Disparity and constraint in olenelloid trilobites and the Cambrian radiation

The Cambrian Radiation marks the appearance of representatives of virtually all major skeletonized phyla in the fossil record and clearly represents a fundamental episode in the history of life. Furthermore, the tempo and mod e of this evolutionary event have been the subject of intense debate. One a rea that has been debated is how so many phylum-level body plans can have e volved in such a geologically brief period. Some have argued that there was enhanced morphological flexibility and fewer evolutionary constraints at t his time, leading to greater morphological disparity of Early Cambrian faun as. Others have claimed that this is not true because the evolution of most of the animal phyla significantly predates the radiation or because they f ailed to detect a signature of decreasing morphological disparity through t ime. At present, the higher-level patterns of diversification during this t ime period and the relevant implications for Early Cambrian uniqueness are areas of active research interest and debate. Recognizing this debate, we u sed both a phylogenetic and a morphometric framework to study whether there is a signature of increasing morphological constraint and decreasing flexi bility through time within one of the clades that is a significant constitu ent of the Early Cambrian biota, specifically, the olenelloid trilobites. I n this species-rich clade, we found no evidence that morphological changes were becoming either increasingly constrained or less flexible in one of th e dominant Early Cambrian metazoan clades as it passed through the Cambrian Radiation.