SIGNIFICANCE OF THE 'ORSTEN' FOR UNRAVELING ARTHROPOD PHYLOGENY

About 40 years ago, the late Klaus J. Müller from Bonn, Germany, discovered an unusual type of preservation yielding tiny, about 500 million years old, three-dimensionally preserved arthropods. This exceptional preservation is an impregnation of the outer cuticular layer by calcium phosphate. By this, such fossils exhibit finest morphological details, such as limbs, setae and all surface structures from fine denticles to membranous areas and pores, still in their correct topological order. This so-called 'Orsten' type of preservation yielded mainly fossils that can be assigned to various evolutionary lineages of Arthropoda, from "Lobopodia", Tardigrada, Pentastomida, to Chelicerata and other representatives of Euarthropoda such as the rock-forming Agnostus pisiformis and a larger number of Crustacea from different levels of crustacean evolution. The crustacean material is by far the most diverse one, both in individual abundancy and species richness. Several fossil species are part of the crown group (Eucrustacea) and share characters either with Maxillopoda or Branchiopoda. Another eight species are derivatives of the early crustacean stem lineage, while the direct sister taxon to the Eucrustacea, the exclusively Palaeozoic Phosphatocopina, are known by more than 30 species, half of these occurring in rocks with 'Orsten' type preservation. Even more, the preservation also of early postembryonic stages (larvae, immature specimens), sometimes in sets of successive instars, provides access to the growth of features and structural complexes in the life cycle of a species. Accordingly, many data are available from crustacean morphologies and ontogenies back to the Cambrian permitting to reconstruct the early evolution of Crustacea sensu lato in much detail. In line with this, several traditional views concerning crustacean morphologies could be demonstrated to be misconceptions, e.g., it became clear that coxae on limbs are a specific feature of a crustacean ingroup, first restricted to two pairs of limbs. As a consequence, the limb basis of, e.g., chelicerates, represents the "old" basipod. The 'Orsten' is not only an excellent source for the diversity of crustaceans already in the Cambrian, but also of significance for our understanding of the evolution and phylogeny of Arthropoda sensu lato in general.