Halkieria

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Halkieria is a fossil organism from the Lower to Middle Cambrian. It has been found on almost every continent in deposits of this age in socalled small shelly fossil assemblages. Halkieria is best known 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 larger evolutionary implications was posed.

[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 was originally of calcium carbonate.

[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 zones. They also noticed the similarity of the shellplates to inarticulate brachiopods. This led them to suggest that Halkieria is a stemgroup brachiopod which had evolved into the bivalved brachiopod by gradually getting inclosed within the two valves and starting a sessile lifemode. Wiwaxia should be the stemgroup of Annelids or Polychaetes according to Nicholas Butterfield (1990) which is led by the recognition of its sclerites having a similar ultrastructure as polychaete chaetae secreted by numerous microvillae. A number of other scientists has been stressing molluscan affinites of Halkieria. According to Vinther and Nielsen (2005) there is no characters supporting either brachiopod or annelid affinity of Halkieria. All characters seem to be compatible with molluscs, especially the Chitons or Polyplacophora.

Lately, a redescription of the Burgess shale fossil Odontogriphus has revealed it as being 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 hereby possibly marks some important links between these Cambrian enigmas. Reference: Vinther, J. and Nielsen, C. (2005) The Early Cambrian Halkieria is a mollusc, Zoologica Scripta, 34 (1) 81-89