Halkieriids are Cambrian animals that looked like slugs in scale mail; often when they died their scales, called sclerites, dissociated and scattered, and their sclerites represent a significant component of the small shelly fauna of the early Cambrian. They typically had their front and back ends capped with shells that resembled those we see in bivalve brachiopods. Wiwaxiids were also sluglike, but sported very prominent, long sclerites, and lacked the anterior and posterior shells; their exact position in the evolutionary tree has bounced about quite a bit, but some argument has made that they belong in the annelid ancestry, and that their sclerites are homologous to the bristly setae of worms. One simplistic picture of their relationship to modern forms was that the halkieriids expanded their shells and shed their scales to become molluscs, while the wiwaxiids minimized their armor to emphasize flexibility and became more wormlike. (Note that that is a very crude summary; relationships of these Cambrian groups to modern clades are extremely contentious. There’s a more accurate description of the relationships below.)
Now a new fossil has been found, Orthozanclus reburrus that unites the two into a larger clade, the halwaxiids. Like the halkieriids, it has an anterior shell (but not a posterior one), and like the wiwaxiids, it has long spiky sclerites. In some ways, this simplifies the relationships; it unites some problematic organisms into a single branch on the tree. The question now becomes where that branch is located—whether the halwaxiids belong in a separate phylum that split off from the lophophorate family tree after the molluscs, or whether the halwaxiids are a sister group to the molluscs.
Here are some photos of the actual fossils.
This work doesn’t yet resolve exactly where to place the halwaxiids on the family tree—there are a couple of possibilities ilusstrated below. (Note also the mention of Odontogriphus, which I wrote about before; it’s like an early Cambrian slug without the armor).
While the relationships are still a bit murky, one important thing is that the murkiness unites three phyla, the molluscs, annelids, and brachiopods, and the difficulty in placing their ancestors into discrete lineages implies a tighter kinship between these groups in the Cambrian than you’d guess from the taxonomy—oh, what we could learn from a few Cambrian DNA samples!
Conway Morris S, Caron J-B (2007) Halwaxiids and the Early Evolution of the Lophotrochozoans. Science 315(5816):1255-1258.