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Bacteria on embryoBacteria played an active role in making this this modern sea urchin embryo into a fossil (inset shows an unfossilized embryo). Such fossilized tissue from past animals is a rare but big find for paleontologistsIMAGE CREDIT: F.R. Turner, E.C. Raff and R.A. Raff; INSERT: Raff and Raff

Paleontologists were stunned when fossils appearing to belong to the soft-tissued embryos of marine creatures were unearthed in Chinese sediments a decade ago (SN: 2/7/98, p. 84).

Now, lab tests hint that a process of three stages, two of which directly involve bacteria common in marine sediments, can reproduce the same fossilization effect in modern embryos, say Rudy Raff, an evolutionary biologist at Indiana University in Bloomington.

First, the fragile embryonic tissues must end up in an oxygen-poor environment that shuts down autolysis, the enzyme-driven process that, in essence, digests the cell from the inside out. Then, bacteria in the environment colonize the surface of the embryo, break into its cells, and reproduce there en masse, building a biofilm that replicates the embryo’s cells and their contents (see the cross-sectional image of a fossilized embryo of a modern-day sea urchin; inset depicts an unfossilized, four-cell embryo).

Finally, Raff and his colleagues report in the Dec. 9 Proceedings of the National Academy of Sciences, the bacteria deposit long-lasting minerals such as calcium carbonate or calcium phosphate to complete the fossilization process.

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