The Cambrian explosion: the construction of animal biodiversity

Abstract

In 1909 Charles Walcott discovered the remarkable soft-bodied animal fauna of the Middle Cambrian age Burgess Shale in British Columbia. These fossils have become the iconic representation of the astonishing expansion of animal diversity during the Cambrian, including the first appearance in the fossil record of essentially all living phyla, known as the “Cambrian explosion.” Although the Burgess Shale biota is the public face of the Cambrian explosion, even appearing on a 1995 cover of Time, it is only a small part of the story (and somewhat late in the tale). Most readers will be familiar with the Cambrian explosion through Stephen Jay Gould’s 1989 book Wonderful Life (1989) or perhaps Martin Brasier’s more recent Darwin’s Lost World (2009). Both of these books were written to reach a general audience. What has been lacking to date is a more technical book, aimed at students and professionals, which summarizes and introduces the tremendous amount of research that this episode of earth history has received in the last 25 years. The Cambrian Explosion: The Construction of Animal Biodiversity by Douglas H Erwin and James W Valentine more than fills this need. It will become required reading for anyone who wants to understand the multiple lines of evidence, from geology, geochemistry, paleontology, genetics, phylogenetics, comparative morphology, and ecology, that have to be integrated to understand one of the most important episodes in the history of life. The authors, both paleontologists, are deeply respected authorities on evolution and the Cambrian fossil record, with a welldeserved reputation for innovative approaches. This book necessarily covers a tremendous amount of ground; the forty-two pages of references demonstrate the depth and breadth of scholarship that the authors summarize. The first section covers the geological context of the explosion, stretching back to the Snowball Earth episodes of the Cryogenian. This includes summaries of the current status of the absolute and relative time scales for this period and what geochemical proxies reveal about ocean chemistry, including oxygenation states and changes in the carbon cycle. The latter is unfortunately the weakest part of the book; there is very poor integration between figures (which come from external sources) and text; the figure captions and labels are inadequate. The book hits its stride in the next section. It begins with a brief review of phylogenetics and of living metazoan morphology and relationships, a subject Valentine explored in more depth in his comprehensive earlier book, On the Origin of Phyla (2004). This is followed