Multicellularity and fossilization of soft bodies

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. 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 soft-bodied
animals, the comparatively poor fossil record of non-skeletal unicellular
eukaryotes, and the explosive emergence of the Cambrian diversity of soft
bodied fossils.