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

Data from: Ontogeny, morphology and taxonomy of the soft-bodied Cambrian ‘mollusc’ Wiwaxia

Data files

Abstract

Usage notes