What conflicting evidence clouds Wiwaxia's placement outside Mollusca/Annelida?
Answer
Absence of clear internal visceral mass or segmentation
The difficulty in definitively classifying *Wiwaxia* stems from the incomplete nature of the fossil record, which preferentially preserves hard parts. Features that would confirm placement in Mollusca (like a defined internal visceral mass or a true muscular foot) or Annelida (like clear metameric segmentation) are notably absent in the fossil evidence. Consequently, interpretation relies heavily on the external armor—the sclerites and spines—whose homologous nature is debated, leaving the placement contingent on structures that might represent convergent evolution rather than true shared ancestry.
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