Understanding the processes that led to the mineralization of Precambrian metazoan eggs and embryos is essential to informed interpretations of such fossils and their significance in early metazoan evolution. Here we show that rapid mineralization of invertebrate eggs is possible under laboratory conditions. Under anaerobic conditions, eggs become coated in mainly calcium carbonate within three weeks. Preservation of the external morphology is comparable to that of fossil material, but no internal mineralization was observed in the laboratory. This is the first report of the laboratory mineralization of metazoan eggs in the absence of a decaying carcass, and demonstrates that eggs, and probably small embryos, can be preserved in the absence of larger organisms as a source of phosphorus or calcium. Thus, it is possible for organisms of this size to have been fossilized prior to the evolution of large metazoans.