Wiwaxia - Description

Description

Spine Dorsal Upper lateral Lower lateral Ventro-lateral = Frontal Wiwaxia sclerite zones, seen from front
Wiwaxia viewed from top (moving up) and right

This concentrates on the species Wiwaxia corrugata, found in the Burgess Shale, since the other specimens consist only of fragments, while the Burgess Shale has provided at least 138 complete ones.

Wiwaxia was a bilaterally symmetrical animal. Viewed from the top the body was elliptical with no distinct head or tail, and from the front or rear it was almost rectangular. The most complete fossils fall into two size ranges: 2 centimetres (0.79 in) to 5 centimetres (2.0 in) long, which are thought to be adults; and 3.4 millimetres (0.13 in) to 1.5 centimetres (0.59 in), which are thought to be juveniles. Estimating their height is difficult because specimens were compressed after death; a specimen of the average length, 3.4 centimetres (1.3 in), 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. The animal was covered in small ribbed armor plates called sclerites, that 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, with 8-9 rows of sclerites; the upper part of the sides, with 11-12; the lower part of the sides, with 8; the front; and the area nearest the sea-floor, with 12-17 rows. 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 with the tips pointing down. In addition there were 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. Specimens ranging from 11 millimetres (0.43 in) 52 millimetres (2.0 in) have about the same number of ventro-lateral sclerites just above the foot. On the other hand the number of spines seems to depend on the size of the specimen, up to about 12 per side. The number and spacing of the spines is asymmetrical in the specimens found, and this may have been natural rather than a result of events in the animal's life or after death. Although the spines in the middle of each row are usually the longest, up to 5 centimetres (2.0 in), a few specimens have rather short middle spines, perhaps because these were part-grown replacements. The smallest specimens may have lacked the long dorsal spines, which appear to have grown quickly in larger juveniles and then more slowly in adults.

"Blade" = Root Wiwaxia spine, seen from front and side

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. The sclerites and spines were not mineralized, and the frayed appearance of some broken ones suggests a fibrous structure. The way they were preserved suggests they were not made of chitin, from which insects' exoskeletons are formed. They may have been made of tanned proteins or of collagen, which is the main component of cartilages and tendons in humans. Since the body sclerites had bases that were narrower than the hard external parts, it is hard to see how they grew. They may have enclosed soft tissue that could have secreted the hard walls, but there is no convincing evidence for this. 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.

Wiwaxia’s flat underside was soft and unarmored. 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, as it folded and retracted when not in use. It would have had to be pushed forward out of the mouth in order to feed. Even the smallest specimens have this type of apparatus, with two rows containing the same number of teeth as in larger ones. This indicates that Wiwaxia’s feeding habits remained the same throughout its life after the larval stage. 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 that lay on the sea-floor.

Since there is no sign of eyes or tentacles, Wiwaxia may have relied mainly on chemical senses such as smell and taste. Its respiratory system is also unknown.

The long dorsal spines may have been a defense against predators, and finds of broken spines suggest that Wiwaxia was attacked. The animal appears to have crawled on the surface of the sea-floor feeding on particles that fell from higher levels of the sea. Wiwaxia shows no signs of legs and was probably too large to move on cilia, so it probably moved by muscular contraction that made its foot ripple. Juveniles may have burrowed into the sea-floor. 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.

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