The central nervous system (CNS) presents unique insight into the behaviors and ecology of extant and extinct animal groups. However, neurological tissues are delicate and prone to rapid decay, and thus their occurrence as fossils is mostly confined to Cambrian Burgess Shale–type deposits and Cenozoic amber inclusions. We describe an exceptionally preserved CNS in the horseshoe crab <i>Euproops danae</i> from the late Carboniferous (Moscovian) Mazon Creek <i>Konservat-Lagerstätte</i> in Illinois, USA. The <i>E. danae</i> CNS demonstrates that the general prosomal synganglion organization has remained essentially unchanged in horseshoe crabs for >300 m.y., despite substantial morphological and ecological diversification in that time. Furthermore, it reveals that the euarthropod CNS can be preserved by molding in siderite and suggests that further examples may be present in the Mazon Creek fauna. This discovery fills a significant temporal gap in the fossil record of euarthropod CNSs and expands the taphonomic scope for preservation of detailed paleoneuroanatomical data in the Paleozoic to siderite concretion <i>Lagerstätten</i> of marginal marine deposits.