page_title: Wiwaxia
section_title: Classification
hierarchical_section_title: Wiwaxia / Classification
caption_attribution_description: English: See File:Wiwaxia 5c.jpg for details.
context_page_description: Wiwaxia is a genus of soft-bodied animals that were covered in carbonaceous scales and spines that protected it from predators. Wiwaxia fossils – mainly isolated scales, but sometimes complete, articulated fossils – are known from early Cambrian and middle Cambrian fossil deposits across the globe. The living animal would have measured up to 5 cm when fully grown, although a range of juvenile specimens are known, the smallest being 2 millimetres long.
  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; 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.
  The proposed clade Halwaxiida contains Wiwaxia as well as several similar Cambrian animals.
context_section_description: During the Cambrian, most of the main groupings of animals recognised today were beginning to diverge. Consequently, many lineages (that would later become extinct) appear intermediate to two or more modern groups, or lack features common to all modern members of a group, and hence fall into the "stem group" of a modern taxon. Debate is ongoing as to whether Wiwaxia can be placed within a modern crown group and, if it cannot, in which group's stem it falls. When Walcott first described Wiwaxia, he regarded it as a polychaete annelid worm, and its sclerites as similar to the elytra ("scales") of annelids. More recently the debate has been intense, and proposed classifications include: a member of an extinct phylum distantly related to the molluscs; a crown-group polychaete; a stem-group annelid; a problematic bilaterian; a stem- or possibly primitive crown-group mollusc.
  In 1985 Simon Conway Morris agreed that there were similarities to polychaetes, but considered that Wiwaxia's sclerites were different in construction from annelids' elytra. He was more impressed by the similarities between Wiwaxia's feeding apparatus and a molluscan radula, and assigned the animal to a new taxon Molluscata, which he proposed should also contain the molluscs and hyolithids. When he later described the first fairly complete specimens of Halkieria, he suggested that these were closely related to Wiwaxia.
  Nick Butterfield, then a postgraduate paleontologist at Harvard inspired by Stephen Jay Gould's lectures, agreed that the sclerites were not like elytra, which are relatively fleshy and soft. However, since the sclerites were solid, he concluded that Wiwaxia could not be a member of the "Coeloscleritophora", a taxon that had been proposed in order to unite organisms with hollow sclerites, and could not be closely related to the halkieriids, which have hollow sclerites. Instead he thought that they were very similar in several ways to the
  chitinous bristles (setae) that project from the bodies of modern annelids and in some genera form leaf-like scales that cover the back like roof tiles – in composition, in detailed structure, in how they were attached to the body via "follicles" and in overall appearance. Some modern annelids also develop on each side rows of longer bristles, which both Walcott and Butterfield considered similar to Wiwaxia's dorsal spines. including the halkieriids.
  Butterfield also contended that Wiwaxia's feeding apparatus, instead of being mounted in the middle of its "head", was just as likely to be mounted in two parts on the sides of the "head", an arrangement that is common in polychaetes. He went so far as to classify Wiwaxia as a member of a modern order, Phyllodocida, and pointed out that Wiwaxia's lack of obvious segmentation is no barrier to this, as some modern polychaetes also show no segmentation except during development. He later noted that Wiwaxia lack some polychaete features which he would expect to be easily preserved in fossils, and therefore a stem-group annelid, in other words an evolutionary "aunt" of modern annelids.
  Conway Morris and Peel (1995) largely accepted Butterfield's arguments and treated Wiwaxia as an ancestor or "aunt" of the polychaetes, and said Butterfield had informed them that the microscopic structure of Wiwaxias sclerites was identical to that of the bristles of two Burgess Shale polychaetes Burgessochaeta and Canadia. Conway Morris and Peel also wrote that one specimen of Wiwaxia showed traces of a small shell, possibly a vestige left over from an earlier stage in the animal's evolution, and noted that one group of modern polychaetes also has what may be a vestigial shell. However they maintained that Wiwaxia's feeding apparatus was much more like a molluscan radula. They also argued that Wiwaxia was fairly closely related to and in fact descended from the halkieriids, as the sclerites are divided into similar groups, although those of halkieriids were much smaller and more numerous; they also said that in 1994 Butterfield had found Wiwaxia sclerites that were clearly hollow. T
caption: Wiwaxia corrugata from the Burgess Shale.  Mouthparts are visible in enlargement.  From Smith (2013).
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