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
Micrograph of NMNH229901
Low-quality light micrograph of 229901. For scale please see accompanying SEM.
SEM close-up of NMNH 229901, showing two sclerites
SEM enlargement of NMNH 229901, showing pustules on sclerites
SEM enlargement of NMNH 229901, showing relief
BSE in shadow mode.
SEM enlargement of NMNH 229901, showing sclerite with longitudinal pyrite crystals
SEM enlargement of NMNH 229901 (posterior)
Showing ventral sclerites
SEM of NMNH194274
Specimen length ~5mm; dorsal scleritome well preserved, showing ribbing patterns and arrangement of sclerites. Backscatter (compo)
SEM of NMNH194274 (BSE - shadow)
Specimen length ~5mm; dorsal scleritome well preserved, showing ribbing patterns and arrangement of sclerites. Backscatter (shadow)
SEM of NMNH199969 - isolated sclerite containing acicular pyrite
SEM enlargement of NMNH199969 - tip of isolated sclerite containing acicular pyrite
SEM of NMNH198674, showing acicular pyrite in sclerites
SEM of NMNH198674, showing acicular pyrite in sclerites - wide field of view
SEM of NMNH198674, showing acicular pyrite in sclerites - wider field of view
Note three-dimensionality of specimen; lower sclerites are out of focus.
SEM enlargement of NMNH198674, showing well-developed acicular crystals (Fig. 2D)
SEM enlargement of NMNH198674, showing well-developed acicular crystals in multiple sclerites (Fig. 2D)
SEM enlargement of NMNH198674, showing distribution of acicular crystals in sclerite ribs
SEM of NMNH198674, showing sclerites with varying degrees of pyritization; cf. Fig. 2E
Region of Fig. 2E in top centre of image
SEM enlargement of NMNH198674, showing lineations in carbon parallel to acicular pyrite crystals (Fig 2A)
SEM enlargement of NMNH198674, showing lineations in carbon parallel to acicular pyrite crystals (Fig 2B)
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.
SEM of Burgessochaeta, NMNH198634 (Fig. 2G)
Note the framboidal form of pyrite in chaetae.
SEM enlargement of Burgessochaeta, NMNH198634 (Fig. 2H)
SEM of Burgessochaeta, NMNH198634
Further chaetae with similar preservation
SEM enlargement of Burgessochaeta, NMNH198634, showing framboidal pyrite in chaetae
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.
Part of ROM57195, Wiwaxia, containing amorphous pyrite
Emphasizing that pyrite is not always associated with an acicular habit.
ROM 57195 (Fig. 3A), colour image highlighting pyritization
ROM 61512 (Fig. 3B) – 'juvenile' specimen, partly enrolled (colour image file)
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
ROM 56965 (Fig. 5A) – partially disaggregated scleritome
USNM 199953 (Fig. 5B) – lateral group of sclerites
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.
3D model of Wiwaxia infant (Fig. 6A)
3D model file used to generate Figure 6A. Blender file format (blender.org).
3D model of Wiwaxia juvenile (Fig. 6B)
3D model file used to generate Figure 6B. Blender file format (blender.org).
3D model of mature Wiwaxia (Fig. 6C)
3D model file used to generate Figure 6C. Blender file format (blender.org).
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 (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.
NMNH 200101 – SEM of anterior region showing mouthparts
Mouthparts stand out as carbon films.
NMNH 200101 – SEM of posterior region
Sclerites are picked out by faint pyrite layer, but carbonaceous mouthparts are demonstrably absent.
ROM 61701 (Fig. 7B) – showing axial trace (gut)
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 61519 (Fig. 8A) – Wiwaxia, terminal view, showing foot
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
ROM 57725 (Fig. 8D) – Odontogriphus, lit to emphasize longitudinal lineations in medial region