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Concept
Halkieriid
Résumé
The halkieriids are a group of fossil organisms from the Lower to Middle Cambrian. Their eponymous genus is Halkieria hælˈkɪəriə, which 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.
The group is sometimes equated to Sachitida, although as originally envisaged, this group includes the wiwaxiids and is thus equivalent to the Halwaxiida.
Armor plates called "sclerites" had long been known as elements of the small shelly fossils, and detailed analysis showed that some of these belonged to the same animal and how they fitted together. The first articulated specimens of Halkieria evangelista, with all their hard parts together, were collected in 1989 from the Sirius Passet lagerstätte in Greenland, and were described in 1990 by Simon Conway Morris and John S. Peel. H. evangelista is used as a model for identifying and reconstructing as halkieriids other similar shells and sclerites; its epithet evangelista reflects its power to explain the Lower Cambrian fossil record.
Only armor-like sclerites of Australohalkieria have been found, and much of the analysis assumes that these animals were similar to Halkieria. However the sclerites are so similar that this assumption looks fairly safe. In both genera the sclerites are of the type called "coelosclerites", which have a mineralized shell around a space originally filled with organic tissue, and which show no evidence of growth by adding material round the outside.
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