The Cambrian radiation of bilaterians: Evolutionary origins and palaeontological emergence; earth history change and biotic factors

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Abstract

Evidence from a variety of research areas, including phylogenetic palaeobiogeographic studies of trilobites, indicates that there may be a fuse to the Cambrian radiation, with a duration on the order of 20–70 myr. Evolution in trilobites appears to have been powerfully influenced by the tectonic changes occurring at the end of the Neoproterozoic: especially the breakup of Pannotia. This continental fragmentation may have also elevated opportunities for vicariance and speciation in trilobites, and other metazoans, given that speciation rates at this time period were high, though not phenomenally so. This provides clear evidence that abiotic factors played an important role in motivating evolution during this key episode in the history of life; biotic factors probably also played a role. The evidence for the role of biotic factors is considered in light of information from some problematic Cambrian taxa. These may show affinities with modern problematic pseudocoelomate phyla, although Cambrian and modern exponents differ dramatically in body size.

Introduction

Scientific understanding of the status of the Cambrian radiation has grown significantly in the last decade; this growth has been accomplished by data from a variety of sources. These data derive from and include: the spectacular discoveries of metazoan embryos (Xiao et al., 1998, Steiner et al., 2004, Chen et al., 2006), with recognition of their possible triploblastic character (Xiao et al., 2000, Chen et al., 2006); analyses revealing the nature of ecological changes during the early and Middle Cambrian (Bottjer et al., 2000, Dornbos et al., 2005a); more accurate approaches to estimating divergence events in the modern biota using molecular dating techniques (Peterson et al., 2004); new evidence of bilaterians in the pre-Cambrian (Chen et al., 2004, Chen et al., 2006, Dornbos et al., 2005b), although some of these results have been challenged by Bengston and Budd (2004); advances in chronostratigraphy (Bowring et al., 2003, Condon et al., 2005); and new chemostratigraphic studies of the late Neoproterozoic and early Cambrian (Corsetti and Kaufman, 2003, Xiao et al., 2004, Maloof et al., 2005). Here, the growing consensus on the nature and timing of the Cambrian radiation emerging from these results will be considered in light of results derived from phylogenetic biogeographic analyses and studies of rates of evolution in trilobites. Further, the phylogenetic affinities some problematic Cambrian taxa share with modern problematic taxa, and the implications this may have for our understanding of the Cambrian radiation, will also be explored.

Section snippets

Conclusions

It appears that physical, abiotic factors play one of the primary roles in motivating evolutionary patterns during the Cambrian radiation (Lieberman, 2000, Lieberman, 2003a, Lieberman, 2003b, Lieberman, 2003c, Lieberman, 2005), but this does not preclude other factors from playing an important role. For instance, evidence is emerging attesting to the changing ecological dynamics during this time period (e.g., Dornbos et al., 2005a). If the possible changes in body size in the problematic

Acknowledgements

Thanks to S. Dornbos and S. Xiao for inviting me to participate in this volume. Thanks to S. Dornbos, J. Hendricks, J. Meert, and two anonymous reviewers for comments on an earlier version of this manuscript; thanks to J. Meert for contributing the palaeogeographic reconstruction used in Fig. 1; thanks to D. E. G. Briggs for discussions related to the nature of vetulicolians. This research was supported by NSF EAR-0518976, NASA Astrobiology NNG04GM41G and a Self Faculty Fellowship.

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