sometimes replaces the original mineral with another, such as silica). It is thought to also have borne an armour coat consisting of biomineralised sclerites, like Halkieria. These are never found in direct association with the shells, but there are many biostratinomic processes which could account for this fact. The lower Cambrian taxon Ocruranus (=Eohalobia) is putatively equivalent to the shells of Oikozetetes and seemingly belonged to a halkieriid-type body, although an intermediate valve suggests a Palaeoloricate-like body form. Occurrence 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. The earliest known occurrences of Halkieriids sclerites, classified as Halkieria longa, date from the Purella antiqua Zone of the Upper Nemakit-Daldynian Stage in Siberia. The mass extinction at the end of the Cambrian period's Botomian age was thought to have wiped out most of the small shellies, including the halkieriids, but in 2004 Halkieriid fossils classified as Australohalkieria were reported from Mid-Cambrian rocks of the Georgina Basin in Australia. It is not known why this clade would have survived while other halkieriid clades apparently died. It may be significant that the only archaeocyathans known to have survived the end-Botomian extinction also occur in Gondwana, the old super-continent that embraced South America, Africa, India, Australia and Antarctica. Halkieriids and other small shelly fossils are typically, although not always, preserved in phosphate, which may or may not have been their original mineral composition. Preservation by a covering of phosphate only seems to have been common during the early Cambrian, becoming rarer with time as a result of increased disturbance of sea-floors by burrowing animals. Hence it is possible that halkieriids and other small shelly fossils were alive earlier than the earliest known fossils and later than the latest known fossils — paleontologists call this kind of uncertainty the Signor–Lipps effect. Species 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