The first Silurian horseshoe crab reveals details of the xiphosuran ground plan.

Horseshoe crabs are an ancient lineage with an evolutionary history stretching back 450 million years and are generally considered to be examples of 'living fossils' exhibiting slow rates of evolution. Despite this reputation, relatively little is known of the early evolution of the group, with only two species described from the Ordovician and a subsequent 80-million-year gap in their fossil record until xiphosurids appear in the Late Devonian. Furthermore, all described Ordovician species are assigned to a single genus, with their close phylogenetic relatedness rendering it unclear whether their morphology is representative of the horseshoe crab ground pattern or an independently derived condition. Here, a new species of horseshoe crab is described from the Silurian of Indiana, USA. The new species bridges the temporal gap in the xiphosuran fossil record and has an overall morphology similar to that of the Ordovician taxa. These new data provide critical information on the ancestral morphology of horseshoe crabs, showing that xiphosurids evolved from forms with a fused thoracetron exhibiting axial segment boundaries, and demonstrate the persistence of basal Xiphosura into the Silurian. Laser-stimulated fluorescence is also shown to be an effective method for studying and imaging arthropod fossils exhibiting challenging preservation.