TAPHONOMIC AND ECOLOGICAL CONTROLS ON EURYPTERID LAGERSTäTTEN: A MODEL FOR PRESERVATION IN THE MID-PALEOZOIC

The frequent occupation of marginal, often ephemeral environments by many eurypterid taxa requires consideration of conditions favorable to not only habitation, but also preservation. Detailed sedimentological, taphonomic, and paleoecological analyses of Upper Silurian and Lower Devonian eurypterid Lagerstätten provide the basis for an ecological/taphonomic model to explain these rare occurrences. The cyclic occurrences of eurypterids in marginal settings across the Appalachian basin, and to a lesser extent Baltica, appear to be the result of local or regional meter-scale sea level fluctuations in predominantly hypersaline peritidal environments. In these settings, eurypterid occurrences typically occur at or just above flooding surfaces, often in conjunction with transgressive indicators such as stromatolites, thrombolites, and associated bio-lithofacies. Their association with euryhaline taxa (e.g., small gastropods, leperditians, lingulids) may be a reflection of ameliorating salinity conditions concomitant with transgressions. A second observation is that eurypterid horizons frequently underlie beds with evidence for subaerial exposure and/or hypersalinity. In rare cases where eurypterids and obvious paleosalinity indicators such as salt hoppers are found on the same horizon, we suggest that the latter are the result of superimposed post-burial effects rather than indicators of the life environment. Moreover, the rapid superposition of hypersaline waters and/or desiccating conditions on buried eurypterid remains may have aided in their long term preservation. The lack of eurypterids (and generally all other fauna) in otherwise “ideal” transgressive-regressive cycles throughout much of the Salina and Bertie group of the Appalachian basin suggests that subtidal waters commonly remained too hypersaline for temporary colonization by these organisms from nearby estuarine settings. Thus, both temporary freshening and subsequent restriction were necessary for eurypterids to live and then become preserved. We suggest that the tendency of eurypterids to inhabit marginal marine and ephemeral settings may have predisposed them to form Lagerstätten in environments for which an unusual preservational “window” remained open long into the Phanerozoic.