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What Are Orsten Faunas?

Orsten faunas are assemblages – not a single fauna! – of such secondarily phosphatized fossil organisms – and different localities (samples from different nodules) may contain different animals. Initially only arthropods were sorted out (focus laid on these), but later also other organisms were discovered in such preservation, f.e., even cyanobacterians, and, more recently, nemathelminths – remarkably only in Australia and China, but not (yet) in Sweden.

Primarily phosphatic microfossils, which may be similarly three-dimensionally preserved, but are in first instance skeletal elements of organisms – so-called small shellies –, and are also found in the material, but in most cases not considered here. Also the phosphatized embryonic stages discovered in the last decades, mainly from the lower Cambrian, are not considered to represent Orsten faunal elements. Because of the small size of the fossils, these were initially interpreted as representatives of a so-called meiofauna, but this may no longer be the case for the majority of the finds, though not all can be excluded. First, size is a matter of a size selection by the use of only small-sized sieves by the discoverer of the Orsten fossils – a process bias –, and, second, various forms are known only from larval stages, and estimations of their later stages may well have exceeded the range defining membership to a meiofauna (discussed elsewhere in more detail).