Ontogeny, morphology and taxonomy of the soft‐bodied Cambrian ‘mollusc’ Wiwaxia

Abstract The soft‐bodied C ambrian organism W iwaxia poses a taxonomic conundrum. Its imbricated dorsal scleritome suggests a relationship with the polychaete annelid worms, whereas its mouthparts and naked ventral surface invite comparison with the molluscan radula and foot. 476 new and existing specimens from the 505‐Myr‐old B urgess S hale cast fresh light on W iwaxia 's sclerites and scleritome. My observations illuminate the diversity within the genus and demonstrate that W iwaxia did not undergo discrete moult stages; rather, its scleritome developed gradually, with piecewise addition and replacement of individually secreted sclerites. I recognize a digestive tract and creeping foot in W iwaxia , solidifying its relationship with the contemporary O dontogriphus . Similarities between the scleritomes of W iwaxia , halkieriids, P olyplacophora and A placophora hint that the taxa are related. A molluscan affinity is robustly established, and W iwaxia provides a good fossil proxy for the ancestral aculiferan – and perhaps molluscan – body plan.