EXPERIMENTAL PHOSPHATISATION OF CRUSTACEAN EMBRYOS: THE POTENTIAL KEY TO UNDERSTAND EXCEPTIONAL MORPHOLOGY PRESERVATION

A range of decay experiments was developed to investigate the external influences (e.g. time, redox conditions, pH) that promote preservation and mineralization of metazoans eggs, in order to get a better understanding about exceptional 3-D fossil preservation of Neoproterozoic to Early Cambrian phosphatized eggs and embryos. We embedded eggs of the marbled crayfish (Marmorkrebs) in a gel-like medium of synthetically manufactured calcite, phosphatic acid, as a source of phosphorus, distilled water and activated coal. Duration of the experiments was generally between 7 and 44 days.

Fresh Marmorkrebs eggs that were embedded in treatments with excess phosphatic acid could be preserved neither in an oxygenated nor a reduced atmosphere. Unlike to the findings at low pH, fresh eggs embedded in the medium with excess calcium carbonate were preservable over several weeks in an oxygenated, as well as an N2-atmosphere. The original spherical shape was however only partly retained. Patchy mineralization on the outer surface was evident after 6 days and coverage of the complete topography and in places dense crystal accumulations up to 150µm in diameter occurred after 2 weeks.

The effect of pre-boiling in distilled water at 80°C lead to complete preservation of the spherical morphology with no visible external decay features. Furthermore, almost complete mineralization of the outer membrane was evident after 7 days. After 2 weeks, the crystal layer accumulated to a thickness of approximately 100µm, and after another week white crystals with a columnar habitus were precipitated. Stained histological sections revealed that the embryonal tissue covering the embryo and the yolk, and various cell types and components (e.g. cell nuclei, segmented caudal papillae, muscle cells) are exceptionally well preserved. They further displayed that the fine-grained crystal layer completely cover the outer membrane of the embryos. Powdered X-ray diffraction with Rietveld refinement revealed bruschite as well as calcite as the dominating mineral phases of the embedding medium and the mineralized layer of Marmorkrebs eggs.

Our findings indicate that natural conditions, such as hydrothermal activity, which might result in the heating of ambient organisms, might considerably contribute to the fossilisation of embryos and small organisms.