The Cambrian Period, the earliest period of the Palaeozoic Era and of the Phanerozoic Eon, saw the end of the evolutionary event commonly referred to as the Cambrian explosion (e.g., Conway Morris, 2000). This event is described as the sudden appearance of a large variety of complex body plans (in particular the metazoans) and new life strategies in an apparently brief span of geological time (Valentine et al., 1999). Over a period of a few tens of millions of years almost all modern phyla were established, although these animals were not the crown-group forms seen today (Budd and Jensen, 2000). Even so, the early history of animals is still very imperfectly known, and there is a great deal of speculation as to what caused or triggered the metazoan radiation (e.g., Conway Morris, 2003), as well as why it happened when it did.
Discoveries during the past ten years have revealed exquisitely preserved fossil embryos in Cambrian and Late Neoproterozoic deposits. In 1994 the first article regarding egg-like structures was published, in which Zhang and Pratt identified what they considered to be traces of cell boundaries on the surface of Middle Cambrian globular microfossils, which they interpreted as arthropod eggs. Bengtson and Yue (1997) subsequently discovered that embryos of several kinds of Early Cambrian marine metazoans had been preserved through fossilization by early diagenetic phosphatization. The specimens showed the full developmental sequence from early cleavage through hatching. These first findings, in the lowermost Cambrian limestones/phosphorites of southern Shaanxi, China, and the Aldan River basin, Siberia, quickly led to other discoveries of phosphatized embryos in the Neoproterozoic Doushantuo limestones/phosphorites of Weng’an, Guizhou, China (Li et al., 1998; Xiao et al., 1998). Since then, a large number of papers concerning fossilized embryos have been published (e.g., Yue and Bengtson, 1999; Kouchinsky et al., 1999; Chen et al., 2000; Chen et al., 2004b; Dong et al., 2004; Dong et al., 2005; Donoghue et al., 2006; Hagadorn et al., 2006; Steiner et al., 2004 a,b; Yin et al., 2007).
Previous work on fossilized embryos has been limited to analysis of their three-dimensional external morphology using scanning electron microscope (e.g., Bengtson and Yue, 1997) or their two-dimensional internal morphology using thin-section observation (e.g., Chen et al., 2004a). Recent studies, where fossil embryos were analysed with synchrotron-based X-ray tomographic microscopy (srXTM), have yielded details of cellular and sub-cellular information of the internal embryonic structures (e.g., Donoghue et al., 2006). The srXTM slices (pictures) prove that the preservation of these embryo fossils is quite extensive and not only limited to surface structures, but also that it is possible to distinguish between taphonomic and diagenetic features.
In this study, fossil embryos from both the Early Cambrian Kuanchuanpu Member of the Dengying Formation and the Neoproterozoic Doushantuo Formation have been identified for analysis of their complete three-dimensional morphology. The specimens, preserved as diagenetically phosphatized replacements and encrustations, represent different developmental stages from early cleavage through hatching.
The aim of this study is to depict the different cleavage patterns and the cell geometry of the fossil embryos from the two localities. Thus far, the apparent difference, when comparing the fossil embryos found in the early Cambrian with the Neoproterozoic ones, is that the Cambrian embryos have preserved the developmental stages from blastula to hatching, whereas the Neoproterozoic ones generally seem to be restricted to early cleavage embryos that show only pre-blastula features (Yin et al., 2001).
The results from these ongoing experiments confirm that synchrotron-based X-ray tomographic microscopy is a powerful tool, non-destructively yielding three-dimensional information at micrometer resolution and in so, revealing internal details of sub-cellular and other embryonic structures. Clearly this technique has the potential to increase our understanding of developmental cycles of the early animals, and thereby providing valuable new insights in phylogenetic and evolutionary studies.