The ‘Orsten’ window — a three-dimensionally preserved Upper Cambrian meiofauna and its contribution to our understanding of the evolution of Arthropoda

‘Orsten’-type preservation, i. e., phosphatisation of cuticles without further diagenetic deformation, has yielded three-dimensional fossils at a scale of 0.1–2.0 mm. Such fossils, first described from Upper Cambrian limestone nodules found in Sweden, have been reported from several continents and from the early Cambrian (approx. 520 M. y. BP) to the early Cretaceous (approx. 100 M. y. BP). Fossils from Cambrian ‘Orsten’-type lagerstatten are mainly representatives of different euarthropod groups and also of different evolutionary levels. This allowed the reconstruction of the early phylogeny particularly of Crustacea in great detail and the recovery of major evolutionary traits within this group, i. e., in the progressive modification of the locomotory and feeding apparatus of the head region. More recently, derivatives also of the early stem lineage toward the Euarthropoda have been discovered. These include apparently parasitic larvae of stemlineage Pentastomida (tongue worms) today living in various tetrapods, a minute fossil related to the equally minute tardigrades (water bears), and fragments of a small tubular organism with segmental tubular limbs, interpreted as the first lobopodian in an ‘Orsten’-type preservation. Lobopodians are worm-like derivatives of the earliest phase in the evolution of arthropods before the development of a sclerotic, segmented dorsal cuticle (arthrodized tergum) and similarly segmented limbs (arthropodia), hitherto known only from the Lower to Middle Cambrian. The presence of these “pre-euarthropods,” which lack, or partly lack, characteristic features developed later in the arthropod evolutionary lineage, and the recent record of phosphatocopine Crustacea in the earliest Palaeozoic are regarded as a support for the view that the ancestry of Arthropoda lies much further back, possibly well in the late Pre-Cambrian. This does not support a “Cambrian explosion”.