Untangling the knots of the arthropod tree of life: three-dimensional insights from the Early Devonian Hunsrück Slate fauna

The relationships among arthropods (e.g. scorpions, millipedes, shrimps, insects) have long been debated, but refinements in molecular phylogenetics and the inclusion of fossil data facilitated by X-ray computed tomography (XCT) have led to an emerging consensus. Nevertheless, there are still big knots in the arthropod tree of life, namely (i) the position of pycnogonids (‘sea spiders’) and relationships among Chelicerata (e.g. horseshoe crabs, scorpions, spiders, mites), and (ii) the origin of insects and relationships among Pancrustacea (e.g. branchiopods, malacostracans, remipedes). Here we propose to apply hierarchical phase-contrast XCT to Devonian (400-myr-old) fossils from Germany, whose level of preservation including soft-tissues is unique for that critical period of arthropod evolution. Anatomical features revealed by XCT (part. the recognition of ancestral morphologies and transitional forms) will provide new phylogenetic insights into the arthropod tree of life.