Wiwaxia
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Wiwaxia is genus of soft-bodied, scale-covered animals known from Burgess shale type Vorlage:Lagerstatte dating from the Early to Middle Cambrian.[1][2] The organisms are mainly known from dispersed sclerites; articulated specimens where found can reach a length of around 21mmVorlage:Verify source. The precise taxonomic affinities of the genus are a matter of ongoing debate amongst palaeontologists.
The organism
Wiwaxia was covered in small sclerites (scale-like plates) that were originally composed of an organic material, probably chitin. The scales form three main regions — the top, longer spines, and sides — and their layout suggests that they served a defensive covering for a bottom-dwelling organism. Wiwaxia also possessed a feeding apparatus situated towards the front of its bottom surface, suggesting that it grazed on the sea floor.
Classification
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 more 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. If it cannot, in which group's stem does it fall?
Evolution of ideas
When Wiwaxia was first described, it was compared to the polychaete subgroup of the annelid worms.[3] Simon Conway Morris later agreed that while there were similarities to polychaetes, Wiwaxia's sclerites were not homologous with the elytra ("scales) of annelids, to which the original comparison was made.[4]
Nick Butterfield, then a postgraduate paleontologist at Harvard inspired by Stephen Jay Gould's lectures, agreed that the sclerites were not like elytra, and instead drew analogy to chitinous bristles (chaetae) that project from the bodies of other annelids.[5] He later noted that Wiwaxia does not appear to fit into the polychaetes, but appears rather to be a stem-group annelid.[6]
However, Butterfield's view was eventually challenged by the Danish zoologist Danny Eibye Jacobsen, who argued that Wiwaxia lacks any characters that would firmly place it as a polychaete or annelid.[7]
Current interpretations
At present, there are two conflicting opinions on the relationships of Wiwaxia. Conway Morris champions the idea that it is a mollusc, while Butterfield maintains an annelid affinity.
References
External links
- ↑ Zhao, Y.L., Qian, Y.; Li, X.S.: Wiwaxia from Early-Middle Cambrian Kaili Formation in Taijiang, Guizhou. In: Acta Palaeontologica Sinica. 33. Jahrgang, Nr. 3, 1994, S. 359–366.
- ↑ Conway Morris, S.: The Middle Cambrian metazoan Wiwaxia corrugata (Matthew) from the Burgess Shale and Ogygopsis Shale. In: British Columbia, Canada: Philosophical Transactions of the Royal Society of London, Series B. 307. Jahrgang, 1985, S. 507–582.
- ↑ Walcott, C.D.: Middle Cambrian annelids. Cambrian geology and paleontology, II. In: Smithsonian Miscellaneous Collections. 57. Jahrgang, 1911, S. 109–144.
- ↑ Conway Morris, S.: The Middle Cambrian metazoan Wiwaxia corrugata (Matthew) from the Burgess Shale and Ogygopsis Shale, British Columbia, Canada. In: Phil. Trans. Roy. Soc. London. 307. Jahrgang, 1985, S. 507–586.
- ↑ Butterfield, N.J.: A reassessment of the enigmatic Burgess Shale fossil Wiwaxia corrugata (Matthew) and its relationship to the polychaete Canadia spinosa. Walcott. In: Paleobiology. 16. Jahrgang, 1990, S. 287–303.
- ↑ Butterfield, N.J.: Exceptional Fossil Preservation and the Cambrian Explosion. In: Integr. Comp. Biol. 43. Jahrgang, 2003, S. 166–177 (oxfordjournals.org [abgerufen am 2. Dezember 2006]).
- ↑ Eibye-Jacobsen D: A reevaluation of Wiwaxia and the polychaetes of the Burgess Shale. In: Lethaia. 37. Jahrgang, Nr. 3, September 2004, S. 317–335, doi:10.1080/00241160410002027.