Re-assessment of the aberrant brittle star genus Astrosombra (Echinodermata, Ophiuroidea), based on an exceptionally preserved specimen from the Upper Cretaceous of Rügen, north-east Germany

Amongs the most extraordinary extinct ophiuroids is the genus Astrosombra from the Upper Cretaceous of Germany. Initially known from isolated ossicles only, the extremely expanded lateral arm plates suggested an aberrant body shape comparable to that seen in the extraordinary astrophiurid genus Astrophiura . Here, we describe the first articulated specimen of Astrosombra rammsteinensis , the type and only known species of the genus, from the type locality of the taxon, showing the highly modified lateral arm plates in their anatomical context and providing insights into the general skeletal morphology. The new specimen confirms the presence of extremely widened arms, formed by the aberrantly expanded lateral arm plates articulating with unusually wide vertebrae and covered dorsally by multiple rows of dorsal arm plates. The width of the arms is further increased by large, flat, feather-like arm spines. MicroCT images of the articulated disc revealed morphological details, in particular high oral plates with wing-like expansions, suggesting transfer of Astrosombra from the Amphilimnidae to the Ophiopsilidae. In analogy with Astrophiura , the extremely widened arms, the fringe of flat arm spines, the large tentacle pores and the overall flat body shape suggest that Astrosombra inhabited the small, patchy hard substrates formed by the skeletal remains of other organisms.