The soft-bodied Cambrian organism Wiwaxia poses a taxonomic conundrum. Its imbricated dorsal scleritome suggests a relationship with the polychaete annelid worms, whereas its mouthparts and naked ventral surface invite comparison with the molluscan radula and foot. 476 new and existing specimens from the 505-Myr-old Burgess Shale cast fresh light on Wiwaxia's sclerites and scleritome. My observations illuminate the diversity within the genus and demonstrate that Wiwaxia did not undergo discrete moult stages; rather, its scleritome developed gradually, with piecewise addition and replacement of individually secreted sclerites. I recognize a digestive tract and creeping foot in Wiwaxia, solidifying its relationship with the contemporary Odontogriphus. Similarities between the scleritomes of Wiwaxia, halkieriids, Polyplacophora and Aplacophora hint that the taxa are related. A molluscan affinity is robustly established, and Wiwaxia provides a good fossil proxy for the ancestral aculiferan – and perhaps molluscan – body plan.
SEM of NMNH229901
The smallest known specimen of _Wiwaxia corrugata_; relative to body length, dorsal sclerites are vast
229901_0056.g.tif
Micrograph of NMNH229901
Low-quality light micrograph of 229901. For scale please see accompanying SEM.
USNM_229901_X31_006.tif
SEM close-up of NMNH 229901, showing two sclerites
229901_0058.tif
SEM enlargement of NMNH 229901, showing pustules on sclerites
229901_0059.tif
SEM enlargement of NMNH 229901, showing relief
BSE in shadow mode.
229901_0060.tif
SEM enlargement of NMNH 229901, showing sclerite with longitudinal pyrite crystals
229901_0061.g2.tif
SEM enlargement of NMNH 229901 (posterior)
Showing ventral sclerites
229901_0062.g.tif
SEM of NMNH194274
Specimen length ~5mm; dorsal scleritome well preserved, showing ribbing patterns and arrangement of sclerites. Backscatter (compo)
194724_0041.g.tif
SEM of NMNH194274 (BSE - shadow)
Specimen length ~5mm; dorsal scleritome well preserved, showing ribbing patterns and arrangement of sclerites. Backscatter (shadow)
194724_0042.g.tif
SEM of NMNH199969 - isolated sclerite containing acicular pyrite
199969_0080.g.tif
SEM enlargement of NMNH199969 - tip of isolated sclerite containing acicular pyrite
199969_0082.g.tif
SEM of NMNH198674, showing acicular pyrite in sclerites
198674_0112.g.tif
SEM of NMNH198674, showing acicular pyrite in sclerites - wide field of view
198674_0117.g.tif
SEM of NMNH198674, showing acicular pyrite in sclerites - wider field of view
Note three-dimensionality of specimen; lower sclerites are out of focus.
198674_0116.g.tif
SEM enlargement of NMNH198674, showing well-developed acicular crystals (Fig. 2D)
198674_0113.g.tif
SEM enlargement of NMNH198674, showing well-developed acicular crystals in multiple sclerites (Fig. 2D)
198674_0115.g.tif
SEM enlargement of NMNH198674, showing distribution of acicular crystals in sclerite ribs
198674_0125.g.tif
SEM of NMNH198674, showing sclerites with varying degrees of pyritization; cf. Fig. 2E
Region of Fig. 2E in top centre of image
198674_0127.g.tif
SEM enlargement of NMNH198674, showing lineations in carbon parallel to acicular pyrite crystals (Fig 2A)
198674_0131.g.tif
SEM enlargement of NMNH198674, showing lineations in carbon parallel to acicular pyrite crystals (Fig 2B)
198674_0130.g.tif
SEM of NMNH198662, isolated Wiwaxia sclerite
Showing acicular pyrite and later spheroidal overgrowths; note the shape of the end of the sclerite. The microvillar fabric, although not evident in the carbon film, is picked out by the pyrite needles.
NMNH198662_001.g.tif
SEM of Burgessochaeta, NMNH198634 (Fig. 2G)
Note the framboidal form of pyrite in chaetae.
198634_627.g.tif
SEM enlargement of Burgessochaeta, NMNH198634 (Fig. 2H)
198634_630.g.tif
SEM of Burgessochaeta, NMNH198634
Further chaetae with similar preservation
NMNH198634_620.g.tif
SEM enlargement of Burgessochaeta, NMNH198634, showing framboidal pyrite in chaetae
198634_618.g.tif
Chaetae of a typical Canadia specimen (NMNH199771); SEM
Chaetae are replaced by 'massive' pyrite with no obvious crystals. Note that occasional Wiwaxia specimens showed a similar preservational style.
