Eurypterids are generally rare in the fossil record, but occasionally occur in abundance. The genus Eurypterus, in particular, is well known from certain upper Silurian Lagerstätten of the northern Appalachian basin (New York and Ontario), but occurs far less frequently in the central and southern Appalachian basin (Pennsylvania, Maryland, and West Virginia, respectively). The recent discovery of an exceptionally preserved mass assemblage of Eurypterus in the upper Tonoloway Formation (upper Ludlow–Přídolí) of Pennsylvania provides new information on the behavior and life habitat of the genus in this region. Eurypterids at this locality are found in thinly laminated, calcareous shale deposited within the lower intertidal to shallow subtidal zone of a coastal mudflat or sabkha. Rare associated fauna of limited diversity, and evaporitic and desiccation features in associated beds, suggest a stressed environment with variable salinity and possible hypoxic conditions. Most eurypterids are disarticulated and fragmentary, but several fully articulated, exceptionally preserved specimens are present. Exoskeletal features and taphonomic indices values indicate a molt rather than death assemblage, and the presence of arthropod trackways suggests that Eurypterus sp. may have molted en masse in the vicinity of the burial site. Sequence stratigraphic interpretation of the site suggests that preservation of eurypterid remains is the result of occupation of ephemeral environmental (salinity/oxygen) conditions during a transgression. The occurrence of this new Lagerstätte within the upper Silurian succession of the central Appalachians, an interval which had heretofore yielded only rare, fragmentary remains, indicates that eurypterids were more prevalent in this region than previously thought.

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