Because of their tiny size and precarious preservation, embryos are the rarest and most precious of all fossils. They contain information about the evolutionary changes that have occurred in embryos over the past 500 million years. Donoghue and colleagues had to process 12 tons of rocks in order to find a few hundred embryos.

In one instance they have exposed the internal anatomy of a close relative of the living penis worm. Another case has revealed a unique pattern for making embryonic worm segments, not seen in any animals living today.

The images were revealed using a revolutionary new technique -- synchrotron-radiation X-ray tomographic microscopy.

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