Wiwaxia
Wiwaxia is an extinct species known mainly from fossils found in Canada's Burgess Shale deposits. Although Wiwaxia is similar to a mollusk, it does not really fit this group because of its sclerites (armor). It is still controversial to what group Wiwaxia really belongs.
In 1899, Wiwaxia was first described on the basis of a single spine discovered in a Middle Cambrian bed on Mount Stephen, which lies across the valley from the Burgess Shale site. Later, after Walcott's discovery of the Burgess Shale site, complete specimens became available.
Wiwaxia has long spines that project in two rows along the back. These evidently provided some protection from predators. The rest of the upper (dorsal) surface is covered with small, flat, overlapping hard plates, termed sclerites. Each of these little scales was attached with a root-like base and it is assumed Wiwaxia grew by molting these plates from time to time. Since there are none on the bottom (ventral) surface, the animal partly resembles the slug, a member of the mollusk family. However, mollusks do not have any sclerite armor, so the animal's affinity to present day species is unsettled. It did have an anterior jaw with two rows of teeth on the ventral surface, suggesting it was another bottom feeder. Fossil sizes range from 1/8 to 2 inches.
Wiwaxia has recently been proposed as an annelid or at least a close relative of one. The first breakthrough in establishing Wiwaxia's affinities came from a postgraduate paleontologist at Harvard who was inspired by Stephen Jay Gould's lectures a decade or so ago. This young researcher, Nick Butterfield, managed to extract pieces of the scalelike armour from the fossilized animal. When Butterfield studied their microstructure, he noticed immediately that it was the same as that of the chitinous bristles (chaetae) that project from the bodies of such modern annelids as earthworms. His conclusion, published in 1990, was that Wiwaxia was not a mollusk at all but an annelid.