Analysis of Problematica Ecdysozoa from the Burgess Shale Using Disparity

The Burgess Shale is one of the most famous lagerstätten from the Cambrian and is known for the “bizarre” organisms found therein. Where do these unique taxa fit into the tree of life? Their phylogenetic placement has been complicated by the fact that the organisms may have some, but not all, morphologic characters that are used to identify modern phyla. By understanding if these organisms are stem-groups, crown-groups, or new phyla entirely, we cast new light on the nature of the Cambrian Explosion.

In order to understand how Burgess Shale taxa relate to modern crown-group taxa, an extensive morphologic range for modern taxa is required. Both modern and Burgess Shale ecdysozoans (chosen for their abundance) were coded with a list of morphologic characters and plotted in morphospace. The range of modern taxa's morphology will be used as the proxy for phyla. The Burgess Shale taxa were assigned as stem-group (ancestral), crown-group (within the phylogenetic group), or in new phyla based on the amount of disparity between the Burgess taxa and the ranges of modern taxa. Burgess organisms were placed within these groups based upon the distance between it and the centroid of the range of modern taxa within a phylum. If the distance is comparable to the average distance within the range, the taxa was considered within the crown group. If the distance was somewhat larger, it was a stem group, and much larger indicated a new phylum. Using this method, the number of phyla during the Cambrian can be estimated and conclusions can be drawn about the amount of overall disparity during the Cambrian Explosion.