Halkieria
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| Halkieria Fossil range: Lower to Middle Cambrian |
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| Halkieria evangelista Conway Morris and Peel 1990 |
Halkieria is a genus of fossil organisms from the Lower to Middle Cambrian. It has been found on almost every continent in Lower to Mid Cambrian deposits, forming a large component of the small shelly fossil assemblages. The best known species is Halkieria evangelista, from the North Greenland Sirius Passet Lagerstätte in which complete specimens were collected on an expedition in 1989. The fossils were described by Simon Conway Morris and John Peel in a short paper in 1990 in the journal Nature. Later a more thorough description was undertaken in 1995 in the journal Philosophical Transactions of the Royal Society of London and wider evolutionary implications were posed.
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[edit] Description of the fossil
The animal was bilaterally symmetric, dorso-ventrally flattened with a naked ventral side. The dorsal side was covered with imbricating sclerites in three zones. In either end, there is a larger shellplate with prominent growthlines. The sclerites are distinguished into three types; the palmate sclerites placed dorsally between the two shellplates, surrounding these there is a zone of sclerites termed the cultrates and along the entire margin, there is a zone of spineshaped sclerites called siculates. The shellplates and the sclerites were originally of calcium carbonate.
Note that the smallest increments on the scale in the figure are millimeters.
[edit] Phylogenetic position of Halkieria
When Conway Morris and Peel described Halkieria evangelista from North Greenland, they noticed the similarity to Wiwaxia, known especially from the Burgess shale which has sclerites organised in similar body zones. They also noticed the similarity of the shellplates to inarticulate brachiopods. This led them to suggest that Halkieria is a stem group brachiopod which had evolved into the bivalved brachiopod by gradually getting enclosed within the two valves and starting a sessile lifemode. Wiwaxia should be the stem group of Annelids or Polychaetes according to Nicholas Butterfield (1990). This is supported by the recognition of its sclerites having a similar ultrastructure to that of polychaete chaetae secreted by numerous microvilli. A number of other scientists has been stressing the molluscan affinities of Halkieria. According to Vinther and Nielsen (2005), there is no character supporting either a brachiopod or an annelid affinity for Halkieria. All characters seem to be compatible with molluscs, especially the Chitons or Polyplacophora.
A redescription of the Burgess shale fossil Odontogriphus has revealed that it is a mollusc relative instead of some sort of lophophorate. The tooth apparatus has many compelling similarities with a molluscan radula along with a foot-like area and surrounding gill-like structures (supposed ctenidia). The tooth apparatus is also very similar to Wiwaxia, which thus possibly marks some important links between these Cambrian enigmas.
Wiwaxia is now considered to be related to Halkieria due to the recently discovered Burgess shale organism, Orthrozanclus reburrus. O. reburrus combines features of both Halkieria (scale-mail sclerites and an anterior shell) and Wiwaxia (elongated, spine-like sclerites).
[edit] References
- Vinther, J. and Nielsen, C. (2005) The Early Cambrian Halkieria is a mollusc, Zoologica Scripta, 34 (1) 81-89
[edit] External links
- Palaeos' article on Halkieria [1] & H. evangelista [2]
- Pharyngula entry on Orthrozanclus reburrus
