BREATHING NEW LIFE INTO AN EXTINCT SEA SCORPION: REVEALING THE GILL STRUCTURE OF A THREE-DIMENSIONALLY PRESERVED EURYPTERID THROUGH MICROCT SCANNING

New technologies are now making it possible to digitally observe and reconstruct fossils that are difficult or impossible to expose through mechanical preparation without causing damage. This has prompted renewed research of exceptionally-preserved fossils that provide unique windows into how extinct organisms lived. In particular, these fossils make it possible to reconstruct long-dead animals as living creatures and place them in the tree of life. In this study, we use CT scanning to reconstruct a unique three-dimensionally preserved Adelophthalmus eurypterid from the Carboniferous of Montagne Noire, France. By analogy to scorpion ecdysis, the retracted position of the walking and swimming appendages as well as the straight alignment of the body suggests that the specimen is a carcass rather a molt. This interpretation is confirmed by the presence of an alimentary canal, which also represents the first well- preserved eurypterid digestive system. A short genital appendage and two sets of book gills are preserved. The book gills appear to comprise only six lamellae, which is a low number in comparison to modern xiphosurans and arachnids. However, the attachment of the book gills is more similar to that seen in arachnid book lungs than in xiphosuran book gills. Initial reconstruction also reveals possible pillars between sets of gill lamellae, but these are an order of magnitude larger than the proximal trabeculae seen in scorpions and currently we cannot rule out the possibility that they are diagenetic structures. The morphological details preserved by this unique specimen provide further support that eurypterids are the sister group to Arachnida, and that ‘merostomes’ (aquatic euchelicerates with book gills) are paraphyletic. The relatively few gill lamellae preserved in each book gill also suggests that the ancillary respiratory structures known as kiemenplatten, originally considered to supplement breathing during amphibious terrestrial excursions, may have also had an active role in aquatic respiration.