Synchrotron X-ray tomographic microscopy of fossil embryos

The inside story on microfossils Fossils of exquisitely preserved half-a-billion year old animal embryos impregnated and encrusted with calcium phosphate have created great excitement as they may contain precious information on developmental processes at a crucial period of animal evolution. Their small size makes them hard to study, but a newly developed technique can recover previously inaccessible detail. The new method, which involves submicrometre-scale tomographic analysis using a synchrotron X-ray light source, has been used to resolve some problems at the extreme limits of fossilization. Remarkable data on the internal anatomy of microscopic fossils of the earliest known bilaterian embryos, Markuelia and Pseudooides , clear up some questions about the phylogeny of the former and reveal a previously unknown model of development in the latter. The cover shows a series of virtual slices through a Markuelia embryo, demonstrating both embryonic and post-mortem features. Fossilized embryos from the late Neoproterozoic and earliest Phanerozoic have caused much excitement because they preserve the earliest stages of embryology of animals that represent the initial diversification of metazoans 1 , 2 , 3 , 4 . However, the potential of this material has not been fully realized because of reliance on traditional, non-destructive methods that allow analysis of exposed surfaces only 1 , 2 , 3 , 4 , and destructive methods that preserve only a single two-dimensional view of the interior of the specimen 5 , 6 . Here, we have applied synchrotron-radiation X-ray tomographic microscopy (SRXTM) 7 , obtaining complete three-dimensional recordings at submicrometre resolution. The embryos are preserved by early diagenetic impregnation and encrustation with calcium phosphate, and differences in X-ray attenuation provide information about the distribution of these two diagenetic phases. Three-dimensional visualization of blastomere arrangement and diagenetic cement in cleavage embryos resolves outstanding questions about their nature, including the identity of the columnar blastomeres. The anterior and posterior anatomy of embryos of the bilaterian worm-like Markuelia confirms its position as a scalidophoran, providing new insights into body-plan assembly among constituent phyla. The structure of the developing germ band in another bilaterian, Pseudooides , indicates a unique mode of germ-band development. SRXTM provides a method of non-invasive analysis that rivals the r