PALEOECOLOGICAL AND STRATIGRAPHIC CONTROLS ON EURYPTERID LAGERSTÄTTEN: A MODEL FOR PRESERVATION IN THE MID-PALEOZOIC

Konservat-Lagerstätten are increasingly being shown to be predictable within a sequence stratigraphic framework. In this study, we hypothesize that formation of eurypterid Lagerstätten in the mid-Paleozoic of Laurentia was controlled by an ecological-taphonomic window that recurred predictably in nearshore, marginal environments during transgressions. To test this hypothesis, a taxonomic and environmental survey was performed on all Silurian–Early Devonian-age eurypterid-bearing intervals in the Appalachian basin, the most prolific region for eurypterid remains in the world. Canonical correspondence analysis (CCA) indicates a strong lithological gradient between groupings of eurypterid genera and associated taxa across basin. Results from the CCA and quantitative analyses of sedimentological and faunal associations, in conjunction with field observations, indicate that: a) associated faunal diversity is greatest within eurypterid-bearing units, and b) eurypterids in the basin frequently occur above microbialitic structures (thrombolites, stromatolites) and beneath indicators of increased salinity or sub-aerial exposure (evaporitic minerals, desiccation cracks). Based on these parameters, we present a sequence stratigraphic model in which microbialites represent flooding surfaces and evaporites/sub-aerial exposure features represent the tops of shallowing-upward parasequences. In this scenario, eurypterids primarily occurred within freshening conditions concomitant with minor transgressions; subsequent shallowing-up successions promoted hypersalinity and anoxia, which facilitated soft-tissue preservation. In the central and southern region of the basin, where microbial structures and evidence for hypersalinity are less common, a similar pattern of cyclical shallowing-upward deposition within eurypterid-bearing units holds. Thus, eurypterid preservation appears to reflect a combination of ecological preferences and abiotic conditions that promote inhabitation and eventual preservation within the same setting. Ultimately, this model suggests that formation of eurypterid Lagerstätten in the mid-Paleozoic are controlled primarily by fluctuations in water depth.