Opabinia Anomalocaris, unique Cambrian 'arthropods'

The Cambrian Opabinia Anomalocaris are odd animals known mainly from the Middle Cambrian Burgess Shale. Opabinia has usually been regarded as an arthropod, e.g. as a branchiopod crustacean. Parts of Anomalocaris have been referred to three or four different phyla. Recent redescriptions have clarified much of their morphology resulted in their removal from the arthropods. Additional observations considerations indicate that the two genera have important similarities, including scale-like structures arranged segmentally in transverse dorsal sets, which are separated by transverse tergal plates. Although external views are rare, traces of segmented appendages are identified in Anomalocaris. The animals are therefore again considered as arthropods, although they do not seem to be related to any of the other arthropod phyla.