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The Eurypterida (Arthropoda: Chelicerata) are unique among chelicerates in having undergone a complete marine-to-freshwater transition during their history. Although the general pattern is well documented, habitats during the peak of the transition during the Silurian and Lower Devonian have remained unclear. This is due to the co-occurrence of eurypterids with euryhaline and marine faunas in environments that have been interpreted as hypersaline, and a paucity of detailed bed-level data within eurypterid-bearing occurrences, such within the numerous Lagerstatten of the Laurentia. A high-resolution field- and specimen-based analysis was carried out on eurypterid occurrences in Silurian–Lower Devonian-age exposures in the Appalachian basin. Eurypterids in the upper Silurian Tonoloway Formation of Pennsylvania occur in a transgressive succession above microbial structures (thrombolites) with a marine fauna that suggests near-euhaline conditions. The paucity of occurrences in adjacent hypersaline facies suggests that eurypterids preferentially occupied freshening conditions. A survey of all eurypterid-bearing units in the basin found that the co-occurrence of eurypterids and hypersaline indicators (evaporites) are common, but only in the northern basin. Here, eurypterid and associated fauna frequently co-occur with disruptive, isolated salt hoppers (pyramidal-shaped halite pseudomorphs), which suggests intra-sedimentary formation. Thus, eurypterid-evaporite associations appear to reflect early-stage diagenesis rather than burial conditions, and it is unlikely that eurypterids inhabited hypersaline conditions. Analysis of eurypterid-bearing strata at the centimeter-scale reveals strong sequence stratigraphic controls on preservation. Eurypterid preservation appears to be principally controlled by water depth in nearshore settings. In carbonate-dominated environments, eurypterids often occur within or above beds containing microbial structures (thrombolites, stromatolites) that are interpreted as the flooding surface within small-scale transgressive events. Shallowing-upward successions above these beds, indicated by desiccation features (evaporites, desiccation cracks), are the result of regressions and/or evaporation. A preservational model is proposed whereby eurypterids entered nearshore settings during transgressive freshening events; subsequent hypersalinity and/or dysoxia or anoxia in post-burial sediments following regression was favorable to excellent preservation of cuticle. In settings lacking microbial structures or hypersaline indicators, eurypterids occur in similar transgressive successions. Such stratigraphic constraint on eurypterid occurrences permits detailed assessment of morphological variation among equivocal species. A combined landmark and semi-landmark-based geometric morphometric analysis of eurypterids from contemporaneous upper Silurian populations indicates that eurypterid morphospecies cannot be distinguished using isolated carapaces, unless identifiable macro-scale characters are present. In well-defined species, however, a combined landmark/semi-landmark approach allows regional-scale variance to be quantified. |
