differ from molluscs in having muscular stalks and a distinctive feeding apparatus, the lophophore. Another halkieriid genus, Thambetolepis, was a "great aunt" of annelids and Wiwaxia was an "aunt" of annelids. Marine biologist Amélie H. Scheltema et al. (2003) argued that Wiwaxias feeding apparatus is very similar to the radulas of some modern shell-less aplacophoran molluscs, and that the sclerites of the two groups are very similar. They concluded that Wiwaxia was a member of a clade that includes molluscs. Scheltema has also highlighted similarities between Wiwaxia and the larvae of certain solenogaster molluscs, which bear iterated calcareous sclerites arranged into three symmetrical lateral zones. Danish zoologist Danny Eibye-Jacobsen argued in 2004 that Wiwaxia lacks any characters that would firmly place it as a polychaete or annelid. Eibye-Jacobsen regarded bristles as a feature shared by molluscs, annelids and brachiopods. Hence even if Wiwaxia's sclerites closely resembled bristles, which he doubted, this would not prove that Wiwaxia's closest relative was the annelids. He also pointed out that the very different numbers of sclerites in the various zones of Wiwaxias body do not correspond to any reasonable pattern of segmentation; while Eibye-Jacobsen did not think that this alone would prevent classification of Wiwaxia as a polychaete, he thought it was a serious objection given the lack of other clearly polychaete features. In his opinion there were no strong grounds for classifying Wiwaxia as a proto-annelid or a proto-mollusc, although he thought the objections against classification as a proto-annelid were the stronger. Butterfield returned to the debate in 2006, repeating the arguments he presented in 1990 for regarding Wiwaxia as an early polychaete and adding that, while bristles are a feature of several groups, they appear as a covering over the back only in polychaetes. A 2012 study redescribing the mouthparts found a number of similarities with the molluscan radula, and overthrew some of the better arguments for an annelid affinity, seemingly demonstrating that Wiwaxia was indeed a mollusc. Occurrence Wiwaxia was originally described by G. F. Matthew in 1899, from an isolated spine that had been found earlier in the Ogyopsis Shale, and classified as "Orthotheca corrugata". Further specimens were found by American paleontologist Charles Doolittle Walcott in 1911 as a result of one of his field trips to the nearby Burgess Shale (Miaolingian, ) in the Canadian Rocky Mountains; he classified it as a member of the polychaete group of annelid