Halwaxiida or halwaxiids is a proposed clade equivalent to the older orders Sachitida He 1980[2] and Thambetolepidea Jell 1981,[3] loosely uniting scale-bearing Cambrian animals, which may lie in the stem group to molluscs or lophotrochozoa.

[6] Some scientists are unhappy with this loose definition, arguing that such traits may have arisen convergently rather than being inherited from a common ancestor.

[6] New finds of Odontogriphus, reported in 2006, put this animal into play as well – despite its lack of sclerites or shells, its feeding apparatus looks very like Wiwaxia’s.

A possible reason for this is that during the Cambrian substrate revolution, the microbial mat's disappearance triggered an alternating pattern in the fauna that were present for the Halwaxiids to eat.

Instead he thought Wiwaxia was very similar to shell-less aplacophoran molluscs, that it must have moved on a mollusc-like muscular foot, and that its feeding apparatus looked like a primitive form of the molluscan radula, a tooth-bearing chitinous "tongue".

[16] When he briefly described the first articulated specimens of Halkieria in 1990, Conway Morris wrote of "the halkieriid-wiwaxiid body plan" and that the halkieriids might be close relatives of molluscs.

[8] Conway Morris and Peel (1995) largely accepted Butterfield's arguments and treated Wiwaxia as an ancestor or "aunt" of the polychaetes.

[14] Danish zoologist Danny Eibye-Jacobsen (2004) regarded bristles as a feature shared by molluscs, annelids and brachiopods.

They thought the teeth on the feeding apparatus of both Wiwaxia and Odontogriphus strongly resembled those of a modern group of molluscs, Neomeniomorpha.

…the seemingly trivial distinction between these two taxa is exactly what is expected at the divergence points leading from a last common ancestor to extant phyla."

However the simplest "family tree" faces an obstacle: the siphogonuchitids appear in earlier rocks and had mineralized sclerites.

[12] 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.

[22] However Conway Morris (2006) criticized Vinther and Nielsen's 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 molluscs are not at all like the halkieriid sclerites with their complex internal channels.

They proposed the two "family trees" described above: Porter (2008) revived an early 1980s idea that the sclerites of Halkieria are extremely similar to those of chancelloriids.

Butterfield and Nicholas (1996) argued that they were closely related to sponges on the grounds that the detailed structure of chancellorid sclerites is similar to that of fibers of spongin, a collagen protein, in modern keratose (horny) demosponges.

[25] Porter (2008) found that the sclerites of halkieriids and chancelloriids resemble each other at all levels: both have an internal "pulp cavity" and a thin external organic layer; the walls are made of the same material, aragonite; the arrangement of the aragonite fibers is in each is the same, running mainly from base to tip but with each being closer to the surface at the end nearest the tip.

It is extremely improbable that totally unrelated organisms could have developed such similar sclerites independently, but the huge difference in the structures of their bodies makes it hard to see how they could be closely related.