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

The soft-bodied Cambrian organism Wiwaxia 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 Burgess Shale cast fresh light on Wiwaxia's sclerites and scleritome. My observations illuminate the diversity within the genus and demonstrate that Wiwaxia 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 Wiwaxia, solidifying its relationship with the contemporary Odontogriphus. Similarities between the scleritomes of Wiwaxia, halkieriids, Polyplacophora and Aplacophora hint that the taxa are related. A molluscan affinity is robustly established, and Wiwaxia provides a good fossil proxy for the ancestral aculiferan – and perhaps molluscan – body plan.

寒武纪软体生物威瓦亚虫（Wiwaxia）长期以来是分类学领域的难解谜题。其覆瓦状排布的背部骨片套（scleritome）暗示其与多毛类环节动物（polychaete annelid worms）存在亲缘关联，而其口器与裸露的腹面则让人联想到软体动物的齿舌（radula）与腹足（foot）。本研究依托采自5.05亿年前布尔吉斯页岩（Burgess Shale）的476件新发现及已有的标本，为威瓦亚虫的骨片与骨片套研究提供了全新视角。观测结果揭示了该属内的形态多样性，并证实威瓦亚虫并未经历离散的蜕皮阶段（moult stages）；相反，其骨片套的发育过程呈渐进式，通过分段添加与替换个体分泌的骨片完成。本研究团队在威瓦亚虫体内识别出消化道（digestive tract）与匍匐足（creeping foot），进一步明确了其与同期生物齿形虫（Odontogriphus）的亲缘关系。威瓦亚虫、哈尔克虫类（halkieriids）、多板纲（Polyplacophora）与无板纲（Aplacophora）的骨片套之间存在诸多相似性，提示这些类群具有演化关联。其软体动物亲缘关系已得到确凿证实，威瓦亚虫可为原始针壳类（aculiferan）乃至软体动物的躯体构型提供优质的化石替代模型（fossil proxy）。