New insights into the Devonian sea spiders of the Hunsrück Slate (Arthropoda: Pycnogonida).

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 five Hunsrück pycnogonids: Poschmann & Dunlop, 2006; Broili, 1928; Broili, 1929; and Kühl, Poschmann & Rust, 2013; as well as a few unidentified specimens. Using X-ray microtomography and Reflectance 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 Bergström, Stürmer & Winter, 1980.

Results: We provide the first detailed description of the cephalic appendages of 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 , the first description of the body of and describe a new specimen of , demonstrating that it had eight legs, in contrast to previous interpretations. We argue that, contrary to previous suggestions, probably had a different body plan from extant pantopods. We discuss the ecological traits of the Hunsrück pycnogonids based on their morphological adaptations, and conclude that there is no compelling evidence of Pantopoda in the Devonian. Through comparative interpretation of the legs as well as general morphology, we can divide the Hunsrück pycnogonids into two morphological groups, while Pantopoda constitutes a third morphological group.