Abstract: A remarkable new fossil horseshoe crab, Lunataspis aurora gen. et sp. nov., from recently discovered Upper Ordovician (c. 445 Ma) shallow marine Konservat-Lagerstätten deposits in Manitoba (Canada), is characterized by fusion of opisthosomal tergites into two sclerites. A broad mesosoma of six or seven fused segments, followed by a narrow metasoma of three reduced segments, represents an advanced transitional condition in the development of the xiphosurid thoracetron. Lunataspis further possesses a large crescentic prosomal shield bearing lateral compound eyes on weak ophthalmic ridges that flank a low cardiac lobe, and a keeled lanceolate telson. Lunataspis is much older than the proposed ‘synziphosurine’ stem lineage of Carboniferous and post-Palaeozoic Xiphosurida, yet is strikingly similar to crown group limuline horseshoe crabs, indicating that major features of the distinctive and highly conserved xiphosurid Bauplan evolved considerably earlier in the Palaeozoic than was previously suspected. In addition to establishing a new temporal benchmark for assessing hypotheses of early chelicerate relationships, the discovery of horseshoe crabs in a Late Ordovician marginal marine setting marks the earliest definitive record of this persistent ecological association.