Cambrian explosion still in the water

Abstract

Was Adrian Thomas's short note in the last issue of TREE[1]intended to be provocative? The Cambrian explosion has not been `blown out of the water'. First, the existence of late Precambrian metazoan fossils does not alter the fact that Cambrian fossils record an explosive diversification of animals, although it may make it easier to explain. Second, it has been many years since the fossil record of metazoan animals started with the Cambrian explosion—I first learned about the late Precambrian Ediacara fauna when I was an undergraduate 25 years ago and it was not particularly new to science then. The recent discovery of fossilized metazoan embryos in 570 million-year-old Precambrian rocks in China[2]is tremendously exciting, but it is not the first indication of a metazoan fossil record predating the Cambrian explosion. The Ediacara fauna (now known as Vendozoa or Vendobionta), once shoehorned into extant phyla, has been interpreted as representing an early radiation of metazoans[3]that were not closely related to Cambrian metazoan animals and that flourished during the last 60 million years (very approximately) of the Precambrian but became extinct before the Cambrian explosion. The Chinese fossil embryos may represent the oldest fossils of metazoans that were directly ancestral to Phanerozoic (Cambrian onwards) lineages. A plausible explanation has been put forward (also reported in the same issue of TREE) that the vendozoans occupied the ecological niches available to large animals (several cm), restricting Precambrian (late Proterozoic and Vendian) precursors of Phanerozoic metazoans to small size (mm) and, therefore, little chance of fossilization[4]. The Cambrian explosion can then be attributed partly to a size increase of `Phanerozoic Metazoa' once the vendozoans became extinct. In terms of the fossil record of all metazoans, the vendozoans could even predate the latest Chinese findings. Recent dating of the Ediacara fauna of the English Midlands suggests that it has a minimum age of 603±2 million years, although the authors[5]are cautious about the accuracy of this figure.