similarities Butterfield (1990) found between Wiwaxia’s sclerites and the bristles of polychaete annelids. Canadia is a Burgess Shale fossil that is widely agreed to be a polychaete. Halkieria evangelista, which Conway Morris had found in Greenland's Sirius Passet lagerstätte, was a "sister" group" to brachiopods, animals whose modern forms have bivalve shells but differ from molluscs in having muscular stalks and a distinctive feeding apparatus, the lophophore. Brachiopods have bristles that are similar to those of annelids and hence to Wiwaxia’s sclerites, and hence to halkieriid sclerites. A brachiopod affinity seemed plausible because brachiopods pass through a larval phase that resembles a halkieriid, and some isolated fossil shells thought to belong to halkieriids had a brachiopod-like microstructure. In 2003 Cohen, Holmer and Luter supported the halkieriid-brachiopod relationship, suggesting that brachiopods may have arisen from a halkieriid lineage that developed a shorter body and larger shells, and then folded itself and finally grew a stalk out of what used to be the back. Vinther and Nielsen (2005) proposed instead that Halkieria was a crown group mollusc, in other words more similar to modern molluscs that to annelids, brachiopods or any intermediate groups. They argued that: Halkieria’s sclerites resembled those of the modern solenogaster aplacophoran shell-less molluscs (see ), of some modern polyplacophoran molluscs, which have several shell plates, and of the Ordovician polyplacophoran Echinochiton; Halkieria’s shells are more similar to the shells of conchiferan molluscs, since shells of both of these groups show no trace of the canals and pores seen in polyplacophoran shell plates; the bristles of brachiopods and annelids are similar to each other but not to Halkieria’s sclerites. Caron, Scheltema, Schander and Rudkin (2006) also interpreted Halkieria as a crown group mollusc, with Wiwaxia and Odontogriphus as stem group molluscs, in other words "sister" and "aunt" of the crown group molluscs. Their main reason for regarding Halkieria as crown group molluscs is that both possessed armor mineralized with calcium carbonate. They treated Wiwaxia and Odontogriphus as stem group molluscs because in their opinion both possessed the distinctive molluscan radula, a chitonous toothed "tongue". Also in 2006, Conway Morris criticized Vinther and Nielsen's (2005) classification of Halkieria as a crown group mollusc, on the grounds that the growth of the spicules in the aplacophorans and polyplacophorans is not similar to the method of growth deduced for the complex halkieriid sclerites; in particular, he said, the hollow spines of various