Experimental mineralization of crustacean eggs: new implications for the fossilization of Precambrian–Cambrian embryos

D Hippler,G Scholtz,M Steiner,N Hu,G Franz

doi:10.5194/bg-9-1765-2012

Abstract

Abstract. Phosphatized globular microfossils from the Ediacaran and lower Cambrian of South China represent an impressive record of early animal evolution and development. However, their phylogenetic affinity is strongly debated. Understanding key processes and conditions that cause exceptional egg and embryo preservation and fossilization are crucial for a reliable interpretation of their phylogenetic position. We conducted phosphatization experiments on eggs of the marbled crayfish Procambarus that indicate a close link between early mineralization and rapid anaerobic decay of the endochorional envelope. Our experiments replicated the different preservational stages of degradation observed in the fossil record. Stabilization of the spherical morphology was achieved by pre-heating of the eggs. Complete surface mineralization occurred under reduced conditions within one to two weeks, with fine-grained brushite (CaHPO4 · 2H2O) and calcite. The mechanisms of decay, preservation of surface structures, and mineral replacement in the experiment were likely similar during fossilization of Cambrian embryos.

Highlights

IntroductionExceptional three-dimensional preservation of fossilized eggs and embryos from the Neoproterozoic, the Cambrian and the Ordovician provide direct insight into embryology during the eve of animal radiation (Bengtson and Zhao, 1997; Li et al, 1998; Xiao et al, 1998, 2000; Chen et al, 2004; Steiner et al, 2004a, b; Donoghue and Dong, 2005; Donoghue et al, 2006; Liu et al, 2007; Dong et al, 2010)
The terminal Neoproterozoic Doushantuo Formation (635 to 551 Ma; Condon et al, 2005) of South China has attracted considerable attention, first because fossilized algae and embryos occur in nearly rock-forming abundance, and second because radiometric ages of the Doushantuo Formation imply that the putative animal embryos predate the Precambrian-Cambrian boundary (542 Ma) by more than 20 million years
Our laboratory phosphatization experiments show that complete surface mineralization of crayfish eggs, which were exposed to an inorganic two-component system consisting of calcite + H3PO4, may occur under anoxic conditions after one or two weeks, producing different grades of preservation in a manner similar to those described from the fossil record

Summary

Introduction

Exceptional three-dimensional preservation of fossilized eggs and embryos from the Neoproterozoic, the Cambrian and the Ordovician provide direct insight into embryology during the eve of animal radiation (Bengtson and Zhao, 1997; Li et al, 1998; Xiao et al, 1998, 2000; Chen et al, 2004; Steiner et al, 2004a, b; Donoghue and Dong, 2005; Donoghue et al, 2006; Liu et al, 2007; Dong et al, 2010). 1a–c, 2), are mainly preserved as diagenetically phosphatized encrustations and impregnations in limestone (Xiao and Knoll, 1999; Steiner et al, 2004a; Hubert et al, 2005) and exhibit an extraordinary preservation of cell geometry and cytological structure.

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Discussion

Conclusion