Ostracodes as indicators of brackish water environments in the Catskill Magnafacies (Devonian) of New York State

Abstract

Leperditiid and beyrichiid ostracodes in marine margin deposits of the Devonian Catskill magnafacies of New York State document brackish conditions in what were previously considered freshwater floodplain deposits. The very low diversity ostracode fauna (two species) includes an abundance of anomalously thin-shelled individuals that belong to the genera Leperditia? and Welleria. This ostracode fauna is one of the oldest reported from brackish water in the stratigraphic record, and it documents the early ecological radiation of ostracodes into waters of reduced salinity. Associated fossils, including the bivalve Archanodon and the conchostracan Euestheria, have been considered to be freshwater forms, partly because of their occurrence in these rocks that were assumed to be fluvial in origin. Nearly everywhere, however, the ostracodes are associated with cosmine-bearing fish fragments, lingulid brachiopods, and the feeding trace Spirophyton, which have a marine or brackish-water affiliation. The presence of open carapaces on bedding planes as well as adults and several instar stages of each ostracode species indicate lack of postmortem transport. At least one ostracode-bearing mudstone contains the mineral amesite; associated calcareous mudrocks interbedded with these units contain symmetrical ripple marks, mudcracks, and locally intense manganese-oxide staining. Collectively, these features indicate extreme environmental conditions and suggest highly variable salinities in shallow pools or embayments along the paleoshoreline. The presence of brackish-water faunas, perhaps estuarine in origin, indicates an aspect of the diversity of Devonian paleoenvironments along this portion of the Catskill deltaic complex not previously recognized. The deposits are widespread, although not ubiquitous, within thick sequences of fine-grained flood-basin deposits. Because most previous paleoecological studies of fossils from these rocks assumed the sediments to be of freshwater origin, the new interpretation of brackish-water communities presented here could have important implications for understanding the evolution of early terrestrial organisms. For example, some animals may have evolved in brackish estuarine environments rather than in freshwater rivers as assumed in previous studies. Recognition of this brackish facies elsewhere in the Catskill succession will not only aid future identification of paleoshoreline position but may also help clarify whether early terrestrial organisms made the transition to land via brackish or freshwater environments.