Taphonomic experiments imply a possible link between the evolution of
multicellularity and the fossilization potential of soft-bodied
organisms
Abstract
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.