Halkieria obliqua

The halkieriids are a group of fossil organisms from the Lower to Middle Cambrian. Their eponymous genus is Halkieria, 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.

Features shared by Halkieria and Australohalkieria 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.

Halkieria evangelista The animals looked like slugs in chain mail - 1.5 centimetres to 8 centimetres long, bilaterally symmetric, flattened from top to bottom and unarmored on the bottom. Very near each end there is a shell plate with prominent growth lines rather like the growth rings of trees.

Australohalkieria superstes The name of the most complete and abundant Australian find means "Southern Halkieria the Survivor" because it proves that halkieriids survived the end-Botomian extinction. The sclerites assigned to this species are convex on the upper surface and concave on the lower.

The only reasonably complete specimens, of Halkieria evangelista, were found in the Sirius Passet lagerstätte in Greenland. Fragments which are confidently classified as belonging to halkieriids have been found in China's Xinjiang province and Australia's Georgina Basin, while shells of a possible halkieriid have been found in Canada's Burgess Shale. Halkieriid-like armor plates, called "sclerites" have been found in many other places as part of the small shelly fauna.

Nearly all members of the genera Halkieria are known only from finds of isolated scaly sclerites: Halkieria alata Duan, 1984 Halkieria amorpha Meshkova,1974 Halkieria bisulcata Qian et Yin, 1984 Halkieria costulata Meshkova, 1974 Halkieria curvativa Mambetov in Missarzhevsky and Mambetov, 1981 Halkieria deplanatiformis Mambetov in Missarzhevsky and Mambetov, 1981 Halkieria desquamata Duan, 1984 Halkieria directa Mostler, 1980 Halkieria elonga Qian et Yin, 1984 Halkieria equilateralis Qian et Yin

The evolutionary relationships of the halkieriids are a complex topic which is still being debated. Most of this debate is about their relationship to Wiwaxia and to the three major lophotrochozoan phyla — molluscs, annelids and brachiopods. The question of their relationship to an apparently much more primitive Cambrian group, the chancelloriids is also significant and may raise some difficult questions.