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ORSTEN FOSSILS

The upper Middle Cambrian and especially the Upper Cambrian of the Cambrian continent Baltica is characterized by dark organic-rich shales known as "alum shales." Frequently interspersed in these shales are fossil-rich calcareous nodules (Swedish: orsten). These fossil-rich nodules were known for a long time as a source of excellent macrofossils, but they also provide a number of incredibly well-preserved minute fossils which are studied in detail by Klaus J. Müller and Dieter Waloßek.

Orsten nodule surrounded by black alum shales. Upper Cambrian, Västergötland, Sweden. By courtesy of D. Waloßek, University of Ulm.

The spectacular fossils are phosphatized, serving for a 3D preservation of the original tissue, occasionally with structures such as pores and secondary bristles on filter setae down to less than 1 µm. The fossil remains are dissolved from the rock by organic acids. The Orsten fossils enlarge the window to soft-bodied animals up into the Upper Cambrian and permits a more precise reconstruction of the biota because the usual fossil assemblage presents only the hard-part components of the original fauna. Most of the Orsten fossils are remains of early and often previously unknown arthropod groups, but probably also the earliest known tardigrades and pentastomids (which are extant groups but otherwise unknown from the fossil record). The research on Orsten-type preservation is now enlarged from the Swedish Upper Cambrian to Middle and Upper Cambrian strata in Poland, Russia, Canada and Australia. For further information see Dieter Waloßek's homepage.

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	Agnostus pisiformis (L.). Agnostid trilobite. Anterior view of early larval specimen with gaping shield, shows ballon-shaped hypostome and a preserved left trunk limb. Upper Cambrian, Sweden. By courtesy of D. Waloßek.
	Rehbachiella kinnekullensis Müller, 1983. SEM micrographs of larvae. a) Ventrolateral view od early stage. b) Advanced stage with developed head shield and first functional trunk limbs. c) Penultimate stage, limbs broken off distally. Large head shield covers much of trunk. d) Last instar with distorted head region. Upper Cambrian, Sweden. From Walossek, 1996, by courtesy of D. Waloßek.
	SEM picture of arthropod larva ("starlet"), approx. 100 µm in size. Oblique dorsal view. Upper Cambrian, Sweden. From Müller and Walossek, 1986, by courtesy of D. Waloßek.
	Boeckelericambria pelturae Walossek and Müller, 1994, a stemline pentastomide. Round-headed larva with "palps". Upper Cambrian, Sweden. By courtesy of D. Waloßek.
	Stemline tardigrade. Lateral view, SEM micrograph. Middle Cambrian Kuonamka Formation, Lena River area, Siberia. By courtesy of D. Waloßek.

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