199771_803.g.tif
Part of ROM57195, Wiwaxia, containing amorphous pyrite
Emphasizing that pyrite is not always associated with an acicular habit.
ROM57195.g.tif
ROM 57195 (Fig. 3A), colour image highlighting pyritization
ROM 57195.JPG
ROM 61512 (Fig. 3B) – 'juvenile' specimen, partly enrolled (colour image file)
WT90-0185_614 615.grainextract.JPG
ROM 61649 (Fig. 3D) – Juvenile specimen with faint trace of foot
Juvenile specimen, preserved sublaterally; mouthparts are visible to anterior, with dark transverse trace underneath, interpreted as anterior margin of foot. See also wet image.
ROM 61649 dry.g.JPG
ROM 61649 (Fig. 3D) – Juvenile specimen with faint trace of foot (wet)
Juvenile specimen, preserved sublaterally; mouthparts are visible to anterior, with dark transverse trace underneath, interpreted as anterior margin of foot. See also dry image.
ROM 61649 wet.g.JPG
ROM 61151 (Fig. 3G) – Mature specimen, with incipient spines, and partial scleritome exposing underlying tissue
WQ00-1886_657g ge 658g.g.JPG
ROM 62273 (Fig. 3I) – posterior portion of specimen (dorsal view), showing imbrication of ventral sclerites
WT90-1446_452.g.JPG
ROM 56965 (Fig. 5A) – partially disaggregated scleritome
ROM 56965_652.g.JPG
USNM 199953 (Fig. 5B) – lateral group of sclerites
USNM 199953.g.tif
ROM 61510 (Fig. 5C) – showing depth of scleritome
98-1116B ROM 61510_953.g.JPG
ROM 61510 (counterpart)
The plane of splitting passed close to the dorsal surface – hence the depth of sclerites on the part, and the spaces between sclerites on this counterpart.
ROM 61510.g.JPG
3D model of Wiwaxia infant (Fig. 6A)
3D model file used to generate Figure 6A. Blender file format (blender.org).
Wiwaxia infant.blend
3D model of Wiwaxia juvenile (Fig. 6B)
3D model file used to generate Figure 6B. Blender file format (blender.org).
Wiwaxia juvenile.blend
3D model of mature Wiwaxia (Fig. 6C)
3D model file used to generate Figure 6C. Blender file format (blender.org).
Wiwaxia mature.blend
NMNH 200101 (Fig. 7A) – "moulting" specimen
Note that gut (pinker) can be traced along length of specimen. Mouthparts are only visible in the anterior of the specimen.
NMNH 200101_003.g.tif
NMNH 200101 (Fig. 7A) – Interference image
Interference image highlights distinct preservation of gut, confirming its continuity into the posterior of the specimen. The pronounced nature of the mouthparts heightens confidence that there are genuinely no mouthparts further posteriad.
USNM_200101p_X8_003-BexR.g2.tif
NMNH 200101 – SEM of anterior region showing mouthparts
Mouthparts stand out as carbon films.
200101cp_0035.tif
NMNH 200101 – SEM of posterior region
Sclerites are picked out by faint pyrite layer, but carbonaceous mouthparts are demonstrably absent.
200101cp_0034.tif
ROM 61701 (Fig. 7B) – showing axial trace (gut)
ROM 61701_666.g.JPG
ROM 61515 (Fig. 7C) – Odontogriphus
WQ94-1498A_719 gex 720.JPG
ROM 57714 (Fig. 7D) – Odontogriphus
Showing mouthparts and gut
WQ95-2130 ROM 57714_616 div 617.g.JPG
ROM_57715 (Fig. 7E) – Odontogriphus, partially decayed specimen
ROM_57715.g.JPG
ROM 61519 (Fig. 8A) – Wiwaxia, terminal view, showing foot
WQ00-1260B_670.g2.JPG
ROM 57723 (Fig. 8C) – Odontogriphus
WQ99-0699A ROM 57723_639g gm 638.JPG
ROM 57723 (Fig. 8C, enlargement) – Odontogriphus foot, showing transverse lines
WQ99-0699A ROM 57723_641.g.JPG
ROM 57725 (Fig. 8D) – Odontogriphus
WQ95-1050A_174+175col.JPG
ROM 57725 (Fig. 8D) – Odontogriphus, lit to emphasize longitudinal lineations in medial region
ROM 57725_175.g.JPG
