A Silurian sea spider

Pycnogonids (sea spiders) are marine arthropods numbering some 1,160 extant species. They are globally distributed in depths of up to 6,000 metres, and locally abundant1,2; however, their typically delicate form and non-biomineralized cuticle has resulted in an extremely sparse fossil record that is not accepted universally3. There are two opposing views of their phylogenetic position: either within Chelicerata as sister group to the euchelicerates4,5,6,7, or as a sister taxon to all other euarthropods8. The Silurian Herefordshire Konservat-Lagerstätte9 in England (∼ 425 million years (Myr) bp) yields exceptionally preserved three-dimensional fossils that provide unrivalled insights into the palaeobiology of a variety of invertebrates10,11,12,13,14. The fossils are preserved as calcitic void in-fills in carbonate concretions within a volcaniclastic horizon15, and are reconstructed digitally12. Here we describe a new pycnogonid from this deposit, which is the oldest adult sea spider by ∼35 Myr and the most completely known fossil species. The large chelate first appendage is consistent with a chelicerate affinity for the pycnogonids. Cladistic analyses place the new species near the base of the pycnogonid crown group, implying that the latter had arisen by the Silurian period.