A reassessment of the enigmatic Burgess Shale fossil Wiwaxia corrugata (Matthew) and its relationship to the polychaete Canadia spinosa Walcott

Abstract

The enigmatic fossil Wiwaxia corrugata is organically preserved in the Burgess Shale (Middle Cambrian, British Columbia) and is therefore extractable by careful acid maceration of the mineralic matrix. High magnification transmitted light microscopy and SEM of macerated Wiwaxia sclerites reveal a substantial amount of previously undescribed structural and microstructural detail. Anatomical and histological comparison with modern organisms indicates that Wiwaxia sclerites are polychaete paleae (flattened setae) and that Wiwaxia was a jawed annelid broadly related to the extant polychaete families Chrysopetalidae and/or Aprhoditidae (Palmyra). Canadia spinosa, an uncontested fossil polychaete from the same beds, shows a paleal microstructure identical to that of Wiwaxia, as well as a closely comparable gross anatomy and taphonomic grade. The unique combination of taxonomically significant characters shared by Wiwaxia and Canadia suggest that they are more closely related to each other than either is to any other fossil or extant polychaete. Thus they constitute a separate superfamily, Canadiacea superfam. nov., in the order Phyllodocida.