4.3. Cell adhesion molecules probably play a role in preservation of the soft-bodied organisms.
Despite the popular idea that durable integuments (e.g., chitinous cuticles) are essential for fossil preservation, in some Lagerstätten fossil animals with and without such durable tissues are found in comparable numbers (Fu et al., 2019).
The results of our FT-IR analysis (Figure 3) show that differently preserved specimens have similar spectral portraits, implying that different biochemical components probably had similar rates of decay. More specifically, we found that the best preserved specimens were not those that retained more chitin or proteins. Although we were not able to measure these components precisely, the similarity of the spectra probably indicates that SBO with different bulk biochemistry may have comparable chances of fossilization, and that durable chitinious integuments may play only a secondary role in the preservation of SBO.
Given that Konservat-Lagerstätten are diverse and numerous, and that durable tissues are not an imperative for SBO preservation, then some other biochemical factors may be expected to play the key role in different preservation pathways. The idea of key biochemical factors seems plausible because the mineralization of a dead organism probably should begin with binding of mineral ions to some cell surface molecules. The obvious candidates are acidic and very acidic proteins, glycoproteins and polysaccharides which have been shown to facilitate nucleation of mineral crystals during skeletal biomineralization in eukaryotes (Marin, Luquet, 2007). We hypothesize that there is a special group of these acidic biomolecules that play a crucial role in the fossilization of SBO: cell adhesion molecules and related extracellular matrix molecules (CAMs and ECMs) that evolved in multicellular animals (and some other organisms like social amoebae) to ensure cell-cell and cell-substrate adhesion.
CAMs and ECMs have the ability to adhere to mineral particles and bind ions; many types of CAMs are known to bind inorganic ions as part of their physiological function (Brown et al., 2018; Sotomayor, Schluten, 2008). From this perspective, the fossilization of dead multicellular soft-bodied organisms via fast binding of certain inorganic ions (e.g., aluminium and silicon ions) would be an inevitable consequence of a certain stage of evolution of multicellularity, when a set of CAMs and ECMs became sufficiently developed. If our hypothesis is correct, then the crown groups of metazoans with fully developed sets of CAMs and ECMs would have a higher chance of leaving a trace in the fossil record compared to basal groups with less advanced or numerous CAMs and ECMs. For example, sets of adhesive molecules are poor or lacking in basal holozoans (including choanoflagellates), which, in the absence of a mineral skeleton, makes their fossilization very unlikely.
The hypothesis explains several enigmatic features of the fossil record: the extreme scarcity of the record of basal Metazoa who probably had only a few primitive CAMs and ECMs; the poor record of non-skeletal holozoans which, according to phylogenetic reconstructions, should have been present since the Proterozoic and throughout the Phanerozoic (Sharpe et al., 2015; Dohrmann, Wörheide, 2017; Betts et al., 2018). The rich fossil record of bacterial mats can be explained by their sheaths of acidic polysaccharides and mucins (one class of CAMs). Speculating on the role of CAMs and ECMs in preservation, we need to take into account that cell surface biochemistry and extracellular structures in plants and fungi differ from those in animals. Thus, their preservation is probably governed by other molecular mechanisms and should be addressed separately.
A critical review of the evolution of CAMs and ECMs and its comparison with the trends in the fossil record of Metazoa may bring interesting insights into the chronology of the early evolution of animals. The fast evolution and diversification of CAMs and ECMs probably can, to some extent, account for the explosive emergence of diverse multicellular organisms in the Cambrian fossil record.