Great-appendage arthropods, characterized by a highly modified anterior limb, were previously unknown after the Middle Cambrian. One fossil from the Lower Devonian Hunsrück Slate, Germany, extends the stratigraphic range of these arthropods by approximately 100 million years. Schinderhannes bartelsi shows an unusual combination of anomalocaridid and euarthropod characters, including a highly specialized swimming appendage. A cladistic analysis indicates that the new taxon is basal to crown-group euarthropods and that the great-appendage arthropods are paraphyletic. This new fossil shows that features of the anomalocaridids, including the multisegmented raptorial appendage and circular plated mouth, persisted long after the initial radiation of the euarthropods.

:  Palaeoscorpius devonicus Lehmann, 1944 is known from only a single specimen, found in the Eschenbach Pit near Bundenbach in the Lower Devonian Hunsrück Slate of Germany. It is a key fossil, having been interpreted both as the most basal member of the Scorpiones and as one of the order’s most likely candidates for an aquatic mode of life. Prepared both ventrally and dorsally, some aspects of its morphology remain problematic. Here, with the aid of new techniques, including computed tomography, we present a re‐investigation of this scorpion’s anatomy and a new reconstruction, with a particular focus on the species’ original habitat. On the basis of the environmental interpretation of the Hunsrück Slate and the completeness of the specimen, previous authors concluded that P. devonicus was marine, but none offered convincing morphological evidence. Recent studies of the deposit’s environment suggest that the Hunsrück Sea was part of an intrashelf basin, relatively close to the coastline, and fossils of land plants show that terrestrial wash‐in occasionally occurred. Our revised interpretation of the fossil’s morphology demonstrates that the scorpion was most probably terrestrial. Internal mesosomal organs are interpreted as book lungs, but other terrestrial adaptations are lacking. The absence of both coxapophyses and gnathobases makes determining the scorpion’s feeding mechanism difficult. Interpreting the scorpion’s character states within a phylogenetic framework, especially the possible presence of book lungs, implies either that the plesiomorphic position of P. devonicus is no longer supported or that the development of book lungs had already taken place early in the scorpion lineage.

The arthropod Mimetaster hexagonalis Gürich, 1931 from the Lower Devonian Hunsrück Slate (Germany) is re-described. With 123 available individuals, M. hexagonalis is the most abundant non-trilobite arthropod from this Lagerstätte. New (as well as old) specimen material shows new morphological features of M. hexagonalis. The ontogeny is described based on three juvenile specimens. The growth mode is similar to that in the marrellomorph Vachonisia rogeri Lehmann, 1955. The number of trunk segments increases from juvenile to adult. The mode of life of the arthropod is re-considered. The arthropod is considered to live in groups of several individuals. A mutualistic relationship with sponges of unknown systematic affinities is most likely. An ecological interaction with tentaculitoids is recognized but needs further investigation. The phylogenetic position of M. hexagonalis as a member of the Marrellomorpha is confirmed, but remains to be explored within the broader frame of euarthropod phylogeny. KeywordsMimetaster hexagonalis Á Marrellomorpha Á Lower Devonian Á Hunsrück Slate Á Symbiosis Kurzfassung Der Arthropode Mimetaster hexagonalis Gürich, 1931 aus dem unterdevonischen Hunsrückschiefer (Deutschland) wird neu beschrieben. Mit rund 123 verfügbaren Exemplaren ist M. hexagonalis der häufigste (ausgenommen Trilobiten) Arthropode aus dieser Lagerstätte. Neues sowie bereits bekanntes Fossilmaterial zeigt neue morphologische Merkmale von M. hexagonalis. Die Ontogenie konnte anhand von drei Exemplaren beschrieben werden. Der Wachstumsmodus ähnelt dem marrellomorphen Vachonisia rogeri, Lehmann, 1955. Die Anzahl der Rumpfsegmente steigt von juvenilen zu adulten Stadien an. Die Lebensweise des Arthropoden ist ebenfalls neu betrachtet. Der Arthropode hat wahrscheinlich in Gruppen mit mehreren Individuen gelebt. Eine mutualistische Beziehung zu nicht näher bestimmbaren Schwämmen ist sehr wahrscheinlich. Eine Interaktion mit Tentakulitoiden konnte nachgewiesen werden, bedarf aber weiterer wissenschaftlicher Untersuchungen. Die phylogenetische Position von M. hexagonalis als ein Vertreter der Marrellomorpha ist bestätigt, wird aber in einem breiteren Rahmen der Phylogenie der Euarthropoden weiter erforscht.