What Counts as a Phylum? Systematics, Evolution, and the Burgess Shale  

Stephen Jay Gould argued that the Burgess Shale fossils, remnants of the Cambrian explosion, represent previously unknown (and now extinct) body plans, of equivalent morphological disparity and taxonomic rank to modern animal phyla.  He contrasted his own interpretation with that of Charles Walcott, discoverer of the Burgess Shale, who (Gould said) had “shoehorned” the Burgess fossils into modern animal phyla and classes.

For much of the twentieth century, these two choices seemed to be the only ones available to palaeontologists: either the Burgess fossils were members of the known animal phyla, or they must be classified in new phyla of their own.  However, a third option became available in the late 1980s, when other palaeontologists, including Derek Briggs and Graham Budd, began to recast Burgess and other Cambrian forms as stem-group species.  On this new understanding, the former “weird wonders” of the Burgess Shale were given a new place in the tree of life, ancestral to (but not themselves members of) the modern crown groups.  The debate continues, not only about how to classify the Burgess creatures specifically, but how to incorporate both extinct and extant animal groups in a coherent system of biological classification.

A closer examination of this disagreement reveals it is not merely a dispute over taxonomic ranking, but opens a window onto scientists’ understanding of the fundamental workings of evolution.  I will discuss how the debate over the interpretation of these Cambrian fossils draws on and illuminates larger debates over the tempo and mode of evolution, and the relative roles in evolution of contingency versus convergence, and adaptationism versus structuralism, as well as systematic methodology and the pattern of the history of life.  Another Burgess Shale palaeontologist, Simon Conway Morris, has played a crucial role in these debates.  Decisions about how to classify Burgess Shale organisms are not mere disputes over stamp collecting, but instead reflect such critical biological issues as how to define a phylum, and whether all animal species belong in this most fundamental of taxonomic groups.