Ordovician horseshoe crab body and trace fossil association preserved in a unique taphonomic setting

• Oldest xiphosuran mortichnia shows body and trace fossil association in unique dorsal-down preservation. • Fossil horseshoe crabs returned to spawning sites, signalling modern behaviour with deep evolutionary roots. • Fossils in storm beds plus paleoclimate modelling reveal a seasonal, nearshore setting like that of modern horseshoe crabs. Horseshoe crabs (Xiphosura) have a deep fossil record but are exceedingly rare within early Palaeozoic deposits. Here, we report a new xiphosuran from the Middle Ordovician (Darriwilian) of AlUla, Saudi Arabia—one of the oldest documented horseshoe crabs that also exhibits strikingly large size for the age. The specimens, preserved in multiple lower shoreface and storm-deposited layers, include three-dimensionally preserved prosomae and detailed impressions, all associated with trace fossils assigned to the new ichnospecies, Selenichnites sursumdeorsum . All fossils are preserved dorsal-down—a unique taphonomic mode for xiphosurans—interpreted as storm-mediated transport of individuals followed by unsuccessful escape attempts. The recurrence of these horseshoe crab fossils across multiple stratigraphic surfaces suggests occupation of the same region over the timespan of cyclical storms, analogous to spawning behaviours in modern horseshoe crabs. Palaeoclimatic reconstructions of AlUla in the Middle Ordovician place the new horseshoe crabs within a seasonally variable nearshore environment, reflecting the ecological conditions of modern xiphosurans. These findings evidence the oldest association of horseshoe crab body and trace fossils (mortichnia), and hint at an Ordovician origin of behaviours observed in extant Xiphosura.