1. The meaning of 'Orsten' in the original sense?

'Orsten' are horizontally laminated limestones nodules, which occur embedded in Alum Shales of the upper part of the Cambrian in Sweden (now uppermost Middle Cambrian to Furongian, formerly: zone 1 Agnostus pisiformis to zone 6 Peltura scarabaeoides of the UC sequence).

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 name '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.

 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.

Quarry near Falbygden, Västergötland, Sweden. Exposed are the Furongian and Lower Ordovician. Lower part with black, organic-rich alum shales (alunskiffer in Swedish), topped by a hiatus of 0.5 m, and 1–1.5 m mudstones forming the basis of the Tremadocian (Ceratopyge shales). Above, banked Ordovician limestones (Latorp limestone). 'Orsten' nodules within the shales. In this region, post-Cambrian dolerite caps have protected the less erosion resistant Alum Shales and other Palaeozoic strata, forming the characteristic table mountains ("Tafelberge") of Västergötland, most notably Kinnekulle. Image from 1989 by DW. – site now "destroyed" due to recreation activities after the quarry had been expired.

 'Orsten' nodules in the famous Gum quarry surrounded by alum shales. Layers disintegrating due to weathering looking like leaves in a book (see below, image from 2004 by DW).

 'Orsten' nodule embedded within alum shales (same quarry). Mostly, they are surrounded by a crystallitic crust of limestone. See picture below (DW).  Map of southern Sweden showing alum shale occurrences and localities (1–5) sampled by K.J. Müller. Distance Gum – Öland approx. 330 km.
Swedish 'Orsten' nodule sliced exposing the horizontal banding (dot = 5 mm). The nodules can be more than 1 meter in width.    
Surface of a fractured nodule with countless agnostid shields. Often these are sorted (dot = 5 mm). Walnut-size pieces of this rock have been dissolved to receive the non-calcitic content: rare siliceous microfossils and phosphatic remains, primarily phosphatic fossils. Time table of major sites that yielded fossils in 3D preservation; note the remarkable amount in the Early Palaeozoic.
The famous Gum site at the southern side of the Kinnekulle (photo taken in 2004 by DW), now protected and unavailable to the public and for further research:

 

 

 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, discoverer of this exceptional fauna, is professor emeritus for micropalaeontology. He has worked at the Institution of Palaeontology of the University of Bonn. He started exploration for Orsten arthropod fossils after first by-chance findings in 1975. Besides the discovery of the 'Orsten', he is 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 interested in exceptional preservations. Long having gone into pension, Klaus continued being interested in the 'Orsten' research and, for example, financed the Siberia trip of Andreas Braun and Dieter Waloszek in 2001. Of course he is now our honorary member (photo taken in 2003 by DW). Read more about him in due course.