The secret history of sea spiders (Arthropoda: Pycnogonida)
Résumé
Sea spider fossils (Chelicerata: Pycnogonida) are restricted to eleven species sparsely distributed from
the Silurian to the Jurassic. Their morphology and relationships to extant pycnogonids are poorly
understood. To improve their description, nest them in the chelicerate phylogeny and decipher
the timing of Pycnogonida’s evolution, we conducted a comprehensive reinvestigation of the sea
spider fossil record. We studied approximately 60 fossils, published or new, from the KonservatLagerstätten of Herefordshire, UK (Silurian), the Hunsrück Slate, Germany (Devonian), La Voulte-sur-Rhône, France and Solnhofen, Germany (Jurassic). Using X-ray microtomography, Reflectance
Transformation Imaging, serial grinding and microtopography, we illustrate the past diversities
of Pycnogonida. We show that while Palaeozoic fossils exhibit a stunning diversity of forms and
adaptations unmatched in extant species, Mesozoic fossils share the same body plan as extant sea
spiders, suggesting they belong to the crown-group Pycnogonida, i.e. Pantopoda. By performing the
first total-evidence phylogeny of Chelicerata including the eleven sea spider fossil species, we further
support our taxonomic interpretations. We demonstrate that Mesozoic fossils are nested within
extant families or superfamilies, while Palaeozoic species belong to the stem-group Pycnogonida.
Finally, we date the phylogeny of Pycnogonida using both node-dating and tip-dating strategies and
estimate the divergence time of major Pycnogonida lineages.