of
multicellular SBO is much more rich and diverse.
In order to identify core chemical pathways in SBO fossilization, we
conducted a line of long term taphonomic experiments (1,5 – 5 years:
the longest of this type) with the model multicellular organismArtemia salina buried in different sediments. Some results were
published previously (Naimark et al., 2016a, b, 2018a, b, c). They are
combined with the new ones presented here (Table 1). The results reveal
coordinated changes in pH and mineral composition of the sediments, as
well as in the elemental and chemical composition of the carcasses.
These changes appear to be different facets of the complex
preservational process in sediment. Overall, the results imply that the
early deposition of aluminium and/or silica ions on decaying tissues
significantly enhances SBO preservation.
Based on these results, we asked if the early deposition of aluminium
proceeds differently on the surface of uni- and multicellular organisms.
We used the flagellate Euglena gracilis and the spongeSpongilla lacustris as model systems in these experiments. We
also experimented with the unicellular and multicellular stages of the
colonial amoebae Dictyostelium discoideum . In all cases we found
enhanced deposition of aluminium ions in multicellular organisms/stages
compared to unicellular ones. It is noteworthy that the transition from
unicellular to multicellular stage in D. discoideum is
accompanied by the appearance of “intercellular glue”, that is, by the
expression of cell adhesion molecules (CAMs). The results led us to
speculate that the emergence of CAMs at the early stages of the
evolution of multicellularity might explain why the fossil records of
unicellular and multicellular soft-bodied organisms are so dramatically
different in richness.