The Herefordshire (Silurian) Konservat-Lagerstätte in England yields remarkable, three-dimensional, non-biomineralized fossils in carbonate concretions hosted in a volcaniclastic deposit. Pycnogonids (sea spiders) have an extremely sparse fossil record, and are known globally from just four species based on a few tens of specimens from two localities. A new, exceptionally-preserved sea spider from the Herefordshire deposit represents the oldest known adult by some 35 million years, the most completely known fossil species, and provides new insight into the early morphology of the Pycnogonida and its relationship to other arthropod groups. Cladistic analyses place the new species near the base of the pycnogonid crown group, implying that the latter had arisen by the Silurian. The morphology of this recently discovered form has been digitally reconstructed to produce a ‘virtual fossil’ in the round.