Anomalocaridid Diversity in the Middle Cambrian Burgess Shale, Canada  

The anomalocaridids are one of the most recognizable organisms of the Burgess Shale, owing to their large size, complex morphology, inferred predatory lifestyle and controversial phylogenetic affinity.  The complex history of discovery of these animals can be partly attributed to the variable preservation potential of different parts of the anomalocaridid body.  Frontal appendages and mouthparts preserve much more readily than whole-body assemblages, so the earliest work on these animals examined these structures in isolation.  It was only relatively recently that the three Burgess Shale genera, Anomalocaris, Laggania and Hurdia, were described in full. 

New isolated frontal appendage material of three Burgess Shale taxa  shows that the diversity of anomalocaridids in this formation is higher than previously thought. New specimens housed at the Royal Ontario Museum, Toronto, include (1) the first known occurrence of Amplectobelua outside of China; (2) a new morph of the Hurdia appendage exhibiting high morphological variability; and (3) a completely new anomalocaridid appendage, which is similar to the Anomalocaris appendage but has a straighter outline and a different arrangement of spines.  This new anomalocaridid material was contemporaneous with the previously described taxa Hurdia, Laggania, and Anomalocaris, and differences in frontal appendage morphology of all these taxa are thought to reflect different feeding strategies employed in order to alleviate competition for prey items.  The stratigraphically oldest quarry, S7 on Mount Stephen, yields appendages from all the anomalocaridid taxa, but assemblages found in younger quarries on Mount Field are dominated by Anomalocaris and Hurdia only, which may indicate a decrease in anomalocaridid diversity with time.