1. Meaning of 'Orsten' in the original sense

The term 'Orsten' has been applied to horizontally laminated limestones nodules. These occur embedded in/surrounded by Alum Shales of the upper part of the Cambrian in Sweden (now uppermost Middle Cambrian, formerly: zone 1 Agnostus pisiformis, to the Furongian, and here the former zones 5/6 Peltura scarabaeoides of the UC sequence)(zones 3 and 4 did not yield any 'Orsten' fossils).

The nodules are actually an accumulation of countless disarticulated calcified head and tail shields of the minute arthropod Agnostus pisiformis (Wahlenberg, 1818). The surrounding Alum Shale is a finely layered compaction series of up to 100 m of bituminous shale with a notable heavy metal content (see e.g.: Buchardt, B. 1989. Irradiation of organic matter by uranium decay in the Alum Shale, Sweden. Geochimica et Cosmochimica Acta 53(6),1307-1322.).

The term 'orsten' derives from a local dialect word for pig, 'orne' and the Swedish word for stone, 'sten'. It is assumed that the name derives from the use of ground 'orsten' as veterinary medicine for domestic pigs. It is often translated to 'stinkstone' due to the characteristic smell ("rotten eggs") when struck or split. The nodules have also been burnt to produce fertilizer. Most Alum Shale and 'Orsten' quarries are now abandoned. Nowadays, nodules are frequently uncovered at construction areas, e.g. for roads.

According to our experience, only very few 'orsten' nodules actually yield phosphatized arthropod remains. In fact, it were mainly only two nodules from the Gum quarry at Kinnekulle, Västergötland, have yielded the bulk of our material. Most of the nodules, representing a time frame of at least 1 million years, have yielded few to single specimens, if at all. Accordingly, it is rather unlikely that the fossils represent an allochthonous thanatocoenosis or have been killed in a event comparable to stagnant, anoxic conditions at certain times during a year, as occurring today in the Baltic Sea sometimes or in the Mediterranean Sea.

Residues derived from the etching process contain various non-arthropod fragments, "small shelly" fossils, conodonts, shells of (primary?) phosphatic brachiopods (mainly Obolus; see picture of procedure in the chapter methods) and originally non-phosphatic material, such as sponge spicules.

For information on orsten see also: Berg-Madsen, V. 1989. The origin and usage of the terms orsten, stinkstone, and anthraconite. Archives of Natural History 16(2), 191-208.

Note: The new finds outside Sweden demonstrate larger differences between the various types of rock that yields 'Orsten' preservation than known before. We will add more on these in due course, with special reference to the new finds in China.

Map of southern Sweden showing alum shale occurrences and localities (1–5) sampled by K.J. Müller. Distance Gum – Öland approx. 330 km.




Time table of major sites that yielded fossils in 3D preservation; note the remarkable amount in the Early Palaeozoic.

2. What are 'Orsten'-type fossils?

'Orsten' fossils in the strict sense are spectacular minute secondarily phosphatised (apatitic) fossils. Until now the majority of taxa described has been affiliated with the Crustacea, representing different evolutionary levels. Also other arthropods, representatives of other euarthropod groups and of stem taxa have been described, and, more recently, also rerpesentatives of the Nemathelminthes, a group of worms considered as the sister taxon to all other gastroneuralian animals. No representatives of other animal groups could be detected so far, notably also not the tiniest fragment of trilobites, whcih are, otehrwise, common in the rock. Remarkably, we did not find any fossil fragment larger than 2 mm.

'Orsten' fossils have the great advantage in being three-dimensionally preserved and with all surface structures in place. Accordingly, they are much easier to interpret than any other fossil material. In fact, 'Orsten' fossils are preserved virtually as if they were just critical-point dried extant organisms. Details range down to less than 1 µm and include pores, sensilla and minute secondary bristles on filter setae and denticles. 'Orsten' fossils also give an insight of meiofaunal benthic life (see picture below) at small scale, including preservation larval stages, and hence a life zone inhabited by the earliest metazoan elements of the food chain.

Therefore, it is not surprising that our material also includes many ontogenetic stages. Even more so, the smaller the better! And in some cases, we were able to establish sets of successive stages, the longest being that of Rehbachiella kinnekullensis with 30 stages, and from larger fragments we even know that this series is still not complete.

Possibly due to impregnation by phosphate (fluor apatite like in our teeth), most of the specimens are hollow carcasses, as the one on lower right. Remarkably, it seems that most, if not all, pentastomids from uppermost Cambrian to Lower Ordovician strata and not co-occurring with any other 'Orsten' taxa, are solid. The reasons for this are still unclear. It is also remarkable that they co-occur with conodonts.

As already briefly mentioned above, 'Orsten' rocks contain also other fossils, such as conodonts or the so-called small shellies, horny brachiopods, and there are also shelly fragments of macrofaunal elements in surrounding rocks, such as agnostids, brachiopods and trilobites, but we do not count these as 'Orsten' fossils because of the lack of cuticular preservation. The exception is Agnostus pisiformis, of which we have numerous immature stages with cuticular preservation.

'Orsten'-type fossils other than the ones mentioned above are likewise 3D-preserved phosphatized microfossils, which may range from embryonic stages (e.g., early cleavage in various late Precambrian to Lwer Cambrian rocks, apparently developed egg stages of the putative nemathelminth Markuelia) to arthropods in much younger sediments. Examples are the Triassic ostracodes from Spitsbergen or Lower Cretaceous ostracodes and parasitic copepods from the Santana formation, Brazil. 3D silification is another unusual type of prseravtion of arthropods, from the Tertiary (Neogene), which preserved crustcaens, spiders and insects incl. immature stages.

We will discuss this further in due course.

3. †Klaus J. Müller, Bonn, the discoverer of the 'Orsten'

Klaus Müller, 1923 – March 12, 2010, the discoverer of this exceptional fauna, was professor emeritus for micropalaeontology. He had worked at the Institution of Palaeontology of the University of Bonn. Exploration for Orsten arthropod fossils started after first by-chance findings in 1975 (his assistant Helga Rehbach found the first isolated legs). Besides the discovery of the 'Orsten', Klaus was also a well-known pioneer of the study of conodonts (see e.g. the page of the Pander Society to learn more about these fossils) and he was always interested in exceptional preservations. Long after his retirement, Klaus continued to be interested in the 'Orsten' research and, for example, financed the Siberia trip of Andreas Braun and Dieter Waloszek in 2001, was still co-author of papers and was our first honorary member (photo taken in 2003 by DW). In fall 2009, the Palaeontological Society of Germany honoured Klaus, making him a Honory Member. They also had a special congress symposium at their regular annual meeting, in which some of us gave talks – e.g. Dieter on the Orsten in general and the history of research in Bonn, Ulm. With Klaus we have lost a friend, supporter and a great colleague.