Cambrian Derivatives of the Early Arthropod Stem Lineage, Pentastomids, Tardigrades and Lobopodians An ‘Orsten’ Perspective

Abstract

Abstract ‘Orsten’-type preservation is the phosphatisation of cuticular surfaces without any further deformation and has yielded completely three-dimensional fossils, mainly arthropods at scale of 100 μm—2 mm. Records of such exceptional fossils are now reported from several continents and from the Early Cambrian (approx. 520 Mi. years BC) to the Early Cretaceous (approx. 100 Mi. years BC). ‘Orsten’-type konservat - Lagerstatten from the Middle and Upper Cambrian have brought up mainly crustaceans and representatives of the Euarthropoda, but also derivatives of their early stem lineage. These are larval stages of Upper Cambrian pentastomids, tongue worms, which today are parasites of various tetrapods. A Middle Cambrian representative of the minute tardigrades is currently under description; it possibly represents the adelphotaxon of extant Tardigrada. New to science are fragments of a small tubular, finely annulated organism with similarly annulated segmental tubular limbs. This Upper Cambrian form, currently under investigation, may represent the first lobopodian in an ‘Orsten’-type preservation and expands their record to the late Cambrian. It shares with several of the Lower to Middle Cambrian Chengjiang and Burgess Shale fauna lobopodians the lobopodian design of its limbs, paired segmental dorsal outgrowths, and the finely annulated tubular body, which is smooth in the limb regions. New information presented by this ‘Orsten’ fossil concerns cuticular details undetectable on flattened fossils. Pentastomids, tardigrades and the onychophorans/lobopodians have been called “pro-” or “prot-arthropods” because they still lack, or partly lack, characteristic features known from later derivatives of the evolutionary line of Arthropoda. Among the three taxa, the Pentastomida are assumed to be the latest offshoot because they possess segmented limbs with pivoted joints between the articles. The now established record of all of the “pro-arthropods” and even true crustaceans in the earliest Palaeozoic implies, in our view, that the ancestry of Arthropoda lies even further back well in the Pre-Cambrian.