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Background. The sea spiders (Pycnogonida Latreille, 1810) of the Hunsrück Slate (Lower Devonian,~ 400 million years ago) are iconic in their abundance, exquisite pyritic preservation, and in their distinctive body plan compared to extant sea spiders (Pantopoda Gerstäcker, 1863). Consequently, the Hunsrück sea spiders are important in understanding the deep evolutionary history of Pycnogonida, yet they remain poorly characterised, impacting upon attempts to establish a time-calibrated phylogeny of sea spiders.

Methods. Here we investigated previously described and new material representing four of the ûve Hunsrück pycnogonids: Flagellopantopus blocki Poschmann & Dunlop, 2006; Palaeoisopus problematicus Broili, 1928; Palaeopantopus maucheri Broili, 1929; and Pentapantopus vogteli Kühl, Poschmann & Rust, 2013; as well as a few unidentiûed specimens. Using X-ray microtomography and Reûectance Transformation Imaging, we describe new fossils, provide evidence for newly revealed anatomical features, and interpret these data in comparison to extant species. We also reinterpret the previously published illustration of (the probably lost) holotype of Palaeothea devonica Bergström, Stürmer & Winter, 1980.

Results. We provide the ûrst detailed description of the cephalic appendages of Palaeoisopus

problematicus and revise the interpretation of the organisation of its ocular tubercle. Furthermore, we provide new insights into the structure of the legs and the proboscis of Palaeopantopus maucheri, the ûrst description of the body of Flagellopantopus blocki and describe a new specimen of Pentapantopus vogteli, demonstrating …