RUNNEGAR, B.; GEHLING, J.G.: Understanding the Ediacarans in the context of early animal evolution

In 1983, when Seilacher suggested that the well-known Ediacaran fossils may belong to an extinct monophyletic clade, the vendozoa (subsequently Vendobionta), the idea was roundly resisted by the paleontologists working with these organisms. Two decades later, following proposals that have placed the Ediacarans all over the map of higher eukaryotes, we are finally beginning to understand the common and complex taxa. In part, this success is due to extensive field studies that have focused on finding life and death assemblages preserved on sizeable, well-exposed, bedding surfaces. Only in this way can the taphonomy, morphology, intraspecific variability, paleoecology, and stratigraphic significance of the organisms and their communities be understood. Equally important, however, has been the intellectual methodology, pioneered by Seilacher, which has required the rejection of many cherished preconceptions. Our approach has been to assemble the organisms into unassigned extinct clades (orphan plesions) using characters shared among the fossils. The next step is to “rescue the orphans” by establishing where they should best be grafted on the tree of life. Although this work is still in progress, it is clear that some clades fit uncomfortably into the Metazoa, even as stem groups to the lowest branching living phyla. On the other hand, some are plausibly stem group sponges, cnidarians, and bilaterians. Collectively, they document an increase in biotic complexity not seen in the fossil record prior to the late Precambrian ice ages (snowball Earth). If this increase in complexity is in fact polyphyletic, it suggest that extrinsic forcing factors may have driven a substantial part of the Cambrian explosion of complex organisms.