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.