Wiwaxia
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Extinct (fossil)
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| Wiwaxia corrugata Walcott, 1911 |
Wiwaxia corrugata is an extinct species of animal known only from fossils found in Canada's Burgess Shale deposits. Although Wiwaxia resembles a mollusk in having a well developed radula, it does not really fit the conchifera because of its sclerites (armor of flattened, chitinous spines), but rather the class Aplacophora. The actual classification of Wiwaxia in the animal kingdom is still controversial.
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[edit] Research and Classification
Wiwaxia's taxonomy, even which phylum it belongs to, remains a highly debated topic.
[edit] As An Annelid
Charles Doolittle Walcott first described Wiwaxia in 1911, comparing it to modern polychaete worms. <ref name='butterfield_2003'/>.
Simon Conway Morris in 1985 agreed that there were similarities to polychaetes, but that Wiwaxia's sclerites did not seem to be homologous with the elytra (scales) of the annelids.<ref>Conway Morris, S. (1985). "The Middle Cambrian metazoan Wiwaxia corrugata (Matthew) from the Burgess Shale and Ogygopsis Shale, British Columbia, Canada". Phil. Trans. Roy. Soc. London 307: 507-586.</ref>
Nick Butterfield, then a postgraduate paleontologist at Harvard inspired by Stephen Jay Gould's lectures, agreed that the sclerites were not like elytra, but on the contrary concluded that they resembled the chitinous bristles (chaetae) that project from the bodies of such modern annelids as bristleworms and earthworms. His conclusion, published in 1990, was that Wiwaxia was an annelid.<ref>Butterfield, N. J. (1990). "A reassessment of the enigmatic Burgess Shale fossil Wiwaxia corrugata (Matthew) and its relationship to the polychaete Canadia spinosa. Walcott". Paleobiology 16: 287-303.</ref> In <ref name='butterfield_2003'> (2003) "Exceptional Fossil Preservation and the Cambrian Explosion". Integr. Comp. Biol. 43: 166-177. Retrieved on 2 Dec 2006.</ref> he reiterated this conclusion, but noted that Wiwaxia does not appear to fit into the polychaetes, but appears rather to be a stem-group annelid.
[edit] As A Mollusk
Butterfield's view has more recently been challenged by the Danish zoologist Danny Eibye Jacobsen (2004) who, after detailed studies of all available material of Wiwaxia, argued that there are no characters clearly placing Wiwaxia with the polychaetes or annelids.<ref>Eibye-Jacobsen D (SEP 2004). "A reevaluation of Wiwaxia and the polychaetes of the Burgess Shale". LETHAIA 37 (3): 317-335.</ref> Given as how Wiwaxia has a creeping sole and what appears to be a radula, a placement with the mollusks therefore still seems most likely.
[edit] References
- National Museum of Natural History (2005) Wiwaxia corrugata. Retrieved on Oct. 26, 2005.
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