Orsten Deposits from Sweden:
Miniature Late Cambrian Arthropods
Carol M. Tang
SINCE THE EIGHTEENTH CENTURY, MARINE INVERTEBRATE fossils have been collected from limestone nodules called orsten from the Alum Shale of southern Sweden. However, it was not until more than 200 years later that the unique preservation of very fine morphological details and soft integuments was recognized. In 1975, while searching for conodonts under the microscope, Klaus J. Müller discovered, by chance, a rich fauna of tiny phosphatized arthropods—including agnostids, trilobites, and early crustaceans—preserved in fine detail in three dimensions. Because the unique phosphatic preservation has occurred only on fossil specimens less than 2 mm in length, it is not surprising that this unique conservation Lagerstätte had not been recognized earlier.
Since then, Müller and colleagues have developed special techniques to search for, extract, and study these exceptionally well preserved, phosphatized fossils. Over the years, they have examined over 1,200 kg of limestone, collected about 100,000 specimens, and described dozens of genera and species, almost all of them new.
The deposit provides extraordinary insight into the paleoecological structure of a flocculent-layer community and is the oldest welldocumented example of meiofauna in the fossil record. The flocculent layer is the fine-grained, unconsolidated layer above the seafloor inhabited by very small organisms called meiofauna. Orsten have yielded the oldest fossil members of the Pentastomida, worm-like internal parasites that inhabit most modern terrestrial vertebrates. Although trilobites are abundant in orsten deposits and surrounding shales, the additional pre-