Most of the debate has been about halkieriids' relationship to
molluscs,
annelids and
brachiopods. However a paper published in 2008 argued that halkieriids were closely related to
chancelloriids, which have been generally regarded as
sponges.
Relationship to Molluscs, Annelids and Brachiopods Halkieriid
sclerites were known for a long time as elements of the
small shelly fossils. The first articulated specimen, with all their hard parts together, were collected in 1989 and were described briefly in 1990 by Conway Morris and Peel (1990). 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. 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. Conway Morris repeated his earlier conclusion that halkieriids were close to the ancestors of both molluscs and brachiopods. In their description of the newly discovered
Orthrozanclus (2007), which has similarities to both
Wiwaxia and the halkieriids, Conway Morris and Caron also took account of the siphogonuchitids, a group known only from isolated mineralized sclerites that resemble those of the halkieriids. They proposed the two "family trees" described above: • The halkieriids formed part of a "sister" group to the molluscs. This hypothesis implies that the halwaxiids were not
monophyletic, since the siphogonuchitids appear between
Wiwaxia and the halkieriids. Nevertheless, Conway Morris and Caron found that this
phylogeny fitted the available data better, although it was not robust. • The siphogonuchitids plus the halwaxiids formed a "sister" group to the annelids and brachiopods, while the molluscs were "aunts" of all of these. 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. This dilemma may be resolved in various ways: but shows no evidence of sclerites. ==Notes and references==