Halwaxiida
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| Halwaxiida Fossil range: Early to Middle Cambrian |
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 Halkieria
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Halwaxiida is a proposed clade loosely uniting scale-bearing Cambrian animals, which may lie in the stem group to molluscs or lohpotrochozoa. Some palaeontologists question the validity of the Halwaxiida clade.
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[edit] Definition
The halwaxiids are defined by possessing three principal sclerite zones, and usually at least one shell.[1] Some scientists are unhappy with this loose definition, arguing that such traits may have arisen convergently; this challenges the monophyletic nature of the group.[2]
[edit] Constituents
The clade consists of fossils known from Burgess shale type deposits, including Wiwaxia, Halkieria, and Orthrozanclus, as well as the siphogonuchitids, a group known only from unarticulated scales and a constituent of the small shelly fauna.[3]
[edit] Taxonomic hypotheses
[edit] Monophyly
A clade incorporating the similar looking Wiwaxia and Halkeria was not strongly supported until the discovery of Orthrozanclus. This organism appeared to be an intermediate form, as it contains a shell on one end, and Wiwaxia-like rows of larger spines.[1] However, there are few strong pleisomorphic traits; further, the scale architecture differs widely, in that most Halwaxiids have hollow scales, attached to the organism by the insertion of soft tissue, where Wiwaxia has scales which insert into the organism. On these grounds, it appears that at least Wiwaxia does not fit into a group with the other Halwaxiids; a term "Halsiphonida" has been offered as a name for the reduced clade.[2]
[edit] Similarity to molluscs
Affinity with the molluscs is supported by the presence of a radula in Wiwaxia.[1] However, the feeding apparatus in question is far from identical from the radulæ of modern day molluscs: it contains only two (occasionally three) bars, not the multiple rows that even juvenile molluscs' radulæ bear; the form also bears limited resemblance.[4] While similarity does exist, it also resembles annelid feeding apparatuses to a similar extent.[2] Further, the microstructure of Wiwaxia's sclerites strongly resembles that of polychæte bristles[5][2] - a trait, again, that is pleisomorphic,[6] but which illustrates the point that non-molluscan affinities are equally likely, so a degree of cynicism is due before grouping the organisms as stem group molluscs; indeed, it has been argued that the possession of a "dorsal scleritome of microvillar setæ" is a trait unique to Wiwaxia and the polychæte worms.[2]
[edit] Relationships with other lophotrochozoa
Two interpretations of the taxonomic position of the Halwaxiids within the Lophotrochozoa are posited by Conway Morris and Caron; they reflect differing degrees of certainty of molluscan affinity.[1] Butterfield, chief amongst the dissenters, offers a starkly different view.
Caron's preferred interpretation:
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Conway Morris's preferred interpretation:
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Butterfield's alternative:
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Kimberella |
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Odontogriphus |
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[edit] Notes and references
- ^ a b c d Morris, S.C.; Caron, J.B. (2007). "Halwaxiids and the Early Evolution of the Lophotrochozoans". Science 315 (5816): 1255. doi:.
- ^ a b c d e N.J., Butterfield (2007-18-12). "Lophotrochozoan roots and stems" in Palaeontological Association Annual Meeting 2007. Budd, G.E.; Streng, M.; Daley, A.C.; Willman, S. Programme with Abstracts: 26-7.
- ^ The original, possibly incomplete, list includes Australohalkieria, Drepanochites, Eohalobia, Halkieria, Lomasulcachites, Ninella, Ocruranus, Oikozetetes, Orthrozanclus, Sinosachites, Siphogonuchites, Thambetolepis, and Wiwaxia
- ^ Butterfield, N.J. (2006). "Hooking some stem-group ‘‘worms’’: fossil lophotrochozoans in the Burgess Shale". Bioessays 28 (12): 1161–6. doi:.
- ^ 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 (3): 287–303.
- ^ Eibye-Jacobsen D (September 2004). "A reevaluation of Wiwaxia and the polychaetes of the Burgess Shale". Lethaia 37 (3): 317–335. doi:.
