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.