Wiwaxia

Wiwaxia is a genus of soft-bodied animals that were covered in carbonaceous scales and spines. Wiwaxia fossils – mainly isolated scales, but sometimes complete, articulated fossils – are known from early Cambrian and middle Cambrian fossil deposits across the globe.[2][3][4] The living animal would have measured up to 5 cm (2 in) when fully grown, although a range of juvenile specimens are known, the smallest being 2 millimetres (0.079 in) long.[4]

Wiwaxia’s affinity has been a matter of debate: researchers were long split between two possibilities. On the one hand, its rows of scales looked superficially similar to certain scale worms (annelids); conversely, its mouthparts and general morphology suggested a relationship to the shell-less molluscs. More recently, evidence for a molluscan affinity has been accumulating, based on new details of Wiwaxia’s mouthparts, scales, and growth history.

Description
This article concentrates on the species Wiwaxia corrugata, which is known from hundreds of complete specimens in the Burgess Shale; other species are known only from fragmentary material.

Body
Wiwaxia was bilaterally symmetrical; viewed from the top the body was elliptical with no distinct head or tail, and from the front or rear it was almost rectangular. It reached 5 centimetres (2.0 in) in length. Estimating their height is difficult because specimens were compressed after death; a typical specimen may have been 1 centimetre (0.39 in) high excluding the spines on their backs. The ratio of width to length does not appear to change as the animals grew.[3]

Wiwaxia’s flat underside was soft and unarmored; most of the surface was occupied by a slug-like foot.[4] Little is known of the internal anatomy, although the gut apparently ran straight and all the way from the front to the rear. At the front end of the gut, about 5 millimetres (0.20 in) from the animal's front in an average specimen about 2.5 centimetres (0.98 in) long, there was a feeding apparatus that consisted of two (or in rare large specimens three) rows of backward-pointing conical teeth. The feeding apparatus was tough enough to be frequently preserved, but unmineralized and fairly flexible.

Sclerites
The animal was covered in eight rows of small ribbed armor plates called sclerites; these lay flat against the body, overlapped so that the rear of one covered the front of the one behind, and formed five main regions — the top; the upper part of the sides; the lower part of the sides; the front; and the bottom. Most of the sclerites were shaped like oval leaves, but the ventro-lateral ones, nearest the sea-floor, were crescent-shaped, rather like flattened bananas, and formed a single row. Larger specimens (>~15mm) bear two rows of ribbed spines running from to rear, one along each side of the top surface, and projecting out and slightly upwards, with a slight upwards curve near the tips. Although the spines in the middle of each row are usually the longest, up to 5 centimetres (2.0 in) long, a few specimens have rather short middle spines that represent part-grown replacements.[4]

Each sclerite was rooted separately in the body; the roots of body sclerites are 40% of the external length or a little less, while the roots of the spines are a little over 25% of the external length; all were rooted in pockets in the skin, rather like the follicles of mammalian hair. The roots of the body sclerites were significantly narrower than the sclerites, but the spines had roots about as wide as their bases; both types of root were made of fairly soft tissue. They bore protrusive, presumably structural, ribs on their upper and (seemingly) lower surfaces.[8]:544 The sclerites and spines were not mineralized, but made of a tough organic (carbon-based) biopolymer. Butterfield (1990) examined some sclerites under both optical and scanning electron microscopes and concluded that they were not hollow, and that the bases split and spread to form the blades, a pattern that is also seen in monocot leaves. [9] The sclerites bear an internal fabric of longitudinal chambers, which suggest that they were secreted from their bases in the manner of Lophotrochozoan sclerites.

Ontogeny
Wiwaxia's scleritome comprises eight rows of sclerites, arranged in bundles. Sclerites are periodically shed and replaced during growth, with the number of sclerites in a given bundle increasing as the animal ages to produce a thicker scleritome.

Once specimens reach a certain size, spines are added to the scleritome; this size is ~15 mm in W. corrugata but substantially smaller in W. taijengensis.[4]

One juvenile specimen was originally interpreted as molting,[3] but in fact represents a single, folded, individual.

Ecology
The long dorsal spines may have been a defense against predators. Wiwaxia apparently moved by contractions of a slug-like foot on its underside.[4] In one specimen a small brachiopod, Diraphora bellicostata, appears to be attached to one of the ventro-lateral sclerites. This suggests that adult Wiwaxia did not burrow or even plough much into the sea-floor as they moved. Two other specimens of Diraphora bellicostata have been found attached to dorsal sclerites. Wiwaxia appears to have been solitary rather than gregarious.[3] The feeding apparatus may have acted as a rasp to scrape bacteria off the top of the microbial mat that covered the sea-floor, or as a rake to gather food particles from the sea-floor.