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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.

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Figure 1: Haliestes dasos and the extant pycnogonid Nymphon gracile.
Figure 2: Strict consensus (consistency index = 0.38, retention index = 0.66) of three most-parsimonious trees of length 203 obtained from analysis of data matrix (see Supplementary Table).

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This work was supported by the Leverhulme Trust, NERC and English Nature. K. Saunders is thanked for technical assistance; C. Arango, R. Bamber and D. Waloszek for discussion; G. Boxshall, J. Dunlop and D. Waloszek for pre-prints of their papers; S. de Grave for providing Recent material; and R. Fenn, T. Hall and J. Sinclair for general assistance.

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Correspondence to Derek J. Siveter.

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Supplementary information

Supplementary Figure 1

Expansion of Figure 2; cladogram of pycnogonids including fossil taxa (see Figure 2 for legend). (JPG 117 kb)

Supplementary Methods

Details of methodology for cladistic analyses. (DOC 20 kb)

Supplementary Note 1

Details of functional argument for interpretation of walking-leg segmentation. (DOC 21 kb)

Supplementary Note 2

Details of general functional morphology and mode of life. (DOC 29 kb)

Supplementary Table 1

Data matrix and character definitions for cladistic analyses. (XLS 49 kb)

Supplementary Video 1

Movie comprising serial grinding images from OUM C.29571. Slice interval is 20 µm, field of view horizontally is 5.15 mm. (MP4 912 kb)

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Siveter, D., Sutton, M., Briggs, D. et al. A Silurian sea spider. Nature 431, 978–980 (2004).

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