Both the timing and nature of early Metazoan evolution remain controversial, with complementary, and sometimes conflicting, evidence from molecular data and fossil occurrences. Exceptionally preserved embryos from the Neoproterozoic and early Phanerozoic remain an important source of direct evidence: fossil embryos of complex organisms at a relatively advanced stage of development provide a test of hypotheses based on comparative embryology and the evolutionary development of living forms. Understanding how these fossils are preserved, and what morphological changes are induced by decay, is essential to interpreting the evidence that they provide for early metazoan evolution. A range of decay experiments was performed on eggs of living arthropods to explore the controls on their mineralization and likely fossilization. Surface mineralization occurred within one month, mainly in calcium carbonate or in a combination of calcium carbonate and calcium phosphate. There was no enhancement of mineralization with increased phosphate or calcium concentrations, or with changing oxic/anoxic conditions, and mineralization occurred both in the absence and presence of an associated carcass. The eggs showed remarkable resistance to decay, indicating that an extended time-period (more than a year) would be available for mineralization and fossilization in some settings. Where sediment was available, it could become attached to the egg surface, replicating the morphology in a fashion analogous to mineralization. The interior of the eggs was not mineralized in the experiments. The degree of mineralization was very variable, reflecting conditions both in the natural environment and in the fossil record.

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