INTERPRETING EURYPTERID PALEOECOLOGY FROM GILL MICROSTRUCTURE REVEALED THROUGH MICROCT SCANNING AND TRENDS TOWARDS TERRESTRIALIZATION IN CHELICERATES

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 reconstruct aspects of their paleoecology. 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 than a molt. This interpretation is confirmed by the presence of an alimentary canal, which also represents the first well- preserved eurypterid digestive system. Three pairs of book gills are preserved and appear to comprise only six lamellae each, which is a low number in comparison to modern xiphosurans and arachnids. The relatively few gill lamellae preserved in each book gill 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. However, the occurrence of pillar-like trabeculae and ridges on the dorsal surface of each gill lamella suggests that the eurypterid was fully capable of subaerial breathing, and was capable of spending extended time on land. Despite this, the animal retains gnathobases on the appendages for processing food. Gnathobases require aquatic suspension to function, and so eurypterids were unable to spend vast periods of times out of the water. The morphological details preserved by this unique specimen provide further support that eurypterids are the sister group to Arachnida, that ‘merostomes’ (aquatic euchelicerates with book gills) are paraphyletic, and that each major euchelicerate clade exhibits increasing adaptations to a terrestrial habitat.