The reliability of evolutionary reconstructions based on the
fossil record critically depends on our knowledge of the factors
affecting the fossilization of soft-bodied organisms. Despite
considerable research effort, these factors are still poorly understood.
The extreme rarity of unicellular non-skeletal eukaryotic fossils
compared to multicellular ones is an example of a pattern that
apparently requires taphonomic explanation. In order to elucidate the
main prerequisites for the preservation of soft-bodied organisms, we
conducted long-term (1-5 years) taphonomic experiments with the model
crustacean Artemia salina buried in five different sediments. The
subsequent analysis of the carcasses and sediments revealed that, in our
experimental settings, better preservation was associated with the fast
deposition of aluminium and silicon on organic tissues. Other elements
such as calcium, magnesium and iron, which can also accumulate quickly
on the carcasses, appear to be much less efficient in preventing decay.
Next, we asked if the carcasses of uni- and multicellular organisms
differ in their ability to accumulate aluminium ions on their surface.
The experiments with the flagellate Euglena gracilis and the
sponge Spongilla lacustris showed that aluminium ions are more
readily deposited onto a multicellular body. This was further confirmed
by the experiments with uni- and multicellular stages of the social
amoeba Dictyostelium discoideum. The results lead us to speculate
that the evolution of cell adhesion molecules, which provide efficient
cell-cell and cell-substrate binding, probably can explain the rich
fossil record of multicellular soft-bodied organisms, the poor fossil
record of non-skeletal unicellular eukaryotes, and the explosive
emergence of the Cambrian diversity of soft bodied fossils.
Key words : fossilization, soft bodied fossils, sediment,
multicellularity, Metazoa, cell adhesion molecules